Model Answers Microbiology Written examinations 2007 - RCPA

Question 2 Compare the advantages and disadvantages of:
RTPCR and viral culture
Chromogenic media and selective media
Swabs versus fluid samples for examination for bacteria
Serum versus plasma for antibody detection
RTPCR and viral culture
Confusion over whether RT means Real time or reverse transcriptase therefore you
must define your topic and indicate what you are addressing if there is confusion.
General discussion about PCR is acceptable
PCR- Rapid TAT especially important when infection control public health
intervention required. Detects non cultivable viruses. Can be used to detect dangerous
viruses without the need for P3/4 facility, Sensitive, Transport and storage of
specimens less important than fro culture, Initial set up is expensive, Easier to train
staff in PCR techniques than those for viral culture, Problems with false positives and
inhibitors resulting in false negative results, Can only detect specific viruses tested for
. there is no isolate available for further characterisation or sensitivity testing
although sequencing can reveal some known genes associated with resistance in some
viruses eg CMV, HIV
Viral culture: Slow, need to maintain cell cultures, CPE can be subtle or non specific,
experienced staff needed to undertake viral cultures, increasingly hard to find. Cell
lines prone to contamination, can detect many different viruses including novel
strains, less sensitive but more specific although new PCR techniques to trawl
samples for DNA/RNA have detected several new viruses recently that have not been
identified in culture previously, Virus available for further characterisation eg typing,
resistance testing and to enable fulfilment of Koch‟s postulates
Chromogenic media and selective media
Candidates need to show that they understand the difference between differential and
selective media. Chromogenic media: commercial , less flexible, expensive. Useful
in mixed culture where provisional ID only of pathogen may be sufficient. Advantage
relates to ease opf plate reading and rapid provisional ID. Colour differences may be
sublte, Some important organisms can not be differentiated eg klebsiella/enterobacter.
Expensive if further ID still required. Selective media: often in house so flexible and
cheaper, isolates always require additional ID tests. Some straines of bacteria may be
inhibited.