81 Antibiotics Microbial Assays USP
81 Antibiotics Microbial Assays USP
81 Antibiotics Microbial Assays USP
At intervals during the incubation period and at its conclusion, examine the media for macroscopic evidence of microbial
growth. If the material being tested renders the medium turbid so that the presence or absence of microbial growth cannot be
readily determined by visual examination, 14 days after the beginning of incubation transfer portions (each not less than 1
mL) of the medium to fresh vessels of the same medium, and then incubate the original and transfer vessels for not less than 4
days.
If no evidence of microbial growth is found, the product to be examined complies with the test for sterility. If evidence of
microbial growth is found, the product to be examined does not comply with the test for sterility, unless it can be clearly dem-
onstrated that the test was invalid for causes unrelated to the product to be examined. The test may be considered invalid only
if one or more of the following conditions are fulfilled:
a. The data of the microbiological monitoring of the sterility testing facility show a fault.
b. A review of the testing procedure used during the test in question reveals a fault.
c. Microbial growth is found in the negative controls.
d. After determination of the identity of the microorganisms isolated from the test, the growth of this species (or these spe-
cies) may be ascribed unequivocally to faults with respect to the material and or the technique used in conducting the
sterility test procedure.
If the test is declared to be invalid, it is repeated with the same number of units as in the original test. If no evidence of
microbial growth is found in the repeat test, the product examined complies with the test for sterility. If microbial growth is
found in the repeat test, the product examined does not comply with the test for sterility.
General Chapters
APPLICATION OF THE TEST TO PARENTERAL PREPARATIONS, OPHTHALMIC, AND OTHER
NONINJECTABLE PREPARATIONS REQUIRED TO COMPLY WITH THE TEST FOR STERILITY
When using the technique of membrane filtration, use, whenever possible, the whole contents of the container, but not less
than the quantities indicated in Table 2, diluting where necessary to about 100 mL with a suitable sterile solution, such as ♦Fluid
A (see Diluting and Rinsing Fluids for Membrane Filtration).♦
When using the technique of direct inoculation of media, use the quantities shown in Table 2, unless otherwise justified and
authorized. The tests for bacterial and fungal sterility are carried out on the same sample of the product to be examined.
When the volume or the quantity in a single container is insufficient to carry out the tests, the contents of two or more con-
tainers are used to inoculate the different media.
The minimum number of items to be tested in relation to the size of the batch is given in Table 3.
The activity (potency) of antibiotics can be demonstrated by their inhibitory effect on microorganisms under suitable condi-
tions. A reduction in antimicrobial activity may not be adequately demonstrated by chemical methods. This chapter summari-
zes procedures for the antibiotics recognized in the United States Pharmacopeia (USP) for which the microbiological assay is the
standard analytical method.
Two general techniques are employed: the cylinder-plate (or plate) assay and the turbidimetric (or tube) assay. Table 1 lists
all the antibiotics that contain microbial assays and specifies the type of assay (cylinder-plate or turbidimetric).
Table 1
Antibiotic Type of Assay
Amphotericin B Cylinder-plate
Bacitracin Cylinder-plate
Table 1 (Continued)
Antibiotic Type of Assay
Bleomycin Cylinder-plate
Capreomycin Turbidimetric
Carbenicillin Cylinder-plate
Chloramphenicol Turbidimetric
Chlortetracycline Turbidimetric
Cloxacillin Cylinder-plate
Colistemethate Cylinder-plate
Colistin Cylinder-plate
Cylinder-plate
Dihydrostreptomycin
Turbidimetric
Erythromycin Cylinder-plate
Gentamicin Cylinder-plate
Gramicidin Turbidimetric
Nafcillin Cylinder-plate
Natamycin Cylinder-plate
Cylinder-plate
General Chapters
Neomycin
Turbidimetric
Novobiocin Cylinder-plate
Nystatin Cylinder-plate
Oxytetracycline Turbidimetric
Paromomycin Cylinder-plate
Penicillin G Cylinder-plate
Polymyxin B Cylinder-plate
Sisomicin Cylinder-plate
Tetracycline Turbidimetric
Thiostrepton Turbidimetric
Troleandomycin Turbidimetric
Tylosin Turbidimetric
Vancomycin Cylinder-plate
[NOTE—Perform all procedures under conditions designed to avoid extrinsic microbial contamination. Take adequate safety
precautions while performing these assays because of possible allergies to drugs and because live cultures of organisms are
used in the procedures.]
Cylinder-plate assay: The cylinder-plate assay depends on diffusion of the antibiotic from a vertical cylinder through a solidi-
fied agar layer in a Petri dish or plate. The growth of the specific microorganisms inoculated into the agar is prevented in a
circular area or zone around the cylinder containing the solution of the antibiotic.
Turbidimetric assay: The turbidimetric assay depends on the inhibition of growth of a microorganism in a uniform solution
of the antibiotic in a fluid medium that is favorable to the growth of the microorganism in the absence of the antibiotic.
Units and Reference Standards: The potency of antibiotics is designated in either units (U) or mg of activity. In each case
the unit or mg of antibiotic activity was originally established against a United States Federal Master Standard for that antibiot-
ic. The corresponding USP Reference Standard is calibrated in terms of the master standard.
Originally, an antibiotic selected as a reference standard was thought to consist entirely of a single chemical entity and was
therefore assigned a potency of 1000 mg/mg. In several such instances, as the manufacturing and purification methods for
particular antibiotics became more advanced, antibiotics containing more than 1000 mg of activity/mg became possible. Such
antibiotics had an activity equivalent to a given number of mg of the original reference standard. In most instances, however,
the mg of activity is exactly equivalent numerically to the mg (weight) of the pure substance. In some cases, such as those listed
below, the mg of activity defined in terms of the original master standard is equal to a unit:
1. Where an antibiotic exists as the free base and in salt form and the mg of activity has been defined in terms of one of
these forms
2. Where the antibiotic substance consists of a number of components that are chemically similar but differ in antibiotic ac-
tivity
3. Where the potencies of a family of antibiotics are expressed in terms of a reference standard consisting of a single mem-
ber which, however, might itself be heterogeneous
Do not assume that the mg of activity corresponds to the mg (weight) of the antibiotic substance.
