Bacteriology Lab Flashcards
What are common diagnostic techniques?
Culture – used to determine antimicrobial resistance
- Sterile sites – e.g. blood and CSF
- Non-sterile sites
> However, in non-sterile sites, there is usually a lot of bacteria so culturing is difficult
Serology
- Used to determine the body’s response to an infection – so measuring the body’s response by doing a blood at the beginning of chicken pox infection and at the end of infection
Molecular techniques – detect resistance genes
Antimicrobial susceptibility testing – used to test AB resistance but takes a long time
How do you carry out a blood culture?
There is a broth at the bottom of a tube and bacteria placed in the tube and incubated can change the colour of the broth to indicate whether a bacterium is present
If the blood culture is positive, you can do gram testing.
- Gram POSITIVE = skin and soft tissue
- Gram NEGATIVE = abdomen and urinary tract
- Chocolate agar – cooked blood – let’s bacteria use the blood nutrients to grow.
- *MacConkey agar – designed to grow gram -ve bacteria
How do gram +ve and -ve bacteria stain?
Gram +ve = thick wall, purple stain, retains dye
Gram -ve = thin wall, pink stain, loses dye
What do staphylococci look like?
What type of staphylococcus tests coagulase+?
often form clumps and look like bunches of grapes as they bud divide
\+ = staph aureus - = common skin microbes
What does streptococci look like?
form chains on gram stain
What groups does streptococci separate into on blood agar?
Alpha haemolysis – incomplete haemolysis, turns green
- e.g. Streptococcus pneumoniae.
Beta haemolysis – complete haemolysis, clears agar
- Group A – Streptococcus pyogenes – skin/soft tissue infection
- Group B – Streptococcus agalactiae – sepsis in the young
What does gram negative bacilli look like?
pink (doesn’t take up stain)
e.g E. coli
What are possible causes of diarrhoea?
Bacteria – e.g. Salmonella, Shigella, Campylobacter, E. Coli, C. difficile, cholera.
Parasites – e.g. Amoeba, Giardia, Cryptosporidium
Viruses
What investigations can you do to find the cause of diarrhoea?
Bacteria:
- Culture on agar – only salmonella, Shigella and campylobacter looked for routinely as LOADS of bacteria grow in faeces
- Clostridium difficile is difficult to culture but can use PCR to detect toxin gene.
Parasites:
- Concentration, special stain
What does salmonella, campylobacter and vibrio cholerae look like on agar?
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
What does the positive predictive value depend on?
pre-test probability of sample being positive
The more likely a patient is to have the disease, the more likely a positive test represents a TRUE positive.
I.E. the prevalence may be 12% so 12/100 have it so is a high pre-test positive probability
What is MIC?
Minimum Inhibitory Concentration = the lowest amount of AB required to inhibit growth of bacteria in vitro
- MIC isn’t very useful on its own so we set breakpoints which correlate MIC with clinical success as an AB
- A bacterium with an MIC below the breakpoint means there is a good chance of success with that A.
- A bacterium with an MIC above the breakpoint is RESISTANT
What happens in disc diffusion?
Use a set concentration of AB in each disc and incubate for 24 hours
The zone size is interpreted using he breakpoints in a AB table
Measure zone diameter to determine whether a bacterium is resistant to a AB or not
