The Silly Distinction Between CA-MRSA and HA-MRSA

Over at Shakespeare’s Sister, I came across this link to an announcement of a new Chlorox disinfectant that is “a one-step, labor-saving solution for killing both strains of MRSA.”
There seems to be some confusion here.

Let’s start with the basics. MRSA is the bacterium methicillin resistant Staphylococcus aureus. Traditionally, MRSA has been classified into two kinds: CA-MRSA, community-acquired MRSA, and HA-MRSA, hospital-acquired MRSA*. A CA-MRSA infection is defined as an infection that occurs in a patient who has been in the hospital for less than 48 hours; in other words, they probably carried the infection in with them. An HA-MRSA infection is one that occurs after 48 hours, and, so, is thought to have been contracted while in the hospital.
When we first began to recognize MRSA as a health problem, CA-MRSA and HA-MRSA were very easy to tell apart: CA-MRSA had one type of methicillin resistance gene cassette and produced a particular set of toxins, while HA-MRSA had a different methicillin resistance gene casette and produced a different set of toxins. Why could we distinguish them?
In The Beginning, there were very few CA-MRSA and HA-MRSA strains**, and that combined with epidemic spread of strains (i.e., these strains weren’t found at equal frequencies), meant that these two groups were very distinct. Over time (the last 10-15 years), more strains have acquired methicillin resistance gene cassettes (as well as gained or lost toxin genes***), and over time, the distinctions have been blurred. Another reason is that HA-MRSA strains are able to adapt and spread into the community (and vice versa).
So there is a difference in a statistical sense, but these are no longer clearly demarcated groups that can be distinguished by a couple of characteristics. It’s silly to boldly proclaim that your novel disinfectant kills both CA-MRSA and HA-MRSA: they’re just not that different. I’ll go out on a limb and guess that these strains can also kill non-human associated MRSA (which can wind up sending people to the hospital, and thus are also CA-MRSA, whereafter, if they manage to survive in the hospital, they can become HA-MRSA, even though it’s the same damn strain).
I’ll go farther out on a limb and speculate that quaternary ammonium compounds used in the disinfectant also kill MSSA–methicillin sensitive S. aureus too. Again, it’s silly. Killing ‘staph’ infections would be a more accurate claim. Of course, The Dread MRSA is hot right now, so there you go.
And if you’re wondering, yes, bacteria can evolve resistant to quaternary ammonium compounds too…
*Recently, a third category, HACO-MRSA, healthcare-associated community-onset MRSA, has come into usage. These are isolates found in healthcare-like settings outside the hospital, such as nursing homes. Last I checked, HACO-MRSA seem to be more similar, on average, to CA-MRSA, but new data may suggest otherwise.
**Obviously, at some point, there was only one strain. Unless you think God has an inordinate fondness for MRSA. (to steal a phrase).
***In S. aureus, many toxins are carried by bacteriophage (bacterial viruses). Not only can these viruses ‘infect’ non-toxin producing hosts, allowing them to produce toxins, but these viruses can also be lost in S. aureus lineages, resulting in the loss of toxin production.

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6 Responses to The Silly Distinction Between CA-MRSA and HA-MRSA

  1. PalMD says:

    There are usually some susceptibility differences in CA and HA MRSAs (at least around here), but it is annoying to see some of the silliness.
    It reminds me of the ads for a certain allergy pill that “is the only medicine approved for indoor AND outdoor allergens”, as if there is some sort of difference.

  2. thebaltrus says:

    Wait right there Mike…it sounds like you are using the evolution to describe something medically relevant. I just also want to point out an important distinction between MRSA strains and “regular” staph in that there were no toxin genes or MRSA strains before Eve ate from the tree of knowledge.

  3. microbiologychick says:

    I completely understand your point, especially about the chlorox, but I will tell you this: there is a significant difference in siderophore production between CA-MRSA and HA-MRSA, at least with the strains I’m working with.

  4. Hi Mike,
    Just some more clarification about nomenclature… at least what is it ’round these parts.
    The ABCs network at CDC classifies MRSA in 3 catagories:
    1) Hospital-onset (HO) if MRSA infection was identified more than 48 hours after admission
    2) Healthcare-associated community-onset (HACO) if MRSA infection was identified more than 48 hours after admission and one or more of the following: a) previous positive MRSA culture; b) a history of hospitalization, surgery, dialysis or residence in a long term health facilty in the previous year; or c) the presence of an indwelling catheter or current percutaneous medical device.
    3) If non of the above mentioned criteria are met for an MRSA infection, the case is classified as community-associated (CA).
    Of course, these classifications will likely shift a bit more in time…

  5. mirc says:


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