EGERTON UNIVERSITY

COLLEGE OF OPEN AND DISTANCE LEARNING

E-CAMPUS

ZOOL 143: Biology of HIV/AIDS and Society

Topic 3 Handout

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Published 2020
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Topic Three: Human Microbial Pathogens

Introduction

Disease-causing microbes always want to invade and live in our bodies, thus,
causing damage resulting in a disease. In this topic, we take you through the
nature of pathogens and human-pathogen interaction. Welcome.

Topic Learning Outcomes

By the end of this topic you should be able to:

i. Describe the nature of human pathogens.

ii. Describe the mechanisms of human-pathogen interaction.
iii. Describe the sexually transmitted diseases and their relationship with
HIV/AIDS.

3.1 Nature of Human Microbial Pathogens

Have you ever fallen sick? Did you know the stranger in your body that made you
sick? What did the stranger do in your body to make you sick? In this lesson, we
shall learn about how such organisms cause diseases and our body’s own efforts
to protect us against them.

a) Pathogens

A pathogen is an environmental agent (e.g. a infectious microbes, toxic chemicals)

or other agents capable of producing disease in the host. Infectious pathogen can
invade the body while contagious pathogen can be spread from one animal to
another.

The ability of a pathogen to produce an infectious disease in an organism, i.e. the

ability of a parasite to enter a host and cause disease, is called pathogenicity. The
degree of pathogenicity (i.e. the comparative ability to cause disease) is known as
virulence. The terms pathogenic and nonpathogenic refer to the relative virulence
of the organism or its ability to cause disease under certain conditions. This ability
depends not only upon the properties of the organism but also upon the ability of
the host to defend itself (its immunity) and prevent injury.
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b) Pathogen Characteristics Favoring Environmental Transmission

The properties of pathogens that favor their transmission in the environment are:

• Multiple sources and highly endemic in humans, animals and

environment
• High pathogen concentrations present in environmental media (water,
food, air)
• High carriage rate in human and animal hosts
• Asymptomatic carriage in non-human hosts
• Proliferate in water and other media
• Adapt to and persist in different media or hosts
• Seasonality and climatic effects
• Natural and anthropogenic sources
• Ability to Persist or Proliferate in Environment and Survive or Penetrate
Treatment Processes
• Stable environmental forms as survival mechanism, e.g. spores, cysts,
oocysts, viral protein coat, capsule, etc.
• Resistance to environmental stressors and antagonists, e.g. heat/
freezing, desiccation, UV light, ionizing radiation, pH extremes.
• Possess DNA repair mechanisms and other injury repair processes
• Ability for genetic exchange, mutation and selection
• Antibiotic resistance
• Infects by multiple routes, e.g. ingestion (GI), inhalation (respiratory),
cutaneous (skin).
• Virulence properties e.g. produce endotoxin (fever producer), exotoxins
(Botulinum, Cholera toxins), hydrolytic enzymes (hyaluronidases), and
have morphological features (presence of pili for attachment, invasions
to facilitate cell invasion, and flagella).

c) Common Types of Human Pathogens

Common types of pathogens are: bacteria, viruses, fungi, prions, and protozoans.
When not infecting humans, pathogens hide in the soil, water bodies, surfaces like
farm equipment, the skin of people and animals, in the air, and in body fluids.
Microbes on the human body outnumber human cells at least ten to one. The

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human body provides a wide range of habitats and supports a wide range of non-
pathogenic microbes. Many times bacterial biota benefit the human host by
preventing the overgrowth of harmful microorganisms, a phenomenon called
microbial antagonism.

3.2 Human-Pathogen Interaction

a) Portals of Entry

Pathogens enter the tissues of the body by a portal of entry, usually a cutaneous
or membranous boundary. These portals of entry are normally the same
anatomical regions that support normal non-pathogenic biota. Common portals of
entry are:

• Skin: Intact skin is very tough- few microbes can penetrate. Nicks,
abrasions, and punctures in the skin provide a route for entry. Some
create their own passageways using digestive enzymes or bites.
Examples include Staphylococcus aureus, Streptococcus pyogenes,
Haemophilus aegyptius, Chalmydia trachomatis, and Neisseria
gonorrhoeae.
• Gastrointestinal tract: Pathogens contained in food, drink, and other
ingested substances enter through GIT. They are adapted to survive
digestive enzymes and pH changes. Examples include Salmonella,
Shigella, Vibrio, Poliovirus, Hepatitis A virus, Echovirus, Rotavirus,
Entamoeba histolytica, Giardia lamblia
• Respiratory Tract: This is the portal of entry for the greatest number of
pathogens. Examples include Streptococcal sore throat, Meningitis,
Diphtheria, whooping cough, Influenza, Measles, Mumps, Rubella,
Chickenpox, Common cold, Bacteria and fungi causing pneumonia.
• Urogenital Tract: This is an important portal for pathogens causing
sexually transmitted diseases (STDs) the pathogens enter skin or mucosa
of penis, external genitalia, vagina, cervix, and urethra. Some can
penetrate an unbroken surface. Examples include Syphilis, Gonorrhea,
Genital warts, Chlamydia, Herpes.
• During Pregnancy and Birth: Some microbes can cross the placenta (e.g.
the syphilis spirochete). Other infections occur perinatally when the

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 child is contaminated by the birth canal. E.g. TORCH (toxoplasmosis,
other diseases, rubella, cytomegalovirus, and herpes simplex).

b) Warning Signals of a Disease

Presence of a disease is signaled by various signs and symptoms.