Apparatus: Labware used for the storage and transfer of test dilutions and microorganisms must be sterile and free of inter-
fering residues (see Cleaning Glass Apparatus á1051ñ). Use a validated sterilization method, such as dry heat, steam, or radia-
tion; or use sterile, disposable labware.
Temperature control: Thermostatic control is required in several stages of a microbial assay: when culturing a microorgan-
ism and preparing its inoculum, and during incubation in plate and tube assays. Refer to specific temperature requirements
below for each type of assay.
Test organisms: The test organism for each antibiotic is listed in Table 3 for the cylinder-plate assay and Table 8 for the turbi-
dimetric assay. The test organisms are specified by the American Type Culture Collection (ATCC) number.
In order to ensure acceptable performance of test organisms, store and maintain them properly. Establish the specific stor-
age conditions during method validation or verification. Discard cultures if a change in the organism's characteristics is ob-
served.
Prolonged storage: For prolonged storage, maintain test organisms in a suitable storage solution such as 50% fetal calf
serum in broth, 10%–15% glycerol in tryptic soy broth, defribinated sheep blood, or skim milk. Prolonged-storage cultures are
best stored in the freeze-dried state; temperatures of −60° or below are preferred; temperatures below −20° are acceptable.
Primary cultures: Prepare primary cultures by transferring test organisms from prolonged-storage vials onto appropriate
media, and incubate under appropriate growth conditions. Store primary cultures at the appropriate temperature, usually 2°–
8°, and discard after three weeks. A single primary culture can be used to prepare working cultures only for as many as seven
days.
Working cultures: Prepare working cultures by transferring the primary culture onto appropriate solid media to obtain
isolated colonies. Incubate working cultures under appropriate conditions to obtain satisfactory growth for preparation of test
General Chapters
inocula. Prepare fresh working cultures for each test day.
Uncharacteristic growth or performance of a test organism: Use new stock cultures, primary cultures, or working cul-
tures when a test organism shows uncharacteristic growth or performance.
Assay designs: Suitable experimental designs are key to increasing precision and minimizing bias. Control of the incubation
parameters, temperature distribution and time, is critical for minimizing bias; it can be accomplished by staging the plates and
racks as described for each assay.
Cylinder-plate assay: The comparisons are restricted to relationships between zone diameter measurements within
plates, excluding the variation between plates. Individual plate responses are normalized on the basis of the relative zone size
of the standard compared to the mean zone size of the standard across all plates.
Turbidimetric assay: To avoid systematic bias, place replicate tubes randomly in separate racks so that each rack contains
one complete set of treatments. The purpose of this configuration is to minimize the influence of temperature distribution on
the replicate samples. The turbidimetric assay, because of the configuration of the samples in test tube racks, is sensitive to
slight variations in temperature. The influence of temperature variation can also be decreased by ensuring proper airflow or
heat convection during incubation. At least three tubes for each sample and standard concentration (one complete set of sam-
ples) should be placed in a single rack. The comparisons are restricted to relationships between the observed turbidities within
racks.
Potency considerations: Within the restrictions listed above, the recommended assay design employs a five-concentra-
tion standard curve and a single concentration of each sample preparation.
For the cylinder-plate assay, each plate includes only two treatments, the reference treatment (median level standard, i.e.,
S3) and one of the other four concentrations of the standard (S1, S2, S4, and S5) or the sample (U3). The concentration of the
sample is an estimate based on the target concentration. The sample should be diluted to give a nominal concentration that is
estimated to be equivalent to the median reference concentration (S3) of the standard. The purpose of diluting to the median
reference concentration is to ensure that the sample result will fall within the linear portion of the standard curve. The test
determines the relative potency of U3 against the standard curve. The sample (U3) should have a relative potency of about
100%. The final potency of the sample is obtained by multiplying the U3 result by the dilution factor.
An assay should be considered preliminary if the computed potency value of the sample is less than 80% or more than
125%. In this case, the results suggest that the sample concentration assumed during preparation of the sample stock solution
was not correct. In such a case, one can adjust the assumed potency of the sample on the basis of the preliminary potency
value and repeat the assay. Otherwise, the potency will be derived from a portion of the curve where the standard and sample
responses will likely not be parallel.
Microbial determinations of potency are subject to inter-assay as well as intra-assay variables; therefore two or more inde-
pendent assays are required for a reliable estimate of the potency of a given sample. Starting with separately prepared stock
solutions and test dilutions of both the standard and the sample, perform additional assays of a given sample on a different
day. The mean potency should include the results from all the valid independent assays. The number of assays required in
order to achieve a reliable estimate of potency depends on the variability of the assay and the required maximum uncertainty
for the potency estimate. The latter is assessed by the width of the confidence interval (refer to Calculations, Confidence limits
and combinations of assay calculations). The combined result of a series of smaller, independent assays spread over a number of
days is a more reliable estimate of potency than one from a single large assay with the same total number of plates or tubes.
Note that additional assays or lower variability allows the product to meet tighter specification ranges. Reducing assay variabili-
ty achieves the required confidence limit with fewer assays.
Cylinder-Plate Method
Temperature control: Use appropriately qualified and calibrated equipment to obtain the temperature ranges specified in
Table 3.
Apparatus
Plates: Glass or disposable plastic Petri dishes (approximately 20 × 100 mm or other appropriate dimensions) with lids
Cylinders: Stainless steel or porcelain cylinders; 8 ± 0.1-mm o.d.; 6 ± 0.1-mm i.d.; 10 ± 0.1-mm high. [NOTE—Carefully
clean cylinders to remove all residues; occasional cleaning in an acid bath, e.g., with about 2 N nitric acid or with chromic acid
(see Cleaning Glass Apparatus á1051ñ) is required.]
