Model Answers Microbiology Written examinations 2007 - RCPA

Question 3: Discuss the differences between:
Virulence and pathogenicity
Colonisation and infection
S. aureus and MRSA
Planktonic and sessile bacteria
Virulence and pathogenicity
Pathogenicity is the ability of an organism to cause disease due to structural and
biochemical mechanisms. Pathogenic organisms have the ability to invade tissues and
initiate infection. Involves both host and microbial factors. Pathogen requires ability
to colonise, bypass host defences, invade deeper tissues. Due to the [presence of
adhesions, resistance to phagocytosis, intracellular invasion and survival within cells,
impairment of complement functions, lytic effect on PMN
Virulence is the degree of pathogenicity within a group of organisms. Multifactorial
microbial and host factors. Infectivity is the ability to initiate infection. Severity
depends on the presence of virulence factors. These are substances produced by
bacteria to harm the host. Eg exotoxins, haemolysins, lipases, elastases, proteases,
alginate polysaccharides, antimicrobial resistance factors, leucocidin, hyaluronidase,
streptokinase, staphylokinase, cytolysins, siderophores, protein M, TSST, PVL
Virulence could be manifest in one host (eg immune suppressed vv another).
Microbial virulence factors can be the target of effective immune responses eg
antibody response demonstrating that host immunity influences virulence. Virulence
factors may be used as vaccine antigens to reduce or negate virulence in certain
microbes
Colonisation and infection
Colonisation requires attachment of bacteria via adhesions to receptors in host tissues,
may be part of the normal flora, may be symbiotic with the host, may precede
infection, examples of adhesions include: pili, fimbriae, lipoteichoic acid, protein F,
examples of normal colonisation are skin, GI tract, upper respiratory tract
Normal flora organisms colonise surfaces
Infection is acquisition of a microbe by a host then invasion of the host by organisms
with virulence factors to produce local or systematic disease. Ie there is presence of
the organism, invasion and a host response. Infecting organisms penetrate the skin or
mucous membranes by the production of invasions (spreading factors, hyaluronidase,
collagenase, etc) and more extensive effects if exotoxins are produced.
Clinical examples of infection compared with colonisation, normal flora compared
with pathogens.