Classify bacteria according to the Gram stain system and the shape of the bacteria, and give two examples for each classification (40% of marks).

Outline the different mechanisms of bacterial antibiotic resistance and an antibiotic for which that mechanism may apply (60% of marks).

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College Answer

This question also highlighted the importance of candidates noting the way marks were
proportioned. A good answer required the following points –
Classification and examples - gram-positive cocci (Staphylococcus aureus, Streptococcus
pyogenes or pneumoniae or agalactiae), gram-negative cocci (Neisseria meningitidis, N.
gonorrhoeae), gram-positive bacilli (Bacillus anthracis, Listeria, corynebacteria,
Clostridium difficile, etc), gram-negative bacilli (Escherichia Coli or E. Coli, Proteus,
Yersinia, Salmonella, Shigella, Pseudomonas aeruginosa, Klebsiella pneumoniae,
Mechanisms of resistance include: a) Enzyme inactivation, beta-lactamase or Extended
Spectrum Beta-lactamase b) Enzyme addition: enzyme produced by bacteria that add a
chemical group to the antibiotic to inhibit its activity, aminoglycoside resistance by
Staphylococcus aureus or Pseudomonas. c) Impermeability, anaerobes have no oxygendependent
transport mechanism which stops the penetration of aminoglycosides into the
bacteria. d) Efflux mechanisms by acquisition of an inner member protein which actively
pumps antibiotics out of the cell, E.Coli becomes resistance to tetracycline by this
mechanism. e) Alternative pathway to circumvent the metabolic block impose by antibiotic,
Some Staphylococcus aureus are resistant to methicillin by developing or acquiring the gene
mecA which produces an alternative penicillin binding protein and hence they are not
inhibited by methicillin. f) Alteration of the target site, rifampicin resistance by point
mutations, insertions, or deletions in RNA polymerase gene.
Syllabus: M2a, b, c
Reference Text: Microbiology and Infection at a Glance by Gillespie & Bamford 3rd Ed, 2007
page 8-9,21.