Bacteriology Flashcards
where are cultures extracted from to determine antimicrobial resistance?
sterile sites e.g. blood and CSF
non-sterile sites (difficult due to great numbers of bacteria)
what is serology done for?
determine body’s response to an infection
what are molecular techniques used for?
detect resistance genes
alternatively do antimicrobial susceptibility testing but this takes a long time
what type of bacteria is found on skin and soft tissue culture usually?
gram +
e.g. Staph aureus
what type of culture is found in the abdomen and urinary tract
gram -
what causes a gram +ve stain?
due to thick peptidoglycan wall it retains the dye and shows up purple
what causes a gram -ve stain?
due to thin wall (double membrane with pep membrane in-between)
loses the dye so looks pink
what is the effect of gram -ve on antibiotics?
may not enter due to the other membrane
e.g. vancomycin only works on gram +ve
what is the most common bacterium?
gram + cocci
staphylococci
how do staphylococci look under the microscope?
they clump like a bunch of grapes as they divide
how can you test to distinguish staphylococcus (aureus)?
coagulase test:
+ indicates s.aureus
- indicates common skin microbes
how do streptococci look under the microscope?
form chains in gram stain
what are the groups of streptococci on blood agar?
1) alpha haemolysis- due to incomplete haemolysis it turns green e.g. s.pneumoniae
2) beta haemolysis- due to complete haemolysis, clears the agar e.g. group A s.pyogenes and group B s.agalactiae
how do bacilli look under the microscope
gram -ve so don’t take up the gram stain and appear pink
e.g. E.coli
what are the possible causes of diarrhoea?
1) Bacteria – e.g. Salmonella, Shigella, Campylobacter, E. Coli, C. difficile, cholera.
2) Parasites – e.g. Amoeba, Giardia, Cryptosporidium.
3) Viruses.
what bacteria cause diarrhoea? how can they be cultured?
Salmonella
– XLD agar – the salmonella colonies are black due to hydrogen sulphide produced.
Campylobacter
– 48hours to grow and can survive at 48 degrees so heat to kill other bacteria.
Vibrio cholerae
– TCBS agar – cholera makes the agar turn green.
how can bacteria involved in diarrhoea be investigated?
- agar culture: salmonella, shigella and campylobacter looked for routinely as they are found in faeces
- PCR to detect toxic gene of c.difficile. This bacteria is difficult to culture
how are parasites involved in diarrhoea investigated?
special stains
concentrations
what is the positive predictive value dependent on?
what is the significance of this?
pre-test probability of the sample being positive
the more likely a patient is to have the disease, the more likely a positive test is a true positive
therefore there is no need to test for everything
what is the MIC?
minimum inhibitory concentration
the lowest amount of AB needs to inhibit the growth of bacteria in vitro
how is MIC used in evaluating the success of antibiotics?
As MICs aren’t very useful Breakpoints are used.
This correlates MIC with the clinical success of a AB
Breakpoint= chosen concentration (mg/L) of an antibiotic which defines whether a species of bacteria is susceptible or resistant to the antibiotic.
1) a bacterium with MIC < breakpoint means the AB has a chance of being successfully used
2) MIC > breakpoint indicates that the bacterium is resistant
how can bacteria resistance also be evaluated?
disc diffusion where a set conc of AB on discs are incubated for 24hrs
a zone size is compared to breakpoints on an AB table
diameter is measured
how long are most bacterial samples cultured for agar plates?
24-48 hours
- some need longer e.g. TB, brucella, actinomycetes
- a further 24hr needed for antibiotic sensitivities
when is it optimal to collect the microbial specimen?
- in the acute phase of illness before starting antimicrobials
- at a proper site to avoid contamination with normal flora
