Bacteriology Flashcards
What sort of infections might be confirmed in a bacteriology lab?
Infectious Diarrhoea – by eating/drinking something strange
Endocarditis (SBE) – infection of the heart valves
Syphilis – an STI – can result in rashes
Toxoplasma – picked up in contact with animals
Tuberculosis – an airborne pathogen (you can get TB of the lung, but also atypical mycobacteria)
Brucellosis
Melliodosis – gram negative rod, you may get it if you travel to the far east
What are some common diagnostic techniques in bacteriology labs?
Culture of sterile sites (blood/CSF) and non sterile sites (urinary tract/bowel/skin)
Serology – looking for an amounted immune response to infection
Molecular Techniques – e.g. PCR – looking for presence of bacterial DNA/RNA
Antimicrobial Susceptibility Testing
How are blood cultures perfomed?
Clean skin first (Keep sterile)
Take blood from the patient and place into two different bottles (aerobic and anaerobic)
Discs at the bottom of the bottles
Blood is incubated at a temperature similar to body temperature, and bacteria multiply
When bacteria reproduce, they produce CO2 → this causes a pH and colour change on the disc
This change flags an alarm on the machine → allows technicians to have a look
This usually takes about 5 days but in case of concern the period can be extended
What is done with positive blood cultures?
Blood is removed and put onto different agar plates, includes blood agar, chocolate agar (haemolysed blood), MacConkey agar and neomycin agar
MOST bacteria can grow on blood/chocolate agar, but gram negative bacteria grow well on MacConkey
The presence of bacteria on MacConkey plates produces a colour change
The plates are then incubated for 24 hours
What is the basis for Gram +Ve and Gram -Ve classification of bacteria?
Gram positive bacteria have a thick peptidoglycan wall (purple/blue)
Gram negative bacteria have a thin peptidoglycan wall, sandwiched by two membranes (pink/red)
Peptidoglycan takes up gram stains well, so gram negative don’t really absorb much of the stain
What tests are done on staphylcoccus bacteria?
COAGULASE TEST: staphylococcus strains may contain the enzyme coagulase, do test to identify coagulase if its positive, its staphylococcus aureus (bad - could be MRSA)
If its negative then the staphylococcus is likely a commensal
What tests are done on streptococcus bacteria?
Incubate Strep strains on Blood Agar
Streptococci may produce a green tinge to the blood agar from partial haemolysis – these are alpha haemolytic
If they use up ALL the blood (haemolysis), they are beta-haemolytic (Leaves clear section)
If the strain is non-haemolytic (Gamma Haemolytic), it could be a commensal strain of enterococci
What could the presence of Gram -Ve Bacilli indicate in a blood culture?
Septic shock
Gram negative bacteria have an outer membrane, that can produce toxins and cause shock
What causes diarrhoea?
Bacteria: Salmonella (including S. typhi ), Shigella, Campylobacter, E coli O157, C difficile, Cholera
Parasites: Amoeba, Giardia, Cryptosporidium
Viruses
What is the procedure for a stool sample?
Bacteria in stool cultured on agar plates
Only Salmonella (XD agar) , Shigella and Campylobacter are looked for routinely
Ask for specific one if suspected - different culture requirements
Testing for C.Diff:
Clostridium difficile – toxin detection or PCR for toxin gene (we can’t culture C. difficile)
How is bacteria sensitivity testing performed?
Minimum Inhibitory Concentration (the point at which bacteria is resistant or sensitive to a specific concentration of drug)
There is a control at the end, with no antibiotic. There are test tubes, one with bacteria and no antibiotic, and one with the antibiotic, but no bacteria. You perform a doubling dilution – with decreasing antibiotic at each stage. You are looking for the point at which bacteria
start growing – this is the MIC.
Also Gradient MICs on strips and Disc Diffusion to test bacteria sensitivity to many antibiotics simultaneously.
Which diagnostic tools are used for the following common bacteria? Subacute Bacterial Endocarditis Syphilis Toxoplasma TB Brucellosis
Subacute Bacterial Endocarditis - blood culture
Syphilis - Serum Antibodies
Toxoplasma - Serum antibodies
TB - Culture, Interferon gamma
Brucellosis - Serum antibodies, Blood culture
How is seroconversion in samples used to date infections?
You get initially exposed to infection, and get an IgM response very early on. The IgM response wanes, and you get a rise later on if you are exposed to the antigen again. The IgG response happens much later on
You may get a negative serology result for IgG, because the infection may be present but the sample may have been sent off too early. It depends on when the initial exposure happens. It is important to repeat tests 2-4 weeks in these scenarios.
What should always be included with bacterial samples?
- Always include clinical information including travel history on requests
- Contact Infectious Diseases or Microbiology early if you require advice
- Try and send samples for culture prior to starting antibiotics if possible