Standard solutions: To prepare a stock solution, dissolve a suitable quantity of the USP Reference Standard of a given antibi-
otic, or the entire contents of a vial of USP Reference Standard, where appropriate, in the solvent specified in Table 2; and
dilute to the specified concentration. Store at 2°–8°, and use within the period indicated. On the day of the assay, prepare
from the stock solution five or more test dilutions, in which the successive solutions increase stepwise in concentration, usually
in the ratio of 1:1.25. Use the final diluent specified such that the median has the concentration suggested in Table 2.
Sample solutions: Assign an assumed potency per unit weight or volume to the sample. On the day of the assay prepare a
stock solution in the same manner specified for the USP Reference Standard (Table 2). Dilute the sample stock solution in the
specified final diluent to obtain a nominal concentration equal to the median concentration of the standard (S3).
Table 2
Stock Solution Test Dilution
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dilution of the sample should contain the same amount of dimethyl sulfoxide as the test dilutions of the USP Reference Standard.
e The letter B refers to buffer. See Media and Solutions, Buffers for a description of each buffer listed in this table.
f Each of the standard test dilutions should contain the same amount of hydrochloric acid as the test dilution of the sample.
g The turbidimetric assay can be used as an alternative procedure.
h Further dilute the stock solution with dimethylformamide to give concentrations of 256, 320, 400, 500, and 624 U/mL before making the test dilutions. Pre-
pare the standard test dilutions simultaneously with test dilutions of the sample to be tested. The test dilution of the sample should contain the same amount of
dimethylformamide as the test dilutions of the standard. Use low-actinic glassware.
i Prepare the stock solution by adding 2 mL of water for each 5 mg of the USP Reference Standard.
Inocula: Suspend the test organism from a freshly grown slant or culture in 3 mL of sterile saline TS. Glass beads can be used
to facilitate the suspension. Spread the saline suspension onto the surface of two or more agar plates (covering the entire sur-
face) or onto the surface of a Roux bottle containing 250 mL of the specified medium (see Table 3).
Incubate for the specified time and at the temperature as specified in Table 3, or until growth is apparent.
After incubation, harvest the organism from the plates or Roux bottle with approximately 50 mL of sterile saline TS (except
use Medium 34 for bleomycin; see the section Media and Solutions), using a sterile bent glass rod or sterile glass beads. Pipet
the suspension into a sterile glass container. This is the harvest suspension.
Dilute an appropriate amount of the harvest suspension with sterile saline TS. Using the UV-visible spectrophotometer,
measure % transmittance at 580 nm. The target value is approximately 25% transmittance at 580 nm. This value is used to
standardize the harvest suspension volume added to the seed layer agar.
Starting with the suggested volumes indicated in Table 3, determine during method verification the proportions of stock
suspension to be added to the inoculum medium that result in satisfactory zones of inhibition of approximately 14–16 mm in
diameter for the median concentration of the standard (S3). [NOTE—Zone sizes that are outside the 11 to 19-mm range are not
desirable, because these contribute to assay variability.] If the dilution percentage transmittance is above 25%, a ratio may be
used to normalize the addition of organism to the seed layer. The normalization factor can be determined by dividing the per-
centage transmittance obtained from the dilution by 25. This ratio can then be multiplied by the suggested inoculum amount
to obtain the volume (mL) of harvest suspension that needs to be added to the seed layer. Adjust the quantity of inoculum on
a daily basis, if necessary, to obtain an optimum concentration–response relationship.
Alternatively, determine during method verification the proportion of harvest suspension to be incorporated into the inocu-
lum, starting with the volumes indicated in Table 3, that result in satisfactory demarcation of the zones of inhibition of about
14–16 mm in diameter for the median concentration of the standard (S3) and giving a reproducible concentration–response
relationship. Prepare the inoculum by adding a portion of stock suspension to a sufficient amount of agar medium that has
been melted and cooled to 45°–50°. Swirl the mixture without creating bubbles in order to obtain a homogeneous suspen-
General Chapters
sion.
Table 3
Suggested
Incubation Conditions Inoculum Composition
Tempera-
ATCCa ture Amount
Antibiotic Test Organism Number Mediumb (°) Time Mediumb (mL/100 mL)
Amphotericin B Saccharomyces cerevisiae 9763 19 29–31 48 h 19 1.0
Bacitracin Micrococcus luteus 10240 1 32–35 24 h 1 0.3
Bleomycin Mycobacterium smegmatis 607 36 36–37.5 48 h 35 1.0
Carbenicillinc Pseudomonas aeruginosa 25619 1 36–37.5 24 h 10 0.5
Cloxacillin Staphylococcus aureus 29737 1 32–35 24 h 1 0.1
Colistimethate Bordetella bronchiseptica 4617 1 32–35 24 h 10 0.1
Colistin Bordetella bronchiseptica 4617 1 32–35 24 h 10 0.1
Dihydrostreptomycin Bacillus subtilis 6633 32 32–35 5 days 5 As required
Erythromycin Micrococcus luteus 9341 1 32–35 24 h 11 1.5
Gentamicin Staphylococcus epidermidis 12228 1 32–35 24 h 11 0.03
Nafcillin Staphylococcus aureus 29737 1 32–35 24 h 1 0.3
Neomycin Staphylococcus epidermidis 12228 1 32–35 24 h 11 0.4
Novobiocin Staphylococcus epidermidis 12228 1 32–35 24 h 1 4.0
Nystatin Saccharomyces cerevisiae 2601 19 29–31 48 h 19 1.0
Paromomycin Staphylococcus epidermidis 12228 1 32–35 24 h 11 2.0
Penicillin G Staphylococcus aureus 29737 1 32–35 24 h 1 1.0
Polymyxin B Bordetella bronchiseptica 4617 1 32–35 24 h 10 0.1
Sisomicin Staphylococcus epidermidis 12228 1 32–35 24 h 11 0.03
Vancomycin Bacillus subtilis 6633 32 32–35 5 days 8 As required
a American Type Culture Collection, 10801 University Boulevard, Manassas VA 20110-2209 (http://www.atcc.org)
b See Media and Solutions, Media.
c Use 0.5 mL of a 1:25 dilution of the stock suspension/100 mL of Medium 10.
