Tag Archives: mrsa

“Getting to know Mrs A…”

Antimicrobial Resistance, The Science of Microbiology, ,
"Sensationalist MRSA Headline that we are all too familiar with."
“Sensationalist MRSA Headline that we are all too familiar with.”

I am going to do a few short posts on MRSA over the next couple of weeks, explaining my thoughts on this bacterium and also exploding a few myths surrounding this so-called “superbug”.

So what is MRSA?  (Methicillin Resistant Staphylococcus Aureus)

Or to be more precise, what is methicillin?

Methicillin was first produced commercially in 1959 by Beecham pharmaceuticals. Its main advantage over its predecessors was its activity against penicillinase producing strains of Staphylococcus aureus.

Methicillin is no longer used these days to treat patients. Compared to other β-lactamase-resistant penicillins, it is less active, can only be administered parenterally,  and has a higher frequency of interstitial nephritis, an otherwise rare side effect of penicillins.

I think they stopped producing methicillin commercially around the mid 90s, I am not sure exactly when, and it has since been superseded by flucloxacillin and dicloxacillin.

The name “Methicillin” will not mean much to younger scientists and doctors. Maybe a better name these days for MRSA would be BRSA, reflecting its resistance to beta-lactam antibiotics…..

Michael

“Adding meaning to methicillin”

History of Microbiology,

 

You will all be familiar with the acronym MRSA (Methicillin Resistant Staphylococcus aureus), but you probably won’t be so familar with the methicillin part, unless you are even older than me.

Methicillin was the first semi-synthetic beta-lactam and was manufactured by Beecham and released commercially in 1959. This antibiotic was a marked improvement on the original penicillins such as Penicillin G (benzylpenicillin). Scientists had discovered that by making the side chain of the beta-lactam ring bulkier, the “steric hindrance” produced made the antibiotic more stable to penicillinases. At that time at least 50% already of all Staph aureus isolates produced penicillinase and were thus resistant to straightforward penicillin.

However a major problem with methicillin is that it is poorly absorbed when given orally and it is broken down rapidly by acid in the stomach. In 1961, Beecham released newer penicillinase stable beta-lactams called oxacillin and cloxacillin (flucloxacillin was first used commercially in the early seventies). These newer antibiotics were much more stable when given orally.

And as with all antibiotics, it did not take long before resistance to methicillin in the human population became prevalent. The first Staphylococcus aureus isolate resistant to methicillin (MRSA) was described in 1961 in the UK.

Clinical use of methicillin tapered off during the 1980s and as far as I am aware the antibiotic is no longer available commercially, but the name Methicillin Reistant Staphylococcus aureus continues to this day.

Maybe we should change the name of MRSA to BRSA… The mechanism of resistance in MRSA (mecA gene encoding for altered PBP2a) confers resistance to not just methicillin, but all beta-lactams. For me, the acronym BRSA makes it clear what antibiotics can be excluded for starters in treatment of an MRSA infection.

Michael

p.s. The taxonomical establishment have since changed the name of methicillin to meticillin, for reasons which are best known to themselves….

“Woolly Mammoths or Unknown Soldiers?”

The Science of Microbiology,

Woolly_mammoth

Do MRSA decolonisation regimes rid an MRSA carrier completely of the bacteria?

A person who is colonised with methicillin resistant Staphylococcus aureus bacteria (MRSA), at a rough guess, are likely to harbour between 10,000,000 and 10,000,000,000 MRSA bacteria in their bodies.

Should we believe then that a successful decolonisation regime rids the person completely of MRSA?, or just below the detection limits of the laboratory assay.

There are thus two possible theories:

Woolly mammoth: When unfavourable environmental conditions prevail, the decline in numbers in a population reaches a critical point where the decrease is irreversible and the numbers present in the population/host decline to zero.

Unknown soldier: An army regiment loses a battle. However the surviving soldiers hide away undectected in the population, ready to regroup and fight again when conditions are more favourable.

I have no idea what the answer is, but would be interested in any suggestions. My gut instinct is that MRSA decolonisation falls into the latter (soldier) theory, and that after “successful” decolonisation, a small number of MRSA bacteria lie undetected in the host (?intra-cellular) and undetectable, waiting to make a re-appearance when conditions become more favourable to their proliferation.

I am only making an educated guess here however….

 

Michael