Tag Archives: MSSA

“MRSA, Mortality and Urban Myths”

Despite several claims to the contrary over the years, the consensus now is that MRSA is, in general, not more virulent or pathogenic than its susceptible counterpart, MSSA.

For a bacterium to both acquire a resistance mechanism and to become more virulent at the same time is in contradiction to evolutionary theory, and would not only be surprising but would be a bit scary as well. Most of the time, acquisition of resistance genes by a bacterium comes with a cost both in terms of fitness and virulence, although this cost is often not measurable in practical terms.

So having said all that, why is mortality from MRSA infection still consistently higher than that of MSSA infection?

There are three main reasons:

  • Delay in getting the correct treatment. If the empirical treatment given to a patient (e.g. a cephalosporin), covers MSSA infection but not MRSA infection, then by the time the MRSA infection is identified, there may have been a significant delay in initiating appropriate treatment. For the frankly septic patient, every hour delay before iniating appropriate treatment can increase the mortality.
  • Treatment not as good. The traditional treatments for invasive MSSA infection, flucloxacillin and cloxacillin, are more efficacious antibiotics than the traditional treatment for MRSA infection, which has been vancomycin. Thus cure rates will be higher for the MSSA group. This is why people are looking hard for, and in many cases now using better alternatives than vancomycin for treating invasive MRSA infection.
  • Sicker cohort of patients. Particularly with regards to the “hospital type” MRSA strains E15,E16 etc., the cohort of patients who are colonised and infected by these bacteria tend to be older, frailer and with more co-morbidities than those with MSSA. Thus the mortality rate will naturally differ between the two cohorts.

As you can see, mortality is influenced by many factors, not just the virulence of the bacterium.

More on mortality and MRSA in the next post, when the story gets even more complicated!


“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….