Staphylococcus aureus is a pretty versatile bug, meaning it dedicates a lot of its time and resources to developing ways to live happily on all kinds of tissue surfaces. As you’ve probably heard, we all have bacteria living all over on us and in us at all times. From the time you eat your first meal, you’ve got them throughout your digestive tract, and even earlier than that every inch of your skin is covered with bacterial cells. Fun fact – you have far, far more bacterial cells associated with you than you actually have of your own cells (by about 10 to 1)!
Bacteria generally live on the skin by attaching to receptors on the epithelial cell layer at the outermost skin surface. MRSA/S. aureus is particularly good at competing for these limited binding spaces. Scientists believe that your genetic make-up also has something to do with how well this bacteria likes you, which is why about 30-40% of people are permanently colonized with S. aureus, while the rest are either just transient or non-carriers. By the way, being a non-carrier doesn’t mean that you’re free of bacteria…it just means that your skin is better suited to some other type of bacteria and you’ve got that living on you instead.
Now, to further complicate the story, research studies are showing that even amongst permanent carriers of MRSA/S. aureus there can be big differences in colonization characteristics. While the inside of the nose is commonly accepted to be a major colonization site for MRSA, nasal carriers may or may not have the bacteria at other body sites including the throat, groin, armpits, and perineum. Finding MRSA in any combination of these places does not guarantee that it will be present at other sites if tested, so there really is no good way to predict how much someone is colonized without taking samples from many different areas. And why do we care about the extent of MRSA colonization? The answer, as I addressed in a previous entry, is that there’s a strong correlation between colonization and the development of subsequent infections.
A new study, published online ahead of print this month in the Journal of Clinical Microbiology, takes perhaps the closest look to date at sites of MRSA colonization on the human body.1 This work, funded by 3M Healthcare, looked at MRSA carriage in 60 patients admitted to Rhode Island Hospital over a 1-year period. Skin swab samples were taken from the nose, groin, perineum, and axilla (armpit). Because all the patients in the group evaluated had been previously identified as MRSA carriers at one time or another, 88% of them were confirmed to be positive in at least one site during this study. In this subset of carriers, it was found that an overwhelming majority (91%) were positive for MRSA in nasal samples. Furthermore, almost 25% had MRSA in the nose but not at any other body site sampled – making the nose the highest of any single anatomical colonization site. Finally, positive samples from the nose were found to contain higher numbers of MRSA cells as compared to positive samples from the other body sites. These findings led the investigators to conclude that the nose has the highest sensitivity of any single site for determining MRSA colonization, and that the greater the numbers of MRSA isolated from the nose, the higher the chance of finding colonization at other body sites as well.
While screening patients for MRSA colonization is showing promise as far as reducing hospital-acquired infections, the major barriers to adoption are cost and extra workload associated with putting a screening program in place. Taking a single sample from each patient is burden enough, without even thinking about multiple samples from different body sites. Understanding that the nose is the single greatest “hotzone” for MRSA is important in allowing infection control doctors to develop screening protocols that create the lowest drain on resources. The other alternative would be to simply decolonize all patients entering the hospital, understanding that up to 40% of them are carrying some strain of S. aureus anyway that increases their risk of infection. This approach would require a decolonization treatment that is fast-acting, low cost, and doesn’t lead to the creation of resistant bacterial populations like current antibiotics. More on that concept in an upcoming post…
1 Mermel LA et al. (2011) Methicillin-Resistant Staphylococcus aureus (MRSA) colonization at different body sites: A prospective, quantitative analysis. Journal of Clinical Microbiology, published online ahead of print on 5 January 2011.