MODULE 15

MICROBES AND DISEASES

Andes, A.T.
Department of Zoology, University of Ilorin, Ilorin, Nigeria

Introduction
Microbe is a noun coined from micro-organism, i.e. organisms that cannot be seen with the naked
eye. They exist as single cells or cell clusters and can be appreciated with the aid of special gadgets
such as the microscope. They form a very large and diverse group of organism that combines plant
and animal features. They include bacteria, fungi, viruses and protozooans.
Microbes play very important roles in nature than their small size suggest. They form a significant
integral part of the community that ensure interaction between living and non-living components
and hence the sustenance of all ecosystems. They ensure the synthesis and degradation of special
organic substances in the course of their existence. Man has over the years taken advantage of this
in a number of ways such as the production of beer, yogurt and antibiotics, baking, soak away
system, etc.

Learning Outcomes
By the end of this module, you should be able to:
(i) give the definition and various groups of microorganisms;
(ii) identify various microbial reservoir;
(iii) explain ways by which microbes can invade a susceptible host;
(iv) differentiate between pathogenicity and normal human flora;
(v) establish ways or routes of transmission of microbes; and
(vi) analyse the etiology and prevention of various microbial diseases

Main Body
Introduction
This module introduces you to meaning of Microorganism, Host Invasion, Pathogenicity,
Reservoirs, Transmission and Microbial Diseases. This will be discussed under 2 units.

Unit 1 Microorganisms
Subunit 1 Host Invasion
Subunit 2 Pathogenicity
Subunit 3 Reservoirs
Unit 2 Microorganisms
Subunit 2 Transmission
Subunit 3 Microbial Diseases
Unit 1: Microorganisms
Contents
1.0 Introduction
2.0 Learning Outcomes
3.0 Main Contents
3.1 Host Invasion,
3.2 Pathogenicity
3.3 Subunit 1 Reservoirs,
4.0 Summary
5.0 Self-Assessment Questions
6.0 Tutor Marked Assessment
7.0 Further Reading

1.0 Introduction
This unit introduces Host Invasion and pathogenicity.

2.0 Learning Outcomes

By the end of this unit, you should be able to:
(i) define and various groups of microorganisms;
(ii) explain ways by which microbes can invade a susceptible host;
(iii) Reservoirs; and
(iv) differentiate between pathogenicity and normal human flora

3.0 Main Content

3.1 Host Invasion
Microbes play very important roles in nature than their small size suggest. They form a significant
integral part of the community that ensure interaction between living and non-living components
and hence the sustenance of all ecosystems. They ensure the synthesis and degradation of special
organic substances in the course of their existence. Man has over the years taken advantage of this
in a number of ways such as the production of beer, yogurt and antibiotics, baking, soak away
system, etc.
Human microflora occur on the skin, orifices, i.e. mouth, nose, anus, vagina etc; body fluids such
as saliva, blood, semen etc., as well as, in the tissues. Most of these form normal body microflora,
in which case their presence is not harmful to their host. The detrimental or pathogenic forms,
however, elicit disease condition i.e. an alteration in the host system resulting in the loss of
productivity, in man as a result of the interaction with the microbe. This disease possibility
necessitates a discussion on Microbes and Diseases, believing that a thorough understanding of
the etiology of microbial disease of man will reduce their incidence and therefore enhance human
livelihood.
To elicit disease condition, microbes must establish contact, multiply and colonise its host either
superficially or in the tissue. To achieve this, they require to have conquered the series of defence
mechanisms put up by an unwilling host. The most prominent of this is the impenetrable skin that
serves as barrier. The determined microbes, however, make their way in by adhering to the surface
and producing chemical substances (enzymes) that break the barrier for them. Some of them take
advantage of skin lacerations by way of wounds, while others reside in obscured spaces with
restricted access but clement environment such as the mouth, anus and vagina. Microbes are
known to show preferences for site of occurrence. e.g. Nisseria gonorrhea, a microbe causing
sexually transmitted disease known as Gonorrhea. which sticks strongly to the inner lining of the
urinogenital tract than any other place.

