“MRSA, MSSA and resistance v virulence”

There are three sound reasons why mortality in sepsisStaph aureus culture due to Methicillin Resistant Staphylococcus aureus (MRSA) is higher than that of Methicillin Susceptible Staphylococcus aureus (MSSA).


  • If the patient is put on initial treatment that just covers MSSA and not MRSA, then effective therapy may be delayed.
  • The antibiotic most commonly used for treating sepsis due to MSSA (flucloxacillin, cloxacillin) is a more effective antibiotic than that usually used for MRSA sepsis (vancomycin)
  • The cohort of patients with MRSA sepsis tends to be a “sicker” cohort (ie more co-morbidities, more prior antibiotics, more hospital admissions etc) than that of patients with sepsis due to MSSA. (This is particularly true for “hospital type” MRSA strains.)

Some people/experts suggest a fourth reason, that MRSA may be a more virulent bacterium per se than MSSA. Some research trials have actually claimed this. However such trials are fraught with difficulty. It is hard enough to attribute mortality to a particular micro-organism. Throw in the confounding variables as described above and you are on a hiding to nothing.

However there are theoretical reasons why MRSA should not be more virulent than MSSA. Generally a bacterium utilises energy in order to become resistant. Sometimes, but not always, this energy is the energy used for making virulence factors within the bacterium.

It doesn’t make any logical or evolutionary sense that MRSA should be both more resistant and more virulent per se than MSSA.

On the contrary it may well be that the converse is true, that MRSA is a slightly less virulent bacterium than MSSA.

Trying to convince the “experts” of this is another matter…..


The link between bacterial resistance and virulence is very interesting. Unfortunately the more you read about it, the more complex it gets. More on this topic in later articles….

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