Analysis: Prepare the base layer for the required number of assay Petri plates, using the medium and volume shown in Table
4. Allow it to harden into a smooth base layer of uniform depth. Prepare the appropriate amount of seed layer inoculum (Table
5) as directed for the given antibiotic (Table 3) with any adjustments made based on the preparatory trial analysis. Tilt the
plate back and forth to spread the inoculum evenly over the base layer surface, and allow it to harden.
Drop six assay cylinders on the inoculated surface from a height of 12 mm, using a mechanical guide or other device to
ensure even spacing on a radius of 2.8 cm, and cover the plates to avoid contamination. Fill the six cylinders on each plate
with dilutions of antibiotic containing the test levels (S1–S5 and U3) specified in the following paragraph. Incubate the plates as
specified in Table 6 for 16–18 h, and remove the cylinders. Measure and record the diameter of each zone of growth inhibition
to the nearest 0.1 mm.
Table 6
Antibiotic Incubation Temperature (°)
Amphotericin B 29–31
Carbenicillin 36–37.5
Colistimethate 36–37.5
Colistin 36–37.5
Dihydrostreptomycin 36–37.5
Gentamicin 36–37.5
Neomycin 36–37.5
Novobiocin 34–36
Nystatin 29–31
Paromomycin 36–37.5
Polymyxin B 36–37.5
Sisomicin 36–37.5
Vancomycin 36–37.5
All others 32–35
The standards (S1–S5) and a single test level of the sample (U3) corresponding to S3 of the standard curve, as defined in
Standard solutions and Sample solutions will be used in the assay. For deriving the standard curve, fill alternate cylinders on
each of three plates with the median test dilution (S3) of the standard and each of the remaining nine cylinders with one of the
other four test dilutions of the standard. Repeat the process for the three test dilutions of the standard. For the sample, fill
alternate cylinders on each of three plates with the median test dilution of the standard (S3), and fill the remaining nine cylin-
ders with the corresponding test dilution (U3) of the sample.
Turbidimetric Method
Temperature control: Use appropriately qualified and calibrated equipment to obtain the temperature ranges specified in
Table 8. [NOTE—Temperature control can be achieved using either circulating air or water. The greater heat capacity of water
lends it some advantage over circulating air.]
Spectrophotometer: Measuring absorbance or transmittance within a fairly narrow frequency band requires a suitable spec-
trophotometer in which the wavelength can be varied or restricted by use of 580-nm or 530-nm filters. Alternatively, a varia-
ble-wavelength spectrophotometer can be used and set to a wavelength of 580 nm or 530 nm.
The instrument may be modified as follows:
1. To accept the tube in which incubation takes place (see Apparatus below)
2. To accept a modified cell fitted with a drain that facilitates rapid change of contents
3. To contain a flow cell for a continuous flowthrough analysis
Autozero the instrument with clear, uninoculated broth prepared as specified for the particular antibiotic, including the
same amount of test dilution (including formaldehyde if specified) as found in each sample.
Either absorbance or transmittance can be measured while preparing inocula.
Apparatus: Glass or plastic test tubes, e.g., 16 × 125 mm or 18 × 150 mm. [NOTE—Use tubes that are relatively uniform in
length, diameter, and thickness and substantially free from surface blemishes and scratches. In the spectrophotometer, use
General Chapters
matched tubes that are free from scratches or blemishes. Clean tubes thoroughly to remove all antibiotic residues and traces of
cleaning solution. Sterilize tubes before use.]
Standard solutions: To prepare a stock solution, dissolve a quantity of the USP Reference Standard of a given antibiotic or
the entire contents of a vial of USP Reference Standard, where appropriate, in the solvent specified in Table 7, and dilute to the
required concentration. Store at 2°–8°, and use within the period indicated. On the day of the assay, prepare from the stock
solution five or more test dilutions, the successive solutions increasing stepwise in concentration, usually in the ratio of 1:1.25.
[NOTE—It may be necessary to use smaller ratios for the successive dilutions from the stock solution for the turbidimetric as-
say.] Use the final diluent specified such that the median level of the standard (S3) has the concentration suggested in Table 7.
Sample solutions: Assign an assumed potency per unit weight or volume to the unknown, and on the day of the assay pre-
pare a stock solution in the same manner specified for the USP Reference Standard (Table 7). Dilute the sample stock solution
in the specified final diluent at a nominal concentration equal to the median concentration of the standard (S3) as specified in
Table 7.
Table 7
Stock Solution Test Dilution
Final Stock Median
Initial Initial Further Concentra- Use Final Concentration
Antibiotic Solvent Concentration Diluent tion Within Diluent (S3)a
Capreomycin Water — — 1 mg/mL 7 days Water 100 mg/mL
Chloramphenicol Alcohol 10 mg/mL Water 1 mg/mL 30 days Water 2.5 mg/mL
Chlortetracycline 0.01 N hydrochloric acid — — 1 mg/mL 4 days Water 0.06 mg/mL
Dihydrostreptomycinb Water — — 1 mg/mL 30 days Water 30 mg/mL
Gramicidin Alcohol — — 1 mg/mL 30 days Alcohol 0.04 mg/mL
Neomycinb,d B.3c — — 100 mg/mL 14 days B.3c 1.0 mg/mL
Oxytetracycline 0.1 N hydrochloric acid — — 1 mg/mL 4 days Water 0.24 mg/mL
Tetracycline 0.1 N hydrochloric acid — — 1 mg/mL 1 day Water 0.24 mg/mL
Thiostrepton Dimethyl
— —
Dimethyl sulfoxide 1 U/mL Same day sulfoxide 0.80 U/mL
Troleandomycin Isopropyl alcohol and water
— —
(4:1) 1 mg/mL Same day Water 25 mg/mL
Tylosin Methanol
and B.3c
Methanol 10 mg/mL B.16c 1 mg/mL 30 days (1:1) 4 mg/mL
a mg in this column refers to mg of activity.