3.2 Pathogenicity
Microbes initiate some level of alterations in the host's system as a result of their activities. Some
of these activities are aimed at improving their survival chances in the areas of food acquisition
and avoidance of the host's defense actions. Some of these activities include:
• Production of excretory wastes that may be intolerable to the host and hence referred to as
toxins
• Deprivation of the host of its nutrients
• Confiscation of host tissue for personal use by the microbe. e.g. viral infections
• Destruction of the host tissue, e.g. anemia resulting from malaria infection
• Initiation of tissue changes that may lead to cancers or tumors and
• Reduction of host immune response thereby giving room for opportunistic infections.

3.3 Reservoirs
Microbes reside temporarily in one or more natural environments known as reservoirs. The major
reservoirs are water, soil, atmosphere, humans, domestic and wild animals. In their bid to survive
they move from one reservoir to the other and become successful when established in their host.
Diseases that are contractible by humans from other animals are termed zoonotic diseases.

4.0 Summary
The unit drive the discussed on Host invasion, Pathogenicity and reservoirs.

5.0 Self-Assessment Questions (SAQs)

(i) give the definition and various groups of microorganisms;
(ii) explain ways by which microbes can invade a susceptible host;
(iii) reservoirs; and
(iv) differentiate between pathogenicity and normal human flora

6.0 Tutor Marked Assessment

Write concise notes on:
i. Host invasion
ii. Pathogenicity
iii. Reservoirs

7.0 Further Reading

Brock, T. D., M. T. Martinko and J. Parker (1994) Biology of micro-organisms. Prentice Hall,
New Jersey: 909pp.
Cheng, T. C. (1973) General Parasitology. Academic Press, London, .965pp.
Godman, A. and A. Gutteridge (1978) A New Health Science for Africa. Longman, London.
255pp.
Unit 2 Microorganisms and Microbial Diseases
Contents
1.0 Introduction
2.0 Intended Learning Outcomes (Objectives)
3.0 Main Contents
3.1 Transmission
3.2 Microbial Diseases
4.0 Summary
5.0 Self-Assessment Questions
6.0 Tutor Marked Assessment
7.0 Further Reading

1.0 Introduction
.This unit will further discuss microorganisms and its transmission and the kinds of diseases that
can result as a means of that.

2.0 Learning Outcomes

By the end of this unit, you should be able to:
(i) establish ways or routes of transmission of microbes; and
(ii) analyse the etiology and prevention of various microbial diseases

3.0 Main Contents

3.1 Transmission
The essence of successful transmission bid is to ensure the establishment of the offspring or
progenies of a particular microbe in another host of the same kind; a mission that entails movement
across reservoirs. They, therefore, utilize media that are inevitably used by their prospective host
such as air, water, food, etc... The following four basic ways are often employed:
• Contaminated food and water (Oral-faecal route)
Ingestion of contaminated food and water is a popular mode of microbial transmission. Faecal
contamination is usually the source of such pathogens, as they reside mostly in the alimentary
canal and they are voided out via the anus. They gain access to drinking water or food owing to
inadequate sanitary procedures or sometimes directly when one licks unclean hands after visiting
the toilet.
• Contamination of the respiratory tract (Respiratory route)
Pathogenic microbes may find their way to the host's respiratory tract, irritate the host and force it
to sneeze or cough. These activities, as well as, talking and throwing out sputum are primarily
meant to protect the unwilling host, but they aid the transference of the microbes to other hosts in
form of droplets in the atmosphere. The pathogens are successfully transmitted when a prospective
host inhales such droplet. Most of the pathogens remain active and infective even after the droplet
had dried up and they could be transported by wind.
• Direct contact with infected persons, animals or contaminated objects
Pathogens frequently utilize the opportunity of contact to get across to a new host. Such a pathogen
is described as-a contagion. Such contacts may be direct or indirect, e.g. sexual intercourse,
kissing, sharing of toiletries, renting of dresses, etc. The entry may be via open wound, e.g. Tetanus
bacteria from soil.
• Inoculation through the skin
Pathogens may take advantage of the interaction between blood sucking parasite of man and man.
The microbe makes itself available to the vector when it feeds on human blood. It is then
transmitted to another host while consuming blood from its new host. Such pathogens usually grow
and multiply in the vector. e.g. Plasmodium and Anopheles mosquito.

