INFECTION

Microorganisms and infections 
What is a microorganism? 
▪ Any organism too small to be seen by the naked eye 
What is a pathogen? 
▪ Microorganisms that cause disease 
What is infection? 
▪ The presence of microorganisms causing damage to body tissues 
 

Relationships with humans: 
▪ SYMBIOSIS – benefits human & not harm the microorganism 
▪ MUTUALISM – benefits human & microorganism 
▪ COMMENSALISM – benefits microorganism & not harm the human 
▪ PATHOGENICITY – benefits microorganism & harms the human 
▪ OPPORTUNISM – a benign microorganism becomes pathogenic 
because of decreased human host resistance 

Physical protective barriers & normal flora 
▪ If compromised: 
▪ Cuts in the skin allow normally non-infectious bacteria 
cause local infections/invade further/infect organs 
▪ Damage to intestinal tract releases intestinal bacteria into 
the blood stream sepsis/ shock/ death 
Immune & inflammatory systems maintain much of the symbiotic 
relationship 
▪ If these systems compromised many microorganisms will leave 
their normal site & cause infection elsewhere in the body 
▪ Individuals with an immune deficiency can easily become 
infected – opportunistic microorganisms - seize the 
opportunity when defensive systems are weakened 

Process of infection 
Four stages of progression (from point 
of view of microorganism): 
1. Colonization 
2. Invasion 
3. Multiplication 
4. Spread 

1. Colonization 
▪ Infectious microorganisms exist in reservoirs e.g. 
environment (contaminated water, soil), animals or 
infected human 
▪ Transmitted via direct contact, or indirectly by 
vectors (insects) – bites/stings/passive transfer; 
direct exposure – faecal oral transmission through 
food/water (salmonella, cholera) or soil (tetanus) 
▪ Human-to-human transmission by aerosolized microorganisms in droplets (coughing/sneezing) – primary means of respiratory tract infections; physical contact (sexual contact, blood transfusion, direct contact contaminated materials) 
▪ After deposit in receptive environment -microorganism stabilizes adherence to the tissue through specific surface receptors e.g. infectious agents causing respiratory tract infections bind to molecules on respiratory epithelium – this helps protect removal of the pathogen by mechanical forces e.g. coughing mucus 

2. Invasion 
▪ Infectious agent can invade surrounding tissues/other sites 
▪ Developed mechanisms to penetrate tissues & avoid host’s nonspecific and specific defences (inflammation & immunity) 
 

3. Multiplication 
▪ Warm, nutrient filled host environment 
▪ Undergo rapid multiplication – replicate within…