Or maybe terrifying is a better word. I just returned from the Network on Antimicrobial Resistance in Staphylococcus aureus meeting, where I learned some very interesting things about S. aureus (since I’m going to refer to MRSA, methicillin resistant S. aureus repeatedly, go check this link if you want to know more about MRSA):
1) 43% of all skin infections in the U.S. are the result of one strain of MRSA. Not 43% of staphylococcal infections. All skin infections.
2) According to the NHANES study, the number of people who carry S. aureus asymptomatically (in other words, it lives up your nose and isn’t making you sick) dropped from 34% in 2001-2002 to 28% in 2003-2004. That’s the good news…
3)…the bad news is that the percentage of people who carry MRSA asymptomatically increased from 0.8% to 1.5% (I’ve seen the experimental design; these changes aren’t statistical wobble but real changes based on good sampling and large population sizes).
4) The MRSA and MSSA clones (a ‘clone’ is a group of genetically related strains) that show up in clinical settings and cause disease are actually quite rare in the commensal S. aureus populations.
5) Over the last several years, there has been a huge shift in MRSA. Most MRSA used to be hospital-acquired (the infection began after 48 hours had be spent in the hospital; ‘HA-MRSA’). Now, roughly 75% of MRSA are either community-acquired (the patient enters the hospital infected and onset occurs before 24 hours has been spent in the hospital; ‘CA-MRSA’) or hospital-associated community onset MRSA (onset occurs between 24-48 hours of hospital entry; ‘HACO-MRSA’). What interesting is that these HACO-MRSA, while ‘community acquired’, are associated with people who spend lots of time in medical facilities or have other risk factors (e.g., surgery within the last month). I suspect that the HACO-MRSA strains are adapted to high antibiotic environments (e.g., frequent hospital stays, nursing homes, etc.), even though they often resemble CA-MRSA in other ways.
6) Nobody has any idea whether PVL toxins increase MRSA virulence, decrease MRSA virulence, or have no effect at all (three researchers, three different conclusions). I think all we can say is that we are far better at killing mice than humans with MRSA. Of course, if you were exposed to a quart of dense MRSA culture, you would probably be dead too….