3.2 Microbial Diseases

Microbes and the diseases initiated by them are numerous; however, some of the examples of these
diseases shall be reviewed with emphasis on those that are of common occurrence in Nigeria. The
diseases have been grouped on the basis of their causative agent into bacterial fungal, viral and
protozoan diseases. The name, location in the host, mode of transmission, part of the host body
affected, clinical manifestation, treatment and prevention of each disease are described briefly.
Viral diseases: These are caused by the smallest organism described, i.e. virus. They can not be
seen under the common light microscope. They are obligate parasites that show evidence of living
only when in their host. Outside their host they are inert; hence they me regarded as organisms at
the borderline between living and non-living things. Viruses usually occur, live and multiply
within the cell of its host and they take over the control of such cells from their respective nuclei
and from the host. Thus one of the general characteristics of viruses is their ability to alter the
working method or system of the cells of a host. Table 1 shows a summary of the etiology of six
common viral diseases and possible treatments and preventive measures. Viral diseases are usually
difficult to treat owing to the fact that they reside within the host's cell where drugs cannot be
tolerated. An effective drug at this point may kill the host cell before the virus. More emphasis is,
therefore, placed on prevention, which ensures that the pathogens do not find their way into the
host cell. When they do, however, the microbe is usually left for the host's immune system to
handle.

Table 1: Etiology, treatment and prevention of some viral diseases of man

Causative Part of Method of
Disease Type Clinical body Prevention and
agent manifestation spread treatment
affected
Polio picorna Headache, Intestine Droplet Vaccination, boil
virus fever, Paralysis and nervous infection, water for all
system feacal purpose
contamination
Measles Paramyxo Fever, nose and Skin and Droplet Isolation of patients,
virus eye discharge, Intestine infection avoiding overcrowding,
rashes, improving ventilation,
coughing, death vaccination
Yellow fever Arbovirus Fever, Blood, liver, Mosquito Vaccination, avoiding
headache, kidney vectors mosquito bite
yellow eyes
Smallpox Venola virus High fever, skin Respiratory Droplet Vaccination, improved
blister and tract, blood, infection, ventilation, avoiding
rashes, death skin wound overcrowding, patient
contagion isolation
Common cold Rhinovirus Running nose, Respiratory Droplet Patient isolation,
(catarrh) headache, tract infection improved ventilation
sneezing avoiding overcerowding
AIDS HIV (Human Inability to The immune Contagion in Controlled sexual
Immuno- defend self system body fluid activity, avoidance of
deficiency against other body fluid contact
virus) disease
Bacterial Diseases: Several human diseases are caused by bacteria of various types. Bacteria are
among the smallest living organisms. They are the first group of micro-organisms to be discovered
as a disease causing organism. Most of them gain entry into their host via the mouth, nose, vagina,
anus or lacerated skin. They obtain their food from their host by secreting enzymes that break
down food substances to simpler forms that are readily absorbed by them. They also secrete waste
products that are usually toxic to their host. Table 2 gives specific information on the etiology,
clinical manifestation, treatment and prevention of some common bacterial diseases of man.

Table 2: Etiology, treatment and prevention of some bacterial diseases of man

Causative Part of Method of
Disease Type Clinical body Prevention and
agent manifestation spread treatment
affected
Cholera Vibrio cholorae Gut irritation, Alimentary Faecal Provision of good
severe canal contaminated water, good personal
diarrhea, food and water, hygiene.
deg=hydration, vectors Use of antibiotics
death (housefly) ORT
Leprosy Bacillus sp Loss of Skin and Droplet Isolation of
sensation, sore nervous infection, infected individuals,
on skin, loss of system prolonged feeding well, use
flesh and bone close contact of drugs
Tuberculosis Mycobacterium Irritation of the Lungs Droplet Feeding well,
tuberculosis respiratory infection, Vaccination,
tract, coughing milk sourced (BCG) use of
from infected antibiotics
cattle
Pneumonia Pneumococcus Chest pain, Lungs and Droplet Isolation of infected
sp high fever, respiratory infection individuals, avoid
constant tract overcrowding, use of
coughing antibiotics
Gonorrhea Neisseria Difficulty in Reproductive Contagion Restricted intercourse,
gonourrhoeve urinating in organ, eyes of by sexual avoiding contact with
men, sterility infants contact infected body fluid,
in both sexes use of antibiotics
Syphilis Treponema Mental Reproductive Contagion by Restricted intercourse,
poludium impairment, organ, eyes sexual contact avoiding contact with
skin blisters, bones, joints, infected body fluid, use
brain damage skin and heart of antibiotics
Tetanus Clostridum Lockjam spasm Blood nerves Wound Improved personal
tetanus and muscles contamination hygiene, putting iodine
on fresh wounded
surface

