Bacterial Pathogenesis

Medical Microbiology: the study of relationships between humans and microbes, including:

• Infection
• Infective disease
• Colonisation
• Transmission of infective agents

The central questions:

1. What are infections?
2. Which microbes cause them?
3. Why do particular individuals get particular infections?
4. What influences the outcome of an infection?

• Pathogenesis (A-D)
• Epidemiology (C)
• Diagnosis (A, B)
• Treatment (D)
• Control (C, D)

The Germ Theory of Disease

Classification of Bacteria:

Phenotypic:

• Morphology
• Staining properties
• Growth and biochemical properties.
• The Gram stain
• Non-stainable prokaryotes (e.g. Mycobacteria)

Genotypic:

• 16s ribosomal RNA subunit genes
• other genes (e.g. cytochrome c)

Practical / Medical or Veterinary (this course):

• Pathogens vs Non-pathogens
• Agents associated with specific diseases

Koch - Isolation of a particular organism associated with particular disease.

But the isolation of an organism from an individual does not necessarily imply disease.

Many different forms of association between microbes and humans.

We are born sterile (microbiologically speaking)

After weaning we are hosts to some 10¹⁴ bacteria.

You are an environment - inhabited by your normal flora comprising several hundred different species of bacteria (+ fungi & protozoa)

These organisms are commensals.

Pathogens often have some properties in common with commensals but the associations they form with humans progress beyond commensalism to disease.

Stages of infection:

1. Encounter
2. Entry/Establishment
3. Spread
4. Multiplication
5. Damage
6. Outcome

Medical microbiology is primarily concerned with Pathogens and Commensals.

Commensals (literally eat at the same table):

Some commensals are also pathogens.

Specific composition of the commensal /normal flora depends on:

1. Site
2. Environmental exposure
3. Age
4. Short and long term structural abnormalities

1. We have to know what the normal flora of a particular site is to distinguish between normal and potentially pathological association.
2. Strategy for studying infection at different sites is heavily influenced by whether that site has abonormal flora or not.

Pathogens cause infection and the establishment and outcome of infection is DOSE-RELATED:

ID₅₀ (dose required to produce specified outcome in 50% of target population) is shifted:

• by increased virulence - or decreased host-resistance
• by decreased virulence - or increased host-resistance

An opportunistic pathogen is generally of low virulence (i.e. ID₅₀ is high) and requires decreased host resistance.

SUMMARY:

• Suspicion that organisms cause disease (history and context)
• Recognition of pathogens and non pathogens
• Stages of infection
• Normal flora / commensals
• Infection / infective disease
• Infection is the multiplication of in a host with associated damage to that host
• Infective disease results when the damage is sufficient to produce symptoms
• Capacity of a pathogen to produce disease is referred to as virulence
• Opportunistic pathogens

© MicrobiologyBytes 2007.