b The cylinder-plate assay can be used as an alternative procedure.
c The letter B refers to buffer. See Media and Solutions, Buffers for a description of each buffer listed in this table.
d Dilute the 100-mg/mL stock solution with Buffer B.3 to obtain a solution having a concentration equivalent to 25 mg/mL of neomycin.To separate 50-mL volu-
metric flasks add 1.39, 1.67, 2.00, 2.40, and 2.88 mL of this solution. Add 5.0 mL of 0.01 N hydrochloric acid to each flask, dilute with Buffer B.3 to volume, and
mix to obtain solutions having concentrations of 0.69, 0.83, 1.0, 1.2, and 1.44 mg/mL of neomycin. Use these solutions to prepare the standard response line.
Inocula: Suspend the test organism from a freshly grown slant or culture in 3 mL of sterile saline TS. Glass beads can be used
to facilitate the suspension. Enterococcus hirae (ATCC 10541) and Staphylococcus aureus (ATCC 9144) are grown in a liquid
medium, not on agar. Spread the saline suspension onto the surface of two or more agar plates (covering the entire surface) or
onto the surface of a Roux bottle containing 250 mL of the specified medium (see Table 8). Incubate at the time and tempera-
ture specified in Table 8, or until growth is apparent.
After incubation, harvest the organism from the plates or Roux bottle with approximately 50 mL of sterile saline TS, using a
sterile bent glass rod or sterile glass beads. Pipet the suspension into a sterile glass bottle. This is the harvest suspension.
Determine during method verification the quantity of harvest suspension that will be used as the inoculum, starting with the
volume suggested in Table 8. Prepare also an extra S3 as a test of growth. Incubate the trial tests for the times indicated in
Table 11. Adjust the quantity of inoculum daily, if necessary, to obtain the optimum concentration–response relationship from
the amount of growth of the test organism in the assay tubes. At the completion of the specified incubation periods, tubes
containing the median concentration of the standard should have absorbance values as specified in Table 9. Determine the
exact duration of incubation by observing the growth in the reference concentration (median concentration) of the standard
(S3).
Table 8
Suggested
Incubation Conditions Inoculum Composition
Tempera-
ATCCa Medi- ture Amount
Antibiotic Test Organism Number umb (°) Time Mediumb (mL/100 mL)
Capreomycin Klebsiella pneumoniae 10031 1 36–37.5 16–24 h 3 0.05
Chloramphenicol Escherichia coli 10536 1 32–35 24 h 3 0.7
General Chapters
Table 9
Absorbance,
Antibiotic NLT (a.u.)
Capreomycin 0.4
Chlortetracycline 0.35
Gramicidin 0.35
Tetracycline 0.35
All others 0.3
Analysis: On the day of the assay, prepare the necessary concentration of antibiotic by dilution of stock solutions of the
standard and of each sample as specified under Standard solutions and Sample solutions. Prepare five test levels, each in tripli-
cate, of the standard (S1–S5) and a single test level (U3), also in triplicate, of up to 20 samples corresponding to S3 (median
concentration) of the standard.
Table 10
Volume of Test Volume of
Dilution Inoculum
Antibiotic (mL) (mL)
Gramicidin 0.10 9.0
Thiostrepton 0.10 10.0
Tylosin 0.10 9.0
All others 1.0 9.0
Place the tubes in test tube racks or other carriers. Include in each rack 1–2 control tubes containing 1 mL of the test diluent
(see Table 7) but no antibiotic. Add the volumes of the standard and sample test dilutions as indicated in Table 10. Randomly
distribute one complete set, including the controls, in a tube rack. Add the volume of inoculum specified in Table 10 to each
tube in the rack in turn, and place the completed rack immediately in an incubator or a water bath maintained at 36.0°–37.5°
for the time specified in Table 11.
Table 11
Incubation Time
Antibiotic (h)
Capreomycin 3–4
Chloramphenicol 3–4
Cycloserine 3–4
Dihydrostreptomycin 3–4
Streptomycin 3–4
Troleandomycin 3–4
Tylosin 3–5
All others 4–5
After incubation, immediately inhibit the growth of the organism by adding 0.5 mL of dilute formaldehyde to each tube,
except for tylosin. For tylosin, heat the rack in a water bath at 80°–90° for 2–6 min or in a steam bath for 5–10 min, and bring
to room temperature. Read absorbance or transmittance at 530 or 580 nm, analyzing one rack at a time.
The media required for the preparation of test organism inocula are made from the ingredients listed herein. Minor modifi-
General Chapters
cations of the individual ingredients are acceptable; and reconstituted dehydrated media can be substituted, provided that the
resulting media possess equal or better growth-promoting properties and give a similar standard curve response.
Media: Dissolve the ingredients in water to make 1 L, and adjust the solutions with either 1 N sodium hydroxide or 1 N hy-
drochloric acid as required, so that after steam sterilization the pH is as specified.
