Why are we colonized with Staphylococcus aureus?

A truly successful parasite is commensal, living in amity with its host, or even giving it positive advantages...A parasite that regularly and inevitably kills its host cannot survive long, in the evolutionary sense, unless it multiplies with tremendous rapidity....It is not pro-survival.

Who said that? I will post the answer tomorrow. You can guess by posting a comment or since you're all busy stamping out hospital infections, you can just google it.

I've been thinking a lot about S. aureus recently for some reason. By recently, I mean spring 2009 when I attended a S. aureus conference in St. Augustine and had the chance to speak with Chip Chambers and Heiman Wertheim, among others. The question was and is, why are 30% of us colonized with S. aureus and what possible survival advantage could there be for this colonization status. Sure there are downsides - carriers are more likely to be infected with S. aureus, but are the costs outweighed by the benefits? Does S. aureus colonization prevent colonization and infection with other pathogens? Perhaps even S. aureus colonization prevents the morbidity and mortality associated with S. aureus infection. What? Did I just say that?

A few years ago (2004), before this blog was started so it's fair game, Prof. Wertheim and colleagues published an interesting study in the Lancet. I will just post the findings from the paper's abstract:

Nosocomial S aureus bacteraemia was three times more frequent in S aureus carriers (40/3420, 1.2%) than in non-carriers (41/10588, 0.4%; relative risk 3.0, 95% CI 2.0-4.7). However, in bacteraemic patients, all-cause mortality was significantly higher in non-carriers (19/41, 46%) than in carriers (seven/40, 18%, p=0.005). Additionally, S aureus bacteraemia-related death was significantly higher in non-carriers than in carriers (13/41 [32%] vs three/40 [8%], p=0.006).

Pretty cool. S. aureus carriers were 3 times more likely to have a nosocomial S. aureus bacteremia as non-carriers, but had one-quarter the risk of death from S. aureus bacteremia. So, if you are a S. aureus carrier you have a 3/3420 or 0.000877 chance of death from S. aureus. If you're a non-carrier you have a 13/10588 or 0.001228 chance of death. Yes, I know, sig figs. Amazingly, being colonized, while increasing the risk of infection, drastically cuts the risk of death such that colonized patients are 30% LESS likely to die. Is this enough to explain why 30% of us are colonized? Obviously not, it's just one paper. Their work and other's since has studied factors associated with colonization, but there is more work to do.

I guess, my question is, what if all of the efforts at decolonization actually increased the mortality in the patients we are trying to benefit? Surely, that would be measured in the intervention trials or at least the meta-analyses, or would it? If you don't think of the question, you might not find the answer.

Wertheim 2004 Lancet Paper

Update:  Answer to the question above:  Mr. Spock, Star Trek II.
I found this quote at the beginning of Janice Moore's excellent book "Parasites and the Behavior of Animals"  Thanks to Dave Smith for recommending the book to me many years ago and also for inviting me to the St. Augustine S. aureus conference.