A bacterial infection is defined as the penetration and subsequent multiplication of harmful bacteria into the tissues and/or blood of more complex organisms. In order for an infection to occur in humans, the pathogenicity of the bacteria must exceed the defensive capacity of the organism; otherwise, the germs are destroyed without obvious damage to the host tissues.
An infectious disease is a set of local and/or general symptoms resulting from a bacterial infection, and it is the result of the interaction between the microbial agent and the body’s immune defences.
The most common pathogenic bacteria and the types of bacterial diseases caused by them are:
- Escherichia coli and Salmonella can cause intoxication.
- Helicobacter pylori is responsible for gastritis and ulcers.
- Neisseria gonorrhoeae causes gonorrhoea (sexually transmitted disease).
- Neisseria meningitidis causes meningitis.
- Staphylococcus aureus is responsible for zits, abscesses, cellulitis, wound infections, toxic shock syndrome, pneumonia and food poisoning.
- Streptococcus can cause pneumonia, meningitis, ear infections, and sore throats.
If the pathogenicity is moderate but the body’s immune capacity is lower, there will be an opportunistic infection: pathogens that are completely harmless to healthy subjects can cause infections if the immune system is weakened for any reason. This is the case, for example, of immunodepressed subjects, who are unable to counteract the pathogenic action of the smallest infectious charges.
If the defensive capacity of the immune system is equal to the pathogenicity, the host is known as “carrier”. In this case, pathogens can survive harmlessly and reproduce (without reaching a number that is high enough to induce infection), even for an indefinite time, in the tissue of the host.
All factors that contribute to the weakening of immune barriers and/or cause pathogens to overcome them can favour infections.