bacteriology Flashcards
common diagnostic techniques
cultures- samples taken from STERILE sites (blood/CSF) or NON-STERILE sites (skin), and bacteria grown
2) molecular techniques ie PCR- tell you what bacteria it is, but not what drug will be good for it (ie SENSITIVITY) so cultures used more: molecular techniques more used for viruses (culture hard for viruses)
3) serology- look at bodies response to specific infection ie antibodies
4) antimicrobial susceptibility testing- tells you which drug bacteria sensitive to (not really diagnostic)
why ppl in hospital more susceptible to infection
parts of innate immune system are compromised eg a cannula allows bacteria past the skin into the blood
explain process of blood cultures
blood extracted from patient, and put in bottle in favourable conditions for bacteria to grow (warm, no antibiotics)- if positive, indicator changes colour. blood then put on agar plates to grow, and antibiotics used to determine sensitivity ie antimicrobial susceptibility testing
gram + vs - and clinical relevance
gram+ have thick peptidoglycan wall, gram- has a thin one, as well as a LPS layer: antibiotic treatment is different for the 2
DIAGRAM diagnosing staphylococci
if gram staining method shows DEEP purple, means gram+. the stain can also show cocci (circles) in clumps, which indicate staphylococci
staphylococci coagulase test
ability of the bacteria to form a clot- this test uses blood culture: if COAGULASE POSITIVE, indicates staphylococcus aureus, which is pathogenic (infection of skin, endocarditis)- if COAGULASE NEGATIVE, indicate commensal skin staphylococci which aren’t very harmful (can infect prostethic joints though)
diagnosing streptococci
unlike staphylococci which divide together into lumps, these divide end on end to form gram positive cocci in CHAINS
haemolytic streptococci
beta haemolytic streptococci are either group A (pyogenes) and B mainly cause skin infections (cellulitis)
alpha haemolytic streptococci are streptococcus pneuomoniae
gram - bacilli and relevance
they stain pink, and are rods (bacilli)- gram- tend to be more resistant than gram +
sterile vs non sterile sites and what lab does
sterile sites are simple as shouldn’t be ANY pathogens there- if there are, indicates disease
non sterile sites more complicated as so many bacteria there in a healthy person- if no history given, in eg faeces/diarrhoea, they look for salmonella, shigella, campylobacter (main cause of food poisoning), and E coil- may also look at cholera if travelling, C difficile, as well as parasites/viruses
investigations for stool/diarhhoea: bacteria vs parasites
mostly cultures, sometimes PCR eg C difficile
parasites don’t use cultures, only special staining methods
sensitivity testing
determines how resistant bacteria is to antibiotic- done by minimum inhibitory concentration/ MIC (least amount to kill it), the higher it is, the more resistant it is- to get this number, you antibiotic discs, and measure ZONE OF INHIBITION- the greater it is, the less resistant
causes of diarrhoea and associated symptoms
infection diarrhoea is more acute, doesn’t occur long term, and usually won’t lead to systemic disease- may be
endocarditis (SBE)- causes fever and weight loss as well
syphilis- rashes and fever too, as well as lymphadenopathy (GLANDS in his neck)
toxoplasma- rashes and fever+ lymphadenopathy
TB- fever and weight loss
brucellosis
serology- seroconversion and why not used so much
acute sample negative for antibodies (as no antibodies in primary response), but long term sample positive for antibodies (as antibodies develop in secondary response)- serology not often used due to TIME LAG
