Approching and Studing Bacterial

Diseases

1) Microbes and Diseases

- Establishing a Connection: Koch’s Postulates
- Concepts of Disease

2) Measuring Infectivity and Virulence

- Animal Models
- Competition Assays
- Tissue Culture and Organ Culture Models
- Good Information about the Pathology of a Disease

The discovery that the bacterium Helicobacter pylori
causes most of the ulcers brought to the fore an old
problem in infectious disease research:

How does one prove that a particular microorganism
causes a disease?

How would you test this hypothesis?

How to avoid a possible bias?

Approching and Studing Bacterial Diseases

Microbes and Diseases: Establishing a Connection:

Koch’s Postulates

How to convince proponents of competing views?

1. The microbe must be associated

with the lesions of the disease.
This is, the microbe should be
found in diseased tissue but not in
healthy tissue.

2. The microbe must be isolated from

the lesions of the disease as a
pure culture.

3. A pure culture of the microbe

should cause the symptoms of the
disease if it is inoculated into
humans or into animals.

4. The microbe must be re-isolated in

pure culture from the humans or
animals used to satisfy the third
postulate

Koch’s Postulates

The first Koch’s postulate and relation between

Chlamydia pneumoniae and atherosclerosis

Chlamydia pneumoniae is frequently isolated from
atherosclerotic plaques but only occasionally isolated
from healthy blood vessel tissue.

The fact that C. pneumoniae is occasionally isolated
from healthy blood vessel tissue blurs the clear line
implicit in Koch’s first postulate.

Although C. pneumoniae is isolated from atherosclerotic
plaque samples, it is not isolated from all such samples.
It means, C. pneumoniae is associated with lesions of
the disease most of the time but not all of the time.

The first Koch’s postulate and Helicobacter

pylori as etiologic agent of ulcers

Over half the people in developed countries and nearly
all of the people in developing countries carry H. pylori
in their stomachs, but only a small number of these
people develop ulcers.

The same pattern is seen in a number of bacterial
diseases; the bacterium colonizes many people, but
only those with some predisposing condition develop a
symptomatic infection.

The Second Postulate: Isolating
the bacterium in pure culture

Some bacteria are more difficult to cultivate
than others

- C. pneumoniae only grows inside human cells

- H. pylori requires a special atmosphere to

grow on agar medium

- Treponema pallidium has never been isolated

as a pure culture.

What is a modern alternative to cultivate
disease-causing bacteria?

The Third Postulate: Showing that the

isolated bacterium causes disease in

humans or animals

The postulate that is the most difficult to satisfy

WHY?

H. pylori and C. pneumoniae

Barry Marshall (J.R. Warren)

Rabbit model of atherosclerosis: a special breed of rabbit that is prone to

develop atherosclerosis is fed a high-fat diet to show that infection with C.

pneumoniae increases the development of atherosclerotic plaque

How closely should an animal model mimic the disease in humans?

The Fourth Postulate: Reisolating the

bacterium from the intentionally infected animal

This postulate is fairly easy satisfy and very important to

prove the causative relation between a bacterium and a

disease

WHY IS SO IMPORTANT?

Consider the rabbit model of C. pneumoniae infection

and cardiovascular disease

A Fifth Koch Postulate?

C. pneumoniae – satisfied all four of Koch’s

postulates if one accepts the rabbit model as a good

model for disease. Still there are many skeptics.

H. pylori, even the use of human volunteer did not

convince everyone

THE FIFTH POSTULATE: The information about

the microbes should enable scientists to design

effective therapeutic or preventive measures for

eliminating the disease.

HIV / AIDS and syphilis

Microbiota Shift Disease Problem

- Periodontal disease

- Bacterial vaginosis (associated with higher

risk for preterm birth)

- Inflammatory bowl disease

Concepts of Disease

1. Variety of Human-Microbe Interactions

- Differences in susceptibility from person to person

-Variations among different strains of the same bacterial species

Lost of virulence

Differences between isolates

The outcome of microbe-human encounter depends on the

infected person’s defenses against disease and on the traits

of the infecting strain

1) Disease-causing bacteria evolved specifically to cause human

disease.

2) Disease-causing bacteria are actually trying to achieve an

equilibrium with humans that does not result in disease, and that

disease symptoms result when this equilibrium is not achieved.

3) Humans are more often than not accidental hosts of bacteria that

may be able to cause human disease but have actually evolved to

occupy some other niche.

In this view, bacteria entering the humen body react by activating

stress responses, producing disease symptoms in the process

2. Views of the Microbe-Human Interaction

Ad.1. A bacterium that causes disease only in human and has no external reservoir
Ad.2. A bacterium that causes an asymptomatic carrier state in most of humans it infects
Ad.3. A bacterium that spends most of its time outside human body and only occasionally
causes human disease

3. Terminology

1) The term “host-parasite” is widely used to describe the human-

microbe interaction

2) Colonization of the body by bacteria capable of causing disease

is called infection

3) An infection producing symptoms is called disease

4) The word colonization means that a bacterium occupies and

multiplies in a particular area of the human body. Colonization is

not synonymous with disease, nor it is necessarily synonymous

with infection.

5) The terms colonization, infection, and disease may vary in their

applicability depending on the status of the person colonized.

6) People, who are infected but do not have detectable symptoms

are called asymptomatic carriers (example of Typhoid Mary)

3. Terminology, cont.

7) Symptoms of bacterial infection are defined as effects of

bacterial infection that are apparent to infected person (Example:

Chlamydia trachomatis / cervix infection / no pain and vaginal

discharge / inflammation / fallopian tubes shut / infertility)

8) Virulence (or pathogenicity) is defined as the ability of a

bacterium to cause infection

9) Virulence factor (or mechanism of pathogenesis or virulence

mechanism) denotes a bacterial product or strategy that

contributes to virulence or pathogenicity

Problems:

– Two or more factors

– A virulence factor in one type of bacteria is

not a virulence factor in another type

– Is a housekeeping protein a virulence factor?

– The loss of trait makes a bacterium more

virulant

An experimental definition of virulence factor: A

loss of the factor by the bacterium results in a

decrease in its ability to cause disease

4. Opportunists

Bacteria that normally do not cause disease in healthy people but

can cause disease in people whose defenses have been impaired

Example: Pseudomonas aeruginosa and burns or cystic fibrosis

Opportunists vs. primary pathogens, definition problems

Example: Streptococcus pneumoniae

Measuring Infectivity and Virulence

Animal Models

Human Volunteers

- ethical considerations

Zidovudine (AZT) therapy test in Thailand in 1990s

The Tuskegee experiment

(from the early 1930s to 1972)

“Involunteer studies” – in which infectious

disease outbreaks are studied in retrospect to

obtain information about disease transmission in

humans (Example of a school bus driver)

Nonhuman Animal Models

- Laboratory rodents

Rats do not have a gallbladder

Coprophagy

Different courses of some human diseases in rodents

Example: Salmonella enterica serovar Typhimurium: diarrhea

in humans, systemic disease in mice.

S. typhi cause typhoid fever in humans, does not infect mice

Ferrets as a model for ulcers

Caenorhabditis elegans

A Perfect Animal Model

• Symptoms and distribution

• A route of infection

• The ease with which the animal model can

be manipulated genetically

– knockin and knockout mice

• Lewis b antigen Le

b

mouse

• The sickle-cell trait conferes immunity to malaria

• A defective chemokine receptor confers resistance

to HIV

ID

50

and LD

50

50

100

10

0

10

2

10

4

Competition Assays

Tissue Cultures and Organ Culture Models