Diagnostics 3 - Bacteriology Lab Flashcards
What are common bacteriology diagnostic tests?
- Culture (watch cell growth at 37 degrees for 5 days) - sterile sites (e.g. Blood/CSF - expect no bacterial growth), and non sterile sites
- Serology
- Molecular techniques (PCR)
- Antimicrobial / AB Susceptibility testing
Why do blood culture discs change colour?
Bacteria reproduce –> due to bacteria reproducing –> produce CO2 –> alters pH
Which type of bacteria tend to clump?
Gram positive cocci - e.g. staphylococci or streptococci
Differentiate gram positive and gram negative bacteria
Gram positive = thick peptidoglycan layer - retains dye so stains purple
Gram negative = thin peptidoglycan layer - retains counterstain - stains red. In gram negative, outer cell membrane usually produces bacterial toxins
What is the peptidoglycan layer made up of?
2 different peptides:
- NAM - N-acetylglucosamine
- NAG - N-acetylmuramic acid
Where is gram positive bacteria generally found? What about gram negative?
Gram positive = skin and soft tissue
Gram negative = abdomen and urinary tract
What are the 3 types of agar and explain their use
- Chocolate agar - cooked blood - certain bacteria can’t lyse RBC - cooking releases nutrients in nutrient agar which allows certain bacteria to grow. e.g. Haemophilus influenzae
- MacConkey Agar - designed to grow gram negative organisms
- Neomycin agar - growth of anaerobic microorganisms
In which 2 scenarios must antibiotics be given ASAP?
- Meningitis
2. Meningococcal septicaemia
Explain how the coagulase test can be used to detect staphylococci
Coagulase converts fibrinogen into fibrin. (it is a virulence factor)
Staph. can either be coagulase positive or negative.
Coagulase positive = staph. aureus (potentially MRSA). Positive result = coagulation/clot
Coagulase negative = common skin commensals of low pathogenic potential
Staphylococcus don’t tend to cause infection unless opportunistic circumstances. They can infect prosthetic material which can cause?
Line, pacemaker infections
Streptococci are divided into 2 groups depending on blood agar. They are?
- Alpha haemolysis - incomplete haemolysis - turns agar green (e.g. S. pneumonia - causes pneumonia and meningitis)
- Beta haemolysis - complete haemolysis - clears agar. Subdivided into group A and B.
- Group A (e.g. S. pyogenes). Skin and soft tissue infections, rheumatic and scarlet fever
- Group B (e.g. S. agalactiae). Sepsis in young children. Infections in diabetics.
(3. Non-haemolytic e.g. gut enterococci)
What are the causes of diarrhoea?
- Bacteria (salmonella, shigella, campylobacter. Or e.coli, C. difficile, cholera
- Parasites - amoeba, giardiasis, cryptosporidium
- Viruses
Explain how stool sample investigations are done for bacteria and parasites
Bacteria - cultured on agar plates. Only salmonella, shigella and campylobacter are checked for routinely. C. difficile can’t grow - so toxin detection or PCR is done for toxin gene.
Parasites - concentrate then use special stains.
What type of agar plate is salmonella grown on?
XLD (xylose lysine deoxycholate)
Salmonella can’t ferment xylose –> causes red colour.
Most there coliforms or enteric bacteria ferment xylose causing a yellow colour
Salmonella - forms H2S which forms black colonies
How is campylobacter plate grown?
Little oxygen environment - they can survive at 42 degrees so this temperature is used for incubation - kills other bacteria in stool sample
What type of agar plate is v. cholerae grown on and what colour is a result?
TCBS agar plate and colour is green
Infectious diarrhoea doesn’t usually persist for how long?
4 months
How can it be determined if bacteria are resistant or sensitive
Zone diameter breakpoints
What is MIC and how can gradient MIC strips be used?
Minimum amount of AB needed to inhibit bacterial growth in vitro after overnight incubation
Gradient MIC strips - varying conc of AB - read off conc at edge of no bacterial growth
Disc diffusion can also be used to measure sensitivity
Use AB discs on agar plate - incubate for 24 hours —> measure how far AB spreads out (look at cleared area diameter)
If bacteria have MIC below the breakpoint, what does this mean?
There is a good correlation with clinical success if you use that antibiotic
If bacteria have MIC above the breakpoint, what does this mean
Reported as resistant
What do bacteria have as natural protection?
Beta-lactamases
Describe IgG and IgM relationships in 1st and 2nd infection
1st infection - IgG peaks later than IgM
2nd infection - IgG rises rapidly first and higher than IgM
