Compare and contrast the spectrum of activity and the mechanisms of microbial resistance for the following penicillins; benzyl penicillin, flucloxacillin and ampicillin.

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College Answer

It was expected candidates would specifically address both the spectrum of activity and mechanism of resistance.

Benzyl penicillin is highly active against Gram positive organisms, particularly streptococci but also effective against Meningococcus / Clostridia and other anaerobes, Listeria and is used as treatment for syphilis ( treponemma). It is readily hydrolysed by penicillinases or beta lactamases so any organisms that produce these are resistant ie. most staphylococci.

Flucloxacillin contains a modified beta lactam ring so is not susceptible to hydrolysis by staphylococcal penicillinases, therefore the spectrum is Staphylococci not resistant to methicillin (ie. not MRSA). Extra credit was given for comments that it won't cover Listeria or some other organisms covered by Benzyl penicillin and it is much less active than Benzyl penicillin on organisms sensitive to both.

Ampicillin is an alpha amino benzyl penicillin (aminopenicllin) and has a broader activity than Benzyl penicillin so covers the streptococci but also a variety of gram negative bacteria including some enterobacteriace and Haemophilus influenzae. It also covers Helicobacter and Enterococci (better than Benzylpenicillin). It is destroyed by beta- lactamase.

Additional credit was given for discussion of other mechanisms of bacterial drug resistance.