Medium 1
Peptone 6.0 g
Pancreatic digest of casein 4.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Dextrose 1.0 g
Agar 15.0 g
Water 1000 mL
pH after sterilization 6.6 ± 0.1
Medium 2
Peptone 6.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Agar 15.0 g
Water 1000 mL
pH after sterilization 6.6 ± 0.1
Medium 3
Peptone 5.0 g
Yeast extract 1.5 g
Beef extract 1.5 g
Sodium chloride 3.5 g
Dextrose 1.0 g
Dibasic potassium phosphate 3.68 g
Monobasic potassium phosphate 1.32 g
Water 1000 mL
pH after sterilization 7.0 ± 0.05
Medium 4
Peptone 6.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Dextrose 1.0 g
Agar 15.0 g
Medium 4 (Continued)
Water 1000 mL
pH after sterilization 6.6 ± 0.1
Medium 5
Peptone 6.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Agar 15.0 g
Water 1000 mL
pH after sterilization 7.9 ± 0.1
Medium 8
Peptone 6.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Agar 15.0 g
Water 1000 mL
pH after sterilization 5.9 ± 0.1
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Medium 9
Pancreatic digest of casein 17.0 g
Papaic digest of soybean 3.0 g
Sodium chloride 5.0 g
Dibasic potassium phosphate 2.5 g
Dextrose 2.5 g
Agar 20.0 g
Water 1000 mL
pH after sterilization 7.2 ± 0.1
Medium 10
Pancreatic digest of casein 17.0 g
Papaic digest of soybean 3.0 g
Sodium chloride 5.0 g
Dibasic potassium phosphate 2.5 g
Dextrose 2.5 g
Agar 12.0 g
Water 1000 mL
Polysorbate 80 (added after boiling the medium to dissolve the agar) 10 mL
pH after sterilization 7.2 ± 0.1
Medium 11
Peptone 6.0 g
Pancreatic digest of casein 4.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Dextrose 1.0 g
Agar 15.0 g
Water 1000 mL
pH after sterilization 8.3 ± 0.1
Medium 13
Peptone 10.0 g
Dextrose 20.0 g
Water 1000 mL
pH after sterilization 5.6 ± 0.1
Medium 19
Peptone 9.4 g
Yeast extract 4.7 g
Beef extract 2.4 g
Sodium chloride 10.0 g
Dextrose 10.0 g
Agar 23.5 g
Water 1000 mL
pH after sterilization 6.1 ± 0.1
Medium 32
Peptone 6.0 g
Pancreatic digest of casein 4.0 g
Yeast extract 3.0 g
Beef extract 1.5 g
Manganese sulfate 0.3 g
Dextrose 1.0 g
Agar 15.0 g
General Chapters
Water 1000 mL
pH after sterilization 6.6 ± 0.1
Medium 34
Glycerol 10.0 g
Peptone 10.0 g
Beef extract 10.0 g
Sodium chloride 3.0 g
Water 1000 mL
pH after sterilization 7.0 ± 0.1
Medium 35
Glycerol 10.0 g
Peptone 10.0 g
Beef extract 10.0 g
Sodium chloride 3.0 g
Agar 17.0 g
Water 1000 mL
pH after sterilization 7.0 ± 0.1
Medium 36
Pancreatic digest of casein 15.0 g
Papaic digest of soybean 5.0 g
Sodium chloride 5.0 g
Agar 15.0 g
Water 1000 mL
pH after sterilization 7.3 ± 0.1
Medium 39
Peptone 5.0 g
Yeast extract 1.5 g
Beef extract 1.5 g
Sodium chloride 3.5 g
Dextrose 1.0 g
Dibasic potassium phosphate 3.68 g
Monobasic potassium phosphate 1.32 g
Water 1000 mL
pH after sterilization 7.9 ± 0.1
Medium 40
Yeast extract 20.0 g
Polypeptone 5.0 g
Dextrose 10.0 g
Monobasic potassium phosphate 2.0 g
Polysorbate 80 0.1 g
Agar 10.0 g
Water 1000 mL
pH after sterilization 6.7 ± 0.2
Medium 41
Pancreatic digest of casein 9.0 g
Dextrose 20.0 g
Yeast extract 5.0 g
Sodium citrate 10.0 g
Monobasic potassium phosphate 1.0 g
Dibasic potassium phosphate 1.0 g
Water 1000 mL
General Chapters
Solutions
Buffers: Prepare as directed in Table 12, or by other suitable means. The buffers are sterilized after preparation; the pH
specified in each case is the pH after sterilization.
Table 12. Buffers
Concentration of Volume of 10 N
Concentration of Dibasic Monobasic Potassium
Potassium Phosphate Potassium Phosphate Hydroxide pH after
Buffer (g/L) (g/L) (mL) Sterilizationa
Buffer B.1 (1%, pH 6.0) 2 8 — 6.0 ± 0.05
Buffer B.3 (0.1 M, pH 8.0) 16.73 0.523 — 8.0 ± 0.1
Buffer B.4 (0.1 M, pH 4.5) — 13.61 — 4.5 ± 0.05
Buffer B.6 (10%, pH 6.0) 20 80 — 6.0 ± 0.05
Buffer B.10 (0.2 M, pH 10.5) 35 — 2 10.5 ± 0.1
Buffer B.16 (0.1 M, pH 7.0) 13.6 4 — 7.0 ± 0.2
a Adjust the pH with 18 N phosphoric acid or 10 N potassium hydroxide.
Calculations
Introduction: Antibiotic potency is calculated by interpolation from a standard curve using a log-transformed straight-line
method with a least-squares fitting procedure (see below for calculation details). The analyst must consider three essential con-
cepts in interpreting antibiotic potency results:
1. Biological concentration–response relationships generally are not linear. The antibiotic potency method allows fitting the
data to a straight line by evaluating a narrow concentration range where the results approach linearity. The assay results
can be considered valid only if the computed potency is 80%–125% of that assumed in preparing the sample stock solu-
tion. When the calculated potency value falls outside 80%–125%, the result for the sample may fall outside the narrow
concentration range where linearity has been established. In such a case, adjust the assumed potency of the sample ac-
cordingly, and repeat the assay to obtain a valid result.
