Microbiology Flashcards
What is the process of gram staining a bacterial sample?
1) Crystal violet
2) Wash off
3) Iodine
4) Wash off
5) Decolourise the bacteria
6) Add a counter stain
What colour do gram +ve and gram -ve bacteria go?
- PurPle = Positive
- piNk = Negative
What are some examples of gram +ve bacteria?
- Staphylococci
- Streptococci
What are some examples of gram -ve bacteria?
- Coliforms
- Neisseria
What are round bacteria called?
Cocci
What are rod-shaped bacteria called?
Bacilli
How can gram +ve cocci be further classified?
- Clustered
- Chained
- Paired
What are chains of gram +ve cocci called?
- Streptococci
What are chains of gram -ve cocci called?
- Staphylococci
How can all of the streptococci be further classified?
Haemolysis on blood agar:
- Beta haemolysis (complete clearance) suggests B-haemolytic strep.
- Alpha haemolysis (green) suggests alpha-haemolytic strep.
For all the alpha haemolytic strep, an optochin test should be performed:
- Sensitivity to optochin suggests streptococcus pneumoniae.
- Resistance suggests Viridans streptococci.
How are the staphylococci further classified?
Coagulase DNAse test:
- +ve indicates staphylococcus aureus.
- -ve indicates another form of coagulase -ve staphylococcus (commonly staphylococcus epidermidis).
What is the pathway to reach a conclusion a sample contains staphylococcus aureus?
Gram stain: Gram +ve.
Shape: Round (cocci) and in clusters (staphylo)
Therefore, the sample is a staphylococcus.
Conduct a coagulase DNA test:
- +ve result means it is staphylococcus aureus.
SIDE NOTE: staphylococcus aureus will appear creamy/yellow when cultured on blood agar.
What is the pathway to reach a conclusion a sample contains streptococcus pneumoniae?
Gram stain: Gram +ve
Shape: Round (cocci) and in chains (strepto)
Therefore, the sample is streptococci.
Conduct a blood agar haemolysis test: Alpha haemolysis (green).
Conduct optochin test: If sample is sensitive to optochin, it is probably strep pneumoniae.
What important diseases can be caused by streptococcus pneumoniae?
- Lobar pneumonia
- Meningitis
Why are samples of gram -ve bacteria more likely to be contaminated than gram +ve ones?
- Gram -ve bacteria more commonly come from non-sterile sites (e.g. mouth, skin, vagina, urethra, large intestine).
How are gram -ve bacilli further classified?
Gram -ve bacilli = Pink, rod shaped organisms.
1) MacConkey agar:
- If pink, they are lactose fermenting.
- If yellow/colourless, they are not lactose fermenting.
2) If the bacteria are not lactose fermenting, conduct an oxidase test:
- +ve oxidase example is pseudomonas aeruginosa.
- -ve oxidase example is shigella.
3) If further differentiation is needed, more detailed biological tests will need conducting.
What is the pathway to reach a conclusion a sample contains pseudomonas aeruginosa?
1) Gram -ve bacilli (pink, rod shaped).
2) MacConkey agar: non-lactose fermenting (clear/yellow).
3) Oxidase test: +ve result indicates it could be pseudomonas aeruginosa.
What are the two main approaches to viral diagnosis?
- Virus detection
- Serology
What is the main method of virus detection now used?
What are the advantages and disadvantages?
PCR
+ Highly sensitive.
+ Quick.
- Only works after viral probe has been developed.
- Sensitive to laboratory contamination.
What does serology investigate with regard to viral diagnosis?
- IgM antibodies and IgG antibodies.
- IgM appears within one week of viral infection, and remains positive up to 12 weeks beyond.
- IgG appears later but will remain positive for many years (sometimes a lifetime!). It is IgG that is responsible for providing immunity upon reinfection.
- Therefore, either IgG or IgM virus specific antibodies can be used to diagnose viral infections.
What virus typically causes glandular fever?
Epstein Barr Virus (EBV)
What will serology tests typically show for glandular fever?
- Atypical lymphocytes on blood film.
- Significant IgM antibody titre.
Why is a bacterial swap important if glandular fever is suspected?
- Sometimes there is a bacterial differential diagnosis, so charcoal bacterial swab should be taken to rule these out.
What are the symptoms associated with glandular fever?
- Fatigue
- Malaise
- Loss of appetite
- Cough
- Sore throat
- Tonsils red with white patches, and swollen.