Fungal Diseases: Disease causing fungi in man are few. Trichophyton sp causes 'ringworm' and
'athletes foot' diseases in children and adults, respectively. This fungus obtains its nourishment
from the outer layer of the skin with the aid of root-like structures called hyphae. It gives a small
dark or red patch that grows, but becomes restricted to the outer margin of a portion with restored
skin colouration and without hair strands later, hence the name 'ringworm'. These patches
frequently occur on the scalp, inside the thighs and armpits. On the scalp, it gives a scaly bald
patch without hair strands. Fungal disease spreads by personal or indirect contact achieved by way
of sharing clothing and other personal effects such as sponge, comb, socks etc. The best preventive
measure is to ensure personal cleanliness and discourage the use of damp socks and cover shoes.
They are frequently treated with antiseptic devices.
Protozoan diseases: Protozoans are tiny single celled organisms. Most of them are free living. A
few of them, e.g. Plasmodium sp are however parasitic and they cause malaria and trypanosomiasis
(sleeping sickness), respectively in man.
Malaria is caused by Plasmodium sp, a pathogen that resides in the blood of man. They colonize
the red blood cell, feed within it, reproduce in it and break it open, thus destroying it. This
destruction results in anaemia. The pathogens also produce toxins that initiate rigors associated
with malaria fever. The pathogens gain access into human blood stream through the feeding action
of female Anopheles mosquito, which acts as a vector. The mosquito collects the pathogen from
an infected individual while sourcing for blood, develops the pathogen to an infective form and
subsequently passes the infective pathogen to a healthy individual during blood sucking activity.
The symptoms of the disease include fever, i.e. high body temperature, headache, pains especially
at the joints, little and deeply coloured urine, etc. Due to the high level of debility involved, chronic
sufferers are usually incapable of carrying out their daily chores and so it leads to significant
economic losses. Prevention is only achievable by avoiding contact with mosquitoes. Taking drugs
to prevent malaria lead to undesirable effects such as the development of resistant to Plasmodium
strain that will not respond to the common antimalarial drugs, thus complicating the management
of malaria. Treatment is readily achieved through the use of drugs.
Trypanosomiasis: Trypanosoma resides in the blood stream of man where they obtain their food
and reproduce. They produce toxin which makes the host sick. The microbes are transmitted from
one human host to the other by Tsetse fly (Glossina sp) while prospecting for blood. Signs and
symptoms of the disease include fever, emaciation, sleeping almost always and eventually death.
Prevention involves avoidance of tsetse fly and treatment entails the use of drugs.

4.0 Summary
Transmission is discussed and microbial diseases. The diseases have been grouped on the basis of
their causative agent into bacterial fungal, viral and protozoan diseases. The name, location in the
host, mode of transmission, part of the host body affected, clinical manifestation, treatment and
prevention of each disease are described briefly.

5.0 Self-Assessment Questions (SAQs)

(i) establish ways or routes of transmission of microbes; and
(ii) analyse the etiology and prevention of various microbial diseases

6.0 Tutor Marked Assessment

i. Group the diseases on basis of their causative agent, listing the name, location in the host,
mode of transmission, part of the host body affected, clinical manifestation, treatment and
prevention of each disease.
ii. Analyse the etiology and prevention of various microbial diseases

7.0 Further Reading

Green, N. P., G. W. Stout, D. J. Taylor and R. Soper (1984) Biological Sciences. Cambridge
University Press.; London. 472pp.
Jawetz, E. 1. L. Melnick and E. A. Adeberg (1980) Review of medical microbiology. Lenge
Medical Publication. 593pp .:
Stanier, R. Y., I. L Ingraham, M. L: Wheelis and P. R Painter (1987) General Microbiology.
Macmillan Education Ltd., London, Sill ed. 690pp.
Thomas, C, G A. (1983) Medical Microbiology, Balliere Tindall Cassel Ltd., Eastbourne BN21 3
UN. 405 pp.