MRSA infections have long been associated with health care settings such as hospitals and nursing homes. These settings, characterized by a sick general population coupled with high antibiotic usage which selects for drug-resistance, are a perfect environment for MRSA strains to gain a foothold. Given that I had my stitches done at an urgent care center I just assumed that’s where I came into contact with MRSA.
In recent years, however, community-acquired MRSA (CA-MRSA) infections have been on the rise. These are infections contracted from settings like schools, childcare centers, gyms, and prisons. Infections caused by CA-MRSA strains are a particular concern because they are more virulent, spread more rapidly, and can cause more severe infections than its healthcare-acquired MRSA (HA-MRSA) counterparts.
What’s worse is the line between the two are blurring as HA-MRSA strains move out into the community and CA-MRSA moves into the hospitals. Because of the increased virulence of CA-MRSA strains there are fears that these strains will eventually replace HA-MRSA strains in healthcare settings–although a recent model published in PLOS Pathogens suggests otherwise.
How MRSA developed β-lactam resistance is still unclear. While there are quite a few different strains of MRSA (some of which have also developed resistance to other classes of drugs) they all carry the mecA gene. The mecA gene, in turn, is part of a larger piece of foreign DNA known as the SCCmec element, which is not normally found in S. aureus.
Since bacteria are quite adept at exchanging DNA with each other, scientists speculate that the SCCmec element found its way into a normal staph strain from an as-of-yet identified trading partner. This process of swapping and transferring DNA is known as horizontal gene transfer.
Interestingly, MRSA has a finely-tuned, “on-demand system” that turns mecA expression on in the presence of β-lactam drugs, while keeping expression turned off in the absence of these drugs. This regulation is carried out by proteins whose genes are also found on the SCCmec element.
In the absence of β-lactams–when the bacteria doesn’t need the drug-resistant PBP2a protein around– the expression of mecA is kept in check by the protein MecI. MecI binds to the DNA promoter region of mecAand prevents gene transcription. However, in the presence of β-lactam drugs the bacteria needs PBP2a around in order to survive.