• Sign: any objective evidence of disease as noted by an observer, e.g.

fever, diarrhea.
• Symptom: the subjective evidence of disease as sensed by the patient,
e.g. pain, nausea.
• Syndrome: when a disease can be identified or defined by a certain
complex of signs and symptoms

c) Vacating the Host

Portals of exit from the host include:

• Respiratory and salivary portals, e.g. coughing, sneezing, talking and

laughing
• Skin scales
• Fecal exit
• Urogenital tract
• Removal of blood or bleeding

3.3 Sexually Transmitted Diseases (STDs)

- A sexually transmitted infection (STI) is an infection you can get by having

intimate sexual contact with someone who already has the infection. STIs can be
caused by viruses, bacteria, and parasites.

- Many STIs have mild or no symptoms. So, you can have an STI and not even
know it.

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Most STIs can be found by simple tests, but routine testing is not widespread. So
many cases of STIs go undiagnosed and untreated, which can lead to serious
health problems—particularly for women.

Examples of STIs

VIRAL STIs

-Genital Herpes (Herpes Simplex Virus):

-Although there are two distinct types of this virus, type 2 is most often associated
with genital sores (type 1 is generally associated with oral sores often called “cold
sores”).

-Herpes is characterized by small sores or blisters in the mouth or genital region

that break open and form painful ulcers.

-After the breakage, these small sores generally heal without scarring. Some
people never develop any symptoms of the virus at all.

-The virus stays in nerve endings in the body and may be reactivated due to
compromise of the immune system and several other factors.

-The virus is spread through anal, oral and vaginal sex.

Human Papillomavirus (Human Papillomavirus):

-This infection may be commonly known as “genital warts” and is known to have
over sixty distinct strains of the virus.

-The strain themselves are divided into two groups for classification.

-The first group of strains is considered “low risk” and includes those that cause
outbreaks of genital warts.

-The second group of strains is considered “high - risk” and includes strains that
cause cervical, anal, or penile precancerous cell-growths and cancer itself.

-The virus is spread through skin-to-skin contact with an infected area.

BACTERIAL STIs

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Chancroid (Hemophilus ducreyi):

-This bacterial infection causes ulcers and small lesions on and around the genital
area.

-The ulcers (sometimes an individual may only have one) begin as small pustules
and eventually burst to become open sores.

-This bacterial infection can be very painful, particularly for men, and sometimes
is asymptomatic in women.

-If detected early and treated with appropriate antibiotics, Chanchroid can be
cured completely.

Chlamydia (Chlamydia trachomatis):

-This bacterial infection is often characterized by pain or burning during urination,

inflammation of the urethra, cervix, or rectum, pain and itching of the penis,
vagina, or anus, a white-colored discharge from the penis or vagina, and/or
swollen lymph nodes in the groin area.

-However, up to two-thirds of all Chlamydia infections may actually be

asymptomatic (this is especially true for women).

-Chlamydia can be treated with antibiotics and is curable when detected and
treated during its early stages.

-If Chlamydia is left untreated in an individual, he or she risks pelvic

inflammatory disease in women

-In addition, newborn babies may be at risk of contracting Chlamydia from their
mothers during childbirth. It often appears in the child as conjunctivitis
(commonly known as “pink eye”) or pneumonia.

Gonorrhea (Neisseria gonorrhoeae):

-This bacterial infection is characterized by pain or a burning sensation during

urination and/or defecation, green-coloured discharge from the penis, vagina,

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rectum, or cervix, swelling and tenderness of the vulva, sore throat, and / or the
false sense of needing to urinate of defecate.

-This bacterial can be asymptomatic in men (10%) and women (80%).

-This STI can be treated with antibiotics and can be cured if detection and
treatment is early.

-If left untreated, this bacterial infection can lead to pelvic inflammatory disease
(women) or permanent joint damage.

-Pregnant women can transmit this infection to their children and run the risk of
premature birth, miscarriage, and stillbirth.

Syphilis (Treponema pallidium):

-The infection often starts with a small sore or ulceration on the penis or vulva.
This may develop on almost any part of the body exposed to the bacteria including
the lips and tongue.

-These sores are usually painless and disappear without treatment after several
weeks.