2. The most effective means of reducing the variability of the reportable value (the geometric mean potency across runs and
replicates) is through independent runs of the assay procedure. The combined result of a series of smaller, independent
assays spread over a number of days is a more reliable estimate of potency than that from a single large assay with the
same total number of plates or tubes. Three or more independent assays are required for antibiotic potency determina-
tions.
3. The number of assays needed in order to obtain a reliable estimate of antibiotic potency depends on the required specifi-
cation range and the assay variability. The confidence limit calculation described below is determined from several estima-
ted log potencies that are approximately equal in precision. If the value calculated for the width of the confidence inter-
val, W, is too wide, no useful decision can be made about whether the potency meets its specification.
The laboratory should predetermine in its standard operating procedures a maximum acceptable value for the confidence
interval width. This maximum value should be determined during development and confirmed during validation or verifica-
tion. If the calculated confidence interval width exceeds this limit, the analyst must perform additional independent potency
determinations to meet the limit requirement. Note that the decision to perform additional determinations does not depend
on the estimated potency but only on the uncertainty in that estimate as determined by the confidence interval width. Assay
variability has a greater impact on the calculated confidence limit than does the number of independent potency determina-
tions. As a result, the analyst should first consider decreasing variability to the extent possible before conducting potency de-
terminations.
The following sections describe the calculations for determining antibiotic potency as well as for performing the confidence
limit calculation. Methods for calculating standard error are also shown in order to allow estimates of assay variance. Where
logarithms are used, any base log is acceptable. Appendix 1 provides formulas for hand calculations applicable when the con-
centrations are equally spaced in the log scale. Alternative statistical methods may be used if appropriately validated.
Cylinder-plate assay: This section details analysis of the sample data and determination of the potency of an unknown, us-
ing the cylinder-plate assay.
Sample data: Table 13 shows the data from one assay that will be used as an example throughout this section. For each
of the 12 plates, zones 1, 3, and 5 are the reference concentration and the other three zones are for one of the other four
concentrations, as shown. Other columns are needed for calculations and are explained below.
Step 1: Perform initial calculations and variability suitability check.
General Chapters
For each set of three plates, average the nine reference values and average the nine standard values.
Example (see Table 13)
15.867 = X (16.1, 15.6, ¼, 15.8)
14.167 = X(14.6, 14.1, ¼, 14.8)
For each set of three plates determine the standard deviation of the nine reference values and the standard deviation of the
nine standard values. For each standard deviation, determine the corresponding relative standard deviation.
Example (see Table 13)
0.200 = s(16.1, ¼, 15.8)
1.3% = (0.200/15.867) × 100
0.324 = s(14.6, ¼, 14.1)
2.3% = (0.324/14.167) × 100
For a variability suitability criterion, each laboratory should determine a maximum acceptable value for the relative standard
deviation. If any of the eight relative standard deviations (four for the reference and four for the standard) exceed this prede-
termined maximum, the assay data are not suitable and should be discarded. [NOTE—The suggested limit for relative standard
deviation is NMT 10%.]
Step 2: Perform a plate-to-plate variation correction.
This correction is applied to convert the average zone measurement obtained for each concentration to the value it would
be if the average reference concentration measurement for that set of three replicate plates were the same as the value of the
correction point:
XC = XS − (XR − P)
a This is the value of the overall reference mean, referred to as the “correction point” below.
Accessed from 10.6.1.1 by JNJconsumer on Sat May 21 13:08:47 EDT 2016
Copyright (c) 2016 The United States Pharmacopeial Convention. All rights reserved.
USP 39
Accessed from 10.6.1.1 by JNJconsumer on Sat May 21 13:08:47 EDT 2016
USP 39 Biological Tests / á81ñ Antibiotics—Microbial Assays 157
Example: Table 14 summarizes the portion of Table 13 needed for this part of the calculation.
Table 14
Corrected Zone
Measurements Concentration
Standard Set (mm) (U/mL)
S1 14.022 3.2
S2 14.989 4.0
Reference 15.722 5
(S3)
S4 16.511 6.25
S5 17.222 7.8125
General Chapters
the standard and the zone measurements of the sample on the three plates used. Correct for plate-to-plate variation using the
correction point determined above to obtain a corrected average for the unknown, U. [NOTE—An acceptable alternative to
using the correction point is to correct using the value on the estimated regression line corresponding to the log concentration
of S3.] Use the corrected average zone measurement in the equation of the standard curve line to determine the log concen-
tration of the sample, LU, by:
LU = (U − a)/b
Sample concentration:
CU = e1.561 = 4.765
1 0.7130
2 0.7960
U3 unknown 3 0.7201 0.7430 0.0460
%R2 = 93.0%
Sample potency determination: To estimate the potency of the unknown sample, average the three absorbance meas-
urements to obtain an average for the unknown, U. Use this average measurement in the equation of the standard curve line
to determine the log concentration of the unknown sample, LU, by:
LU = (U − a)/b
CU = 101.9696 = 93.2
General Chapters
CU = concentration of the sample
Confidence limits and combination of assays calculations: Because of interassay variability, three or more independent
determinations are required for a reliable estimate of the sample potency. For each independent determination, start with sep-
arately prepared stock solutions and test dilutions of both the Standard and the sample, and repeat the assay of a given sam-
ple on a different day.
Given a set of at least three determinations of the unknown potency, use the method of Appendix 2 to check for any outlier
values. This determination should be done in the log scale.
To obtain a combined estimate of the unknown potency, calculate the average, M, and the standard deviation of the accep-
ted log potencies. [NOTE—Use either the natural log or the base 10 log.] Determine the confidence interval for the potency as
follows:
antilog[M − t(0.05, N − 1) × SD/ÖN], antilog[M + t(0.05,
N − 1) × SD/ÖN]
M = average
SD = standard deviation
N = number of assays
t(0.05, N−1) = the two-sided 5% point of a Student's t-distribution with N−1 degrees of freedom
NOTE—The t value is available in spreadsheets, statistics texts, and statistics software.
