How do bacteria cause diseases (Pathogenesis)?

Pathogen

A pathogen is a microorganism. If the microorganism has an ability to cause a disease in a person, it will be known as a pathogen. There are some microorganisms which are always causing diseases whereas others cause diseases in individuals with low immunity. These organisms are called opportunistic pathogen. These opportunists are usually the members of normal flora.

Virulence

This is a quantitative measurement of ability of microorganisms to cause diseases (pathogenicity).This is measured by the number of organisms. The number of organisms needed to kill half the host is known as the 50% lethal dose (LD₅₀). The number of organisms to cause disease in half of the host is known as the 50% infectious dose. These numbers are varying according to the organisms. The factors which make the organism more virulent are called virulence factors. Pilli, endotoxin and enterotoxin are some of the virulence factors.

Why do people get infectious diseases?

Human immunity system has an ability to provide protection against infection. If a microorganism overpowers our defense, we will get an infection. This is the simple explanation for “why people get infectious diseases?”. But real life is not that easy.

Number of organisms and the virulence of the organism are the main determinates which decides whether the organism is able to invade the human defense system or not.

Types of bacterial infections

1. communicable bacterial infections
2. epidemic bacterial infections
3. endemic bacterial infections
4. pandemic bacterial infections
5. inapparent or subclinical bacterial infections

Stages of bacterial pathogenesis

1. transmission from an external source
2. evasion of primary host defense
3. adherence to mucous membranes
4. colonization
5. invasion and inflammation
6. host response
7. progression or resolution

Important modes of transmission

Mode of transmission	Clinical example	comment
1. human to human
a. direct contact	gonorrhea	Sexual, through birth canal
b. no direct contact	dysentery	Feco-oral
c. transplacental	Congenital syphilis	Crosses the placenta
2.non-human to human
a. soil source	tetanus	Spores in soil
b. water source	Legionnaire’s disease	Bacteria in water
c. animal source
i. directly	Cat scratch fever	Bacteriae enter in cat scratch
ii.via insect vector	Lyme disease	Bacteriae enter in tick bite
iii. via animal excreta	E. coli	Bacteriae in cattle faces

Stages of infectious diseases

1. the incubation period
2. the prodrome period
3. the specific illness period
4. recovery period