-If syphilis remains untreated, however, it can cause damage to the eyes, heart,
bones, joints and nervous system.

-A pregnant woman may transmit this bacterial infection to her child, which may
result in stillbirth or other serious physical and mental disabilities.

-If detected early and treated with antibiotics, syphilis can be treated, however, the
antibodies for syphilis will remain in the human body for life and the infection
may be reactivated at any point during the infected individual’s lifetime.

PARASITIC STIs

Pubic Lice (Phthirus pubis or Pediculosis pubis):

-This STI is often commonly called “crabs” and is caused by small insects.

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-This STI can be transmitted via close sexual contact, but also through shared
underclothes, towels, bedding, or clothing.

-These insects typically live in the pubic hair but may also spread to other hair on
the body.

-This STI is typically characterized by itching.

-It is diagnosed by the appearance of the small grey insects and their white eggs
and treated with topical creams or prescription drugs.

-Additionally, it is necessary to thoroughly clean the infected individual’s

clothing, towels, and bedding with hot water.

Scabies (Sarcoptes scabiei):

-This STI is caused by a small mite that burrows under the skin. The small mites
prefer warm, moist folds of skin, so the genital region and the areas between the
toes are popular areas of infection.

-Transmission can occur through shared clothes, bedding, or towels.

This STI is characterized by extreme itching that often worsens during the night.

-The female mite lays her eggs beneath the skin in the early stages of the
infection, and later the itching is caused by the mite’s bites.

-Diagnosing this STI is tricky and may involve detection of the mites under the
skin or it may involve checking scrapings of the skin under a microscope.

-Treatment usually involves a topical cream.

-In addition, the individual’s clothes, towels, and bedding must be thoroughly
washed with hot water to rid them of any remaining mites.

Relationship between STIs and HIV transmission

-There are strong links between the transmission of STIs and HIV and therefore
an understanding of both is important for preventing their transmission. If an
individual is infected with an STI, he or she must be aware of what scientists call
the “epidemiology synergy” between certain STIs and HIV.
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-First, the presence of an STI has been shown to increase the individual’s
susceptibility to HIV

o The presence of genital ulcers that accompany STIs like syphilis, herpes, or
chanchroid can result in breakages in skin on or around the genitals. These
small openings are “gateways” for HIV’s entrance into the human body. Close
sexual contact with an individual who has HIV may allow the virus to enter the
body through these sores.

o STIs that do not result in ulcers like Chlamydia, gonorrhea, and

trichomoniasis have been shown to lead to increase of cells targeted by HIV in
genital secretions. For example, increased concentrations of CD4+ cells in
genital secretions can cause increase in an individual’s susceptibility to HIV by
providing the HIV with more targets and potential ways to infect the body.
These increased CD4+ cell in the area of the STI as part of the body’s natural
immune response.

-Second, it has been demonstrated by several research studies that the presence of
an STI increases the viral load present in a man’s semen or a woman’s vaginal
fluids. These increased viral loads increase the individual’s infectiousness and
increase their chances of transmitting the virus to a partner during close sexual
contact.

-Third, research has demonstrated that the damage HIV causes to the immune
system makes the symptoms of STIs more severe, longer lasting, and more
difficult to treat. Further, some STIs that are asymptomatic (which means that you
cannot see or tell they are present) in HIV negative individuals can have
symptoms in HIV positive individuals. These symptoms may result in more
lesions or sores, which can contribute to the spread of HIV for the reasons
mentioned above.

-Fourth, the presence of an STI in an individual may, according to recent research,

increase the gaps between the epithelial cells that line the genital and urinary
tracts. These increased gaps make it easier for HIV to enter the body so an HIV
negative individual with an STI will be more susceptible to contracting HIV
because of the presence of an STI.

Studies have shown that individuals who receive treatment for STIs actually shed
smaller amounts of HIV less often than those who receive no treatment if they

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know they are infected with an STI, and individuals who are unsure should seek to
be tested.

Topic Summary

• In this topic, we have learned that a pathogen is an environmental agent

or other agents capable of producing disease.
• Virulence of a pathogen depends not only upon the properties of the
organism but also upon the ability of the host to defend itself (its
immunity) and prevent injury.
• Pathogens enter the tissues of the body through a cutaneous or
membranous boundary such as skin, respiratory, digestive or urogenital
tracts.
• STDs are diseases transmitted via sexual intercourse or close sexual
contact and have a synergistic relationship with HIV/AIDS.
https://www.youtube.com/watch?v=v3RaxCEIRdk

Further Reading

1. Talaro KP, 2007. Foundations in Microbiology, Basic Principles. 6th

edition. McGraw-Hill International.
2. William L, Anderson WL, Janeway C and Travers P. 2001.
Immunology, The immune System in Health and Disease. 5th edition,
Garland Publishing, Inc.

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