Monday, 2 July 2007

Question and Answers: Microbiology

Q1) What does XLD plate stands for?
Ans: Xylose lysine sodium desoxycholate (XLD) plate

Q2) What makes the Salmonella colonies black?
Ans: The Salmonella(non-glucose fermenter) produces H2S when growing which causes the colonies to turn black
Q3) How Rotavirus Quick test works?
Ans: It works by detecting the Rotavirus antigen using monoclonal antibodies (Elisa mtd). The test kit has monoclonal antibody againts the Rotavirus antigen, if the stool contains the virus a coloured pdt will be formed.

Q4) What is a MUG test?
Ans: Is by using the principle of antibody binding to the specific antigen on the Ecoli and under UV light the Fluorence compound labelled on the antibody will emit light(Ecoli positive)

Q5) What do you mean by using 3 parts of the loop?
Ans: The picture shows a inoculation loop which i have colour coded and numbered it. When doing the primary streaking use the Black part of the loop, Green part when streaking the secondary, and red part when doing the tertiary streak.(seen above)

Q6) What is a Primary, Secondary, Tertiary streak?

Ans: Refer to diagram above.

Q7) Why must the stool samples be subcultured onto the 3 different types of media (XLD plate, Campylobacter Plate, Selenite F broth)?
Ans: We want to look out for Salmonella which will grow as black colonies on the XLD plates, Campylobacter plate to look out for Campylobater jejum species(tiny grey colonies on the campy plate), so we will be able know the specific cause of the diarrhoea. The selenite culture is just for subculturing the stool sample on to a XLD plate after incubation for 1day.

Q7) Growth and identifying campylobacter.
Ans: The Campylobacter jejum species looks like pin-point grey colonies on the black background of the campy plate. a test can be done to verify the bacteria(Hippurate test). By adding the colonies into the hippurate reagent and incubate for 10mins and then add 7drops of Ninhydrin reagent and incubate for 2hrs-positive will have a purple coloured ring on the upper part of the solution.

Q8) For the quick test for rotavirus antigen identification.. are there any negative or positive controls to be done when doing it?
Ans: actually the blue band is already a control band so we do not have to run a seprate control. If the blue band does not appear, the test is not considered and the test must be redone.

Q9) For the vp3 test, how useful is it in diagnosis of microorganisms infection in the female reproductive system?
Ans:According to the manufacturer, the test kit's sensitivity is 100% and the specificity is 99%.

Q10)for UTI diagnosis,what would be done if its negative for the test, is there any further investigations that would be performed if the results turned out to be negative (not E.coli)?
Ans: It depends if the FEME count is it high if it is high we must issue a prelim report to the doctors to indicate no significant/no bacteria growth but also reflect the WBC count, so the doctor can monitor the patient's progress

Q11) What is a zig-zag line on the agar?

Ans: Refer to above diagram. the sequence of streaking is written in the picture. for the top part the streak is very close together and then when it comes close to the bottom the space is wider(but remember to maximize the space avalible at the sides. Also no note that the streaking must be done fast to prevent the urine from drying up and the streak must be consistent.
Q12)What are the main differences between MacConkey and CLED?
Ans: for CLED it is a more specialized media used for isolation, presumptive identification, for my lab we use the CLED for those overnight samples that is sent to us after working hours as if urine is not directly plated within 2hours it must be kept at 5degrees C. By using a dip slide which contains 3 types of agar: Mac, CLED, colourless base medium which is for identification of Ecoli. Mac agar inhibits gram pos bacteria which are unlikely causes of UTI thus we plate it on mac to rule out the gram pos bacteria.

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