Priscilla Johanesen; Week 9 MED1011; Microbiology
There is a balance between microorganism attack and host defence. Skin surface is relatively dry and acidic with the main source of nutrition being dead cells. Normal flora is mostly gram positive (Corynebacteria, Staphylococci, Propionibacterium acnes). Bacterial flora in mouth and nose are Staph aureus, Strep pyogenes, Neisseria meningitidis. Saliva contains antibacterial substances, organisms must attach to surfaces. Plaque is a buildup of mainly bacteria on the teeth, main are Strep mutans and sanguis, anaerobic. Caries involves destruction of enamel, bacteria in plaque convert sucrose to lactic acid. Nasal mucus contains lysosyme, URT similar to mouth, pyogenes and H influenzae can be opportunistic pathogens. LRT does not normally have microbiota, microorganisms removed by mucus/alveolar macrophages. Stomach is hostile to bacteria. H pylori is in half the population. Duodenum has 10^4 organisms per gram, jejunum 6, ileum 9. Large intestine has 10^11-12 per gram, Candida albicans and protozoa are harmless commensals. Strictly anaerobes are present. E coli is a facultative anaerobe. Upper urethra usually sterile, lower in females is not.
Pathogenicity is capacity of organism to cause disease, virulence is a measure of pathogenicity and the degree of intensity. Virulence factor is required for organism to cause disease.
Steps in the infectious disease process are existence, reservoir, mode of transmission, means of entry, host susceptibility. Pathogens must enter the body, colonise the host, evade host defences, multiply and disseminate and cause damage to the host. Source of infection can be inanimate and animate. Barriers to entry are physical, innate or adaptive.
Skin disease commonly Staph aureus, Strep pyogenes. Clostridium perfringens is gas gangrene, Mycobacterium ulcerans is Bairnsdale ulcer. S aureus, S epidermidis, group A streptococci can cause necrotising fasciitis. From bites, Pasteurella multocida, S aureus, Clostridium tetani. Burns P aeruginosa, S aureus enter as gram negative rods.
Resp route of infection is through inhalation, rapid spread, increase in secretions (sneezing, coughing), fomites. Common URT diseases are whooping cough Bordetella pertussis, TB mycobacterium tuberculosis, pneumonia strep pneumoniae. Faecal oral common GIT infections are Vibrio cholerae, diarrhoea (E coli, salmonella, Campylobacter jejuni), shigellosis (shigella sonnei), dysentery (shigella dysenteriae), botulism (Clostridium botulinum), giardiasis (giardia lamblia), Hep A virus.
Cystitis is E coli or Staph saprophyticus. In hospitals, some UTIs are from Pseudomonas, Enterococcus and Candida albicans. Pyelonephritis is 90% caused by E coli.
Common STDs are gonorrhoea (Neisseria gonorrhoeae), chlamydia trachtomatis. Perinatal infections can be conjunctivitis (C trachomatis, N gonorrhoeae); pneumonia (group B strep, C trachtomatis) meningitis (strep agalacticae). Congenital is syphilis, listeriosis