Host-microbe relationship

Relationships of organisms
Symbiosis - the coexistence of two or more different organisms.
 * Endosymbiosis x ectosymbiosis
 * Symbiont – any microorganism that shares at least part of its life with another organism
 * Three forms of symbiosis:
 * commensalism – benefits the commensal and does not harm the host (normal gut flora)
 * mutualism – both partners benefit (Escherichia coli and production of vitamin B12 and K, making colonization by pathogens more difficult)
 * parasitism – the organism harms the host - directly or just deprives it of nutrients - intracellular parasites, ectoparasites (lice), endoparasites (tapeworms)

Parasite and saprophyte
 * A parasite is an organism that at least sometimes requires another living organism
 * Saprophyte - a microbe does not need another organism, only non-living organic matter
 * Pathogenic saprophytes – Clostridium botulinum, Histoplasma capsulatum

Infection
There are several definitions for the term infection or contagion:
 * 1) The presence of a microorganism in a particular host.
 * 2) The penetration of an infectious agent into an organism, multiplication inside and adverse effects on its surfaces (in contrast to this stands colonization - colonization by a non-pathogenic or pathogenic microbe without disease symptoms).
 * 3) Conflict between a microbe and its host (even asymptomatic infection).

Process of infection
The process and severity of infection can be influenced by several factors:
 * On the part of the microbe


 * pathogenicity;
 * virulence;
 * infectious dose.
 * From the macroorganism side


 * degree of non-specific resistance - overall fitness, genetic factors (population resistance);
 * specific immunity - primary, secondary reactions;
 * age, nutrition, lifestyle.
 * Other circumstances
 * Environmental influences (e.g. seasonal variations - respiratory infections in cold weather),
 * gateway infection (e.g. tularemia).

Stages of infection

 * 1) Incubation period;
 * 2) period of prodromal symptoms - non-specific symptoms;
 * 3) typical syndrome;
 * 4) recovery - relapse may occur;
 * 5) relapse - after recovery.

Forms of infection

 * According to manifestations


 * 1) Inaparent.
 * 2) Manifest.
 * 3) * Subclinical - non-characteristic symptoms.
 * 4) * Abortive - poorly expressed symptoms.
 * 5) * Clinical form of infection - all symptoms expressed.
 * According to the range
 * 1) Local (possibly also focal).
 * 2) Systemic (sometimes referred to as massive) - when it affects the whole system or a substantial part (e.g. influenza, pulmonary TB, meningitis, pyelonephritis...).
 * 3) Generalized (regularly e.g. typhoid fever, spotted fever, most exanthematous viruses).
 * According to the course
 * 1) Acute (days).
 * 2) Subacute (months).
 * 3) Fulminant (violent).
 * 4) Chronic (years).
 * 5) * Asymptomatic chronic infections.
 * 6) ** Mainly viruses, bacteria – Rickettsia prowazekii, Chlamydia trachomatis, Salmonella typhi, Mycobacterium tuberculosis.
 * 7) *# Persistent - agents present in an infectious detectable form.
 * 8) *# Latent - in the body in a non-infectious form.

Related articles

 * Pathogenicity and virulence of bacteria