W = antilog{[t(0.05, N − 1) × SD/ÖN]}
W = half-width of the confidence interval
Compare the half-width of the confidence interval to a predetermined maximum acceptable value. If the half-width is larger
than the acceptance limit, continue with additional assays.
Example: Suppose the sample is assayed four times, with potency results in the natural log scale of 1.561, 1.444, 1.517,
and 1.535. Then:
N=4
M = X(1.561, 1.444, 1.517, 1.535) = 1.514
SD = s(1.561, 1.444, 1.517, 1.535) = 0.050
t = 3.182
The confidence interval in the log scale is
1.514 ± (3.182 × 0.050/Ö4) = (1.434, 1.594)
Taking antilogs, the estimated potency is
e1.514 = 4.546
with a 95% confidence interval for the potency of e1.434, e1.594 = (4.197, 4.924).
The confidence interval half-width to compare to an acceptance value is the ratio 4.924/4.546 = 1.083.
If the concentrations are equally spaced in the logarithmic scale, the calculations can be performed using the following for-
mula. Let:
Sk = mean corrected zone measurement (cylinder-plate assay) or average absorbance value (turbidimetric assay) for stand-
ard set k
k = 1, 2, 3, 4, 5
S = mean of the five Sk values
Lk = logarithm of the kth concentration. [NOTE—Use either the natural log or the base 10 log. Slope of the regression line
is calculated by:]
b = (Yhigh − Ylow)/(Xhigh − Xlow)
For example, using the data for the cylinder-plate assay in Table 13 and natural logarithms:
b = [(4 × 17.222) + (2 × 16.511) − (2 × 14.989) −
(4 × 14.020)]/{5[ln(7.81)] − ln(3.2)} = 3.551
S = (14.020 + 14.989 + 15.722 + 16.511 + 17.222)/5 = 15.693
Natural log of sample concentration = ln(5) + [(15.522 − 15.693)/3.551] = 1.561
Sample concentration = e1.561 = 4.765
A measurement that is clearly questionable because of a failure in the assay procedure should be rejected, whether it is dis-
covered during the measuring or tabulation procedure. The arbitrary rejection or retention of an apparently aberrant measure-
ment can be a serious source of bias. In general, the rejection of measurements solely on the basis of their relative magnitudes
is a procedure that should be used sparingly.
Each suspected potency measurement, or outlier, may be tested against the following criterion. This criterion is based on the
variation within a single group of supposedly equivalent measurements from a normal distribution. On average, it will reject a
valid observation once in 25 trials or once in 50 trials. Designate the measurements in order of magnitude from y1 to yN, where
y1 is the candidate outlier, and N is the number of measurements in the group. Compute the relative gap by using Table A2-1,
Test for Outlier Measurements, and the formulas below:
When N = 3 to 7:
G1 = (y2 − y1)/(yN − y1)
When N = 8 to 10:
G2 = (y2 − y1)/(yN − 1 − y1)
When N = 11 to 13:
G3 = (y3 − y1)/(yN −1 − y1)
If G1, G2, or G3, as appropriate, exceeds the critical value in Table A2-1, Test for Outlier Measurements, for the observed N,
there is a statistical basis for omitting the outlier measurement(s).
N 8 9 10
G2 0.681 0.634 0.597
N 11 12 13
G3 0.674 0.643 0.617
Example: Estimated potencies of sample in log scale = 1.561, 1.444, 1.517, 1.535.
Check lowest potency for outlier:
G1 = (1.517 − 1.444)/(1.561 − 1.444) = 0.624 < 0.889
General Chapters
G1 = (1.561 − 1.535)/(1.561 − 1.444) = 0.222 < 0.889
♦Portions of this general chapter have been harmonized with the corresponding texts of the European Pharmacopoeia and/or
the Japanese Pharmacopoeia. Those portions that are not harmonized are marked with symbols (♦♦) to specify this fact.♦
The Bacterial Endotoxins Test (BET) is a test to detect or quantify endotoxins from Gram-negative bacteria using amoebo-
cyte lysate from the horseshoe crab (Limulus polyphemus or Tachypleus tridentatus).
There are three techniques for this test: the gel-clot technique, which is based on gel formation; the turbidimetric technique,
based on the development of turbidity after cleavage of an endogenous substrate; and the chromogenic technique, based on
the development of color after cleavage of a synthetic peptide-chromogen complex. Proceed by any of the three techniques
for the test. In the event of doubt or dispute, the final decision is made based upon the gel-clot limit test unless otherwise
indicated in the monograph for the product being tested. The test is carried out in a manner that avoids endotoxin contami-
nation.
APPARATUS
Depyrogenate all glassware and other heat-stable materials in a hot air oven using a validated process.♦1♦ A commonly used
minimum time and temperature is 30 min at 250°. If employing plastic apparatus, such as microplates and pipet tips for auto-
matic pipetters, use apparatus that is shown to be free of detectable endotoxin and does not interfere in the test. [NOTE—In
this chapter, the term “tube” includes any other receptacle such as a microtiter well.]
Amoebocyte Lysate
A lyophilized product obtained from the lysate of amoebocytes (white blood cells) from the horseshoe crab (Limulus polyphe-
mus or Tachypleus tridentatus). This reagent refers only to a product manufactured in accordance with the regulations of the
competent authority. [NOTE—Amoebocyte Lysate reacts to some b -glucans in addition to endotoxins. Amoebocyte Lysate prepa-
rations that do not react to glucans are available: they are prepared by removing the G factor reacting to glucans from Amoe-
♦1For a validity test of the procedure for inactivating endotoxins, see Dry-Heat Sterilization under Sterilization and Sterility Assurance of Compendial Articles á1211ñ.
Use Lysate TS having a sensitivity of not less than 0.15 Endotoxin Unit per mL.♦