Pathogenicity and virulence MUDr. Lenka Černohorská, Ph.D.

Symbiosis Commensalism – 1 partner profits, 2 nd is not damaged (bacteria of intestinal flora) Mutualism – reciprocate profit, for ex. E. coli – vitamine K, protection of body surfaces (Staphylococcus epidermidis) Parasitism – 1 partner profits, 2 nd is damaged (direct or indirect effect – food competition)

Relationships and their dynamics between a microbe and a host – tendence to equilibrate the powers the dynamic and duration of a disease is dependent on these relationships Mutualism Commensalism Parasitism

Parasite Ecological “gangster” – steal and kill! Ecological “gangster” – steal and kill! In microbiology – Parasitology - parasitic protozoa or helmints (Toxoplasma gondii, Ascaris lumbricoides…) In microbiology – Parasitology - parasitic protozoa or helmints (Toxoplasma gondii, Ascaris lumbricoides…) Parasite is a beeing, who needs for life another living organism // an opposite saprophyt – needs only organic substances from not living organisms Parasite is a beeing, who needs for life another living organism // an opposite saprophyt – needs only organic substances from not living organisms

Pathogenicity = Ability to damage and cause a disease 1.species characteristic – is related to whole microbial species 2.natural (Yersinia pestis - pest)/experimental (Treponema pallidum - syfilis in rabbit) 3.must be related to host species (species susceptibility) Microb able to damage and cause a disease - pathogen

Koch‘s postulates 1.The organism must be isolated from every patient with the disease 2.The organism must be isolated and cultured outside the body in pure culture 3.The pure organism must cause the disease in healthy, susceptible animals 4.The organism must be recovered from the inoculated animal 5.The antibody to the organism should be detected in the patient´s serum

Pathogenicity Obligate (primary) pathogen cause disease in healthy persons – typhus, syfilis etc. Facultative (opurtune) – the disease is caused only in imunocompromited patients- for ex. E. coli – normal flora of large intestine, but also causative agens of 80 % urinary tract infections, wound infections...

Virulence = quantitative measure of patogenicity and is related to a strain 1.is an individual characteristic 2.can be measured : the number of organism needed to kill half the host – 50% letal dose - LD 50 and the number needed to cause infection in half the host – 50% infectious dose - ID 50 3.genetic changes are possible as well as 4.changes of virulence made by an environment – pasaging in suboptimal conditions ( virulence, preparation of BCG vaccine - TBC), or on susceptible host ( virulence, danger of laboratory infection!) Analogies: patogenicity/virulence - microbe species/individual susceptibility (resistance) - macroorganism

Factors of pathogenicity 1. Contagiosity – transmissivity – high (respiratory viral infections), low (Clostridium tetani) 2. Invasiveness – interfering with the host defence mechanism – entrance (adherence and penetration), division and dissemination 3. Toxigenicity – ability to destroy – due to microbe himself or host reaction

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