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!