Infection is a process by which the organism enters into a relationship with the host.
Although microbial infections occur frequently, most infections end without occurrence of pathological changes and thus are not manifested as clinical disease.
The outcome of bacterial infections depends on the mutual relationship between bacteria and host.
Accordingly, bacteria could be classified into:
1. Saprophytic bacteria:
Are those which live freely in nature, on decaying organic matter, in soil or water. They do not require a living host.
2.Parasitic bacteria:
Are those which live on or in a living host. They are classified according to their relation to the host into:
- Pathogenic: Bacteria capable of causing disease.
- Non-pathogenic (commensals): Bacteria that do not cause disease, and are part of the normal flora.
- Opportunistic pathogens: These are potentially pathogenic bacteria that do not cause disease under normal conditions but can cause disease in immunocompromised patients, or when they find their way to another site other than their normal habitat. Many of these opportunistic pathogens are originally commensals.
Stages of the Infectious Process
- Source of infection which may be man (case or carrier), animal or soil.
- Mode of transmission e.g. droplet inhalation, ingestion, injection, insects, contact and transplacental.
- Portal of entry e.g. respiratory tract, gastrointestinal tract, skin...etc. The organism then starts to multiply within the host causing tissue damage (disease).
- Portal of exit e.g. urine, stools, blood, respiratory or genital discharge, from which the organism is transmitted to a new host.
Microbial Virulence
While pathogenicity is a qualitative description of a species of bacteria denoting ability to produce disease, virulence is a quantitative character (degree of pathogenicity) of a strain belonging to a pathogenic species.
Virulence is genetically determined by genes carried on plasmids, phages, pathogenicity islands and chromosomes.
Virulence Factors of Bacteria
A virulence factor is either a structure (e.g. capsule) or a product (e.g. toxins) that enables the organism to cause disease.
a. Adherence factors
They enable bacteria to attach to the host surfaces, thus contributing to the establishment of the infection. For example:
- The fimbriae of Neisseria gonorrhoeae and E. coli help the attachment of these organisms to the urinary tract epithelium.
- The glycocalyx of Staphylococcus epidermidis and certain viridans streptococci allows the organisms to adhere strongly to the heart valves.
b. Invasion factors
Invasion of tissue followed by inflammation is one of the main mechanisms by which bacteria can cause disease. This invasion is helped by:
1. Enzymes
- Collagenase and hyaluronidase which degrade collagen and hyaluronic acid and allow the bacteria to spread through subcutaneous tissues.
- Immunoglobulin A protease which degrades IgA.
- Leukocidin which can destroy both polymorphonuclear leucocytes and macrophages.
- Deoxyribonuclease that breaks down DNA.
- Lecithinase that breaks down lecithin of cell membrane.
2. Antiphagocytic factors
- Capsule: The capsule prevents the phagocytes from attachment to the bacteria, e.g. Strept. pneumoniae.
- Cell wall proteins of Gram-positive cocci, such as the M protein of Strept. pyogenes and protein A of Staph. aureus.
- Coagulase: It accelerates the formation of a fibrin clot from fibrinogen. This clot can protect bacteria from phagocytosis, e.g. Staph. aureus.
Toxin production is another mechanism by which bacteria can produce disease. Bacterial toxins are either exotoxins or endotoxins (Table 1).
Table (1): Comparison of the main features of exotoxins and endotoxins:
| Feature | Exotoxins | Endotoxins |
|---|---|---|
| Location | Produced inside the bacterial cell and secreted | Part of the bacterial cell wall |
| Release | Released during bacterial growth and lysis | Released only when the bacterial cell dies |
| Heat stability | Heat sensitive | Heat stable |
| Antigenicity | Highly antigenic | Moderately antigenic |
| Toxicity | Highly toxic | Moderately toxic |
Treatment of exotoxin with formalin (or other agents) removes its toxicity and retains its antigenicity converting it into toxoid, that can be used for immunization.