When is MRSA not MRSA?

…when it contains a weird gene conferring methicillin resistance that many tests miss.

Methicillin-resistant Staphylococcus aureus (MRSA) has become a big issue in the past 15 years or so, as it turned up outside of its old haunts (typically hospitals and other medical facilities) and started causing infections–sometimes very serious–in people who haven’t been in a hospital before. Typically MRSA is diagnosed using basic old-school microbiology techniques: growing the bacteria on an agar plate, and then testing to see what antibiotics it’s resistant to. This can be done in a number of ways–sometimes you can put a little paper disc containing antibiotics right onto a plate where you’ve already spread out a bacterial solution and see which discs inhibit growth, or sometimes you can grow the bacteria in a plate with increasing concentrations of antibiotics, to see when the drugs are high enough to stop growth. Both look at the phenotype of these bacteria–the proteins they’re expressing which lead to the bacteria’s drug resistance.

However, these culture-based methods are slow–they can take days between when the patient first is seen by a doctor and the time the results come back from the clinical lab. For this reason, increasingly labs are moving to molecular methods, which are much quicker than the culture-based methods. Indeed, detection of the gene responsible for methicillin resistance, mecA, has been the gold standard for *really* identifying MRSA, even beyond phenotypic methods.

A new pair of papers demonstrate the limitations of this reliance. Like many science discoveries, this one started with a “huh, weird” moment. Investigators noticed that a number of their S. aureus samples were categorized as MRSA using the traditional phenotypic methods, but were negative when it came to the mecA DNA test. Genetic analysis showed that these isolates carried a different mecA gene, dubbed mecALGA251. The investigators searched their isolate collection in England, and also worked with collaborators in Scotland and Denmark to search through their banks for additional mecA-negative MRSA, and found almost 70 isolates, including one dating back to 1975. (A second paper by a different group examined two isolates in Ireland).

Now is when it starts to get really interesting. (Continued below)
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