Principles of Infection I
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What is symbiosis?
• living together - eg. microbes require human to survive + multiply • does not distinguish between relationships that are harmful or beneficial to either party
What is commensalism?
• microbe derives food/shelter from host • normally does not harm
What is mutualism?
• symbiosis which is beneficial to both organisms involved
What is another term for commensals?
Normal flora
Where are normal flora found?
• nose, nasopharynx + upper resp tract • oral cavity - large bowel • lower female genital tract • skin
What do normal flora/commensal microbes tend to do?
• COLONISE the host
What is a pathogen?
A microbe that causes disease / harm to the host
In which 3 instances can commensals be pathogenic?
• immunosuppression (eg. leukemia) -> OPPORTUNISTIC INFECTION • commensal at 1 site, pathogen at another (eg. SA in nose vs post-op wound) • commensal bypasses normal defences -> enters sterile site
What are some normally sterile sites?
• lower resp tract • blood • bone, joint + subcutaneous tissue • female upper genital tract • urinary tract • CNS - CSF + eye • other viscera eg. liver, spleen, pancreas
What is meant by the term ‘virulent’?
Highly pathogenic microbes
What two things can fall out of balance to result in infection?
When the pathogenic mechanisms are STRONGER than the defensive mechanisms
What are the 6 stages of infection?
1) survival of microbe + spread to host 2) adherence of microbe to host + entry 3) multiplication 4) evasion of host defences 5) damage to host 6) shedding of microbe + spread to env/other host
What are some routes of spread for pathogens?
• airborne/droplet - tb, influenza • GI tract - salmonella, cholera • through skin - malaria • inanimate objects - mrsa, rsv • other epithelial surfaces (oral, genital) - ebv, gonorrhoea
What is a fomite?
any nonliving object or substance capable of carrying infectious organisms eg. a teddy bear
What are some local symptoms of infection?
• redness, swelling, warmth, pain, loss of function • pus - pyogenic infection
What are some systemic symptoms of infection?
• fever, rigors, chills, tachycardia, tachypnoea