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

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Competition Assays

Tissue Cultures and Organ Culture Models