Methicillin-resistant Staphylococcus aureus, more commonly known as MRSA, is a type of bacteria resistant to at least one class of antibiotics. There are many different strains of MRSA, and the bacteria is continuing to evolve and adapt. The effectiveness of antibiotics for MRSA treatment depends on many variables, such as the type of infection, geographic region, and patient’s medical history. Certain drugs remain effective against most strains of MRSA, but laboratory testing has already shown the potential for resistance to these drugs as well.
S. aureus is a form of bacteria that is usually found on human skin. While usually harmless, it can cause infections in open cuts or wounds. These infections had been treated with antibiotic drugs for many years, but in 1961 MRSA, a strain resistant to the commonly used antibiotic methicillin, was identified in Great Britain.
By the mid-1990s, drugs like methicillin, penicillin, and amoxicillin had lost their effectiveness against MRSA, which was infecting an increasing number of people. Most infections were healthcare associated (HA-MRSA), and spread in places like hospitals where the bacteria could prey upon those with weakened immune systems. Community associated MRSA (CA-MRSA) was found in community settings with close contact between individuals, such as day care centers and military boot camp.
Strains of CA-MRSA are often genetically different from strains of HA-MRSA. This means that the effectiveness of antibiotics for MRSA infections can depend on how the infection was acquired. Certain drugs, including clindamycin, rifampin, and trimethoprim/sulfamethoxazole, are much more effective in treating CA-MRSA infections than HA-MRSA infections. CA-MRSA infections also respond better to the fluoroquinolone class of antibiotics, including ciprofloxacin, than HA-MRSA.
Drug resistance among bacteria can also vary regionally. A 2003 study, for example, found that 94% of CA-MRSA samples taken from patients in Chicago had genes responsible for clindamycin resistance, compared to just 8% of samples tested in Houston. This makes it even more difficult to determine the most effective antibiotics for MRSA.
In serious or life-threatening cases, a medicine known as vancomycin is often administered intravenously. Vancomycin, once the only choice in treating strains with multi-drug resistance, remains effective for the majority of infections caused by both CA-MRSA and HA-MRSA. Unfortunately, a few uncommon strains of S. aureus are now resistant to both beta-lactam antibiotics and vancomycin and are difficult to treat.
Recently introduced antibiotics like linezolid are used to treat particularly stubborn infections. Linezolid is mostly used as a last resort because of its high cost and side affects. In addition, overuse of this drug could spur bacterial resistance against one of the most effective antibiotics for MRSA currently on the market.
A patient’s own medical history may also contribute to the effectiveness of treatments for S. aureus infections. About 25 to 30% of people have S. aureus bacteria living on their skin, and these bacteria could become resistant to antibiotics used for other types of infections. Previous hospitalization, surgery, and chronic illness can increase the risk of serious MRSA infections.
MRSA has proven to be extremely adaptable and persistent, and is likely to remain a problem in both healthcare and community settings. In laboratory tests, MRSA has demonstrated resistance to every antibiotic drug currently used to treat S. aureus infections. S. aureus bacteria also seems to have the ability to exchange genetic material with other microorganisms, which could accelerate resistance to drugs. These factors mean that new antibiotics for MRSA outbreaks will almost certainly be needed in the future.
There is evidence, however, that reintroducing certain older antibiotics could be effective. Over time, bacteria may lose its resistance to unused drugs; by cycling drugs in and out of use, healthcare professionals may be able to maintain a more effective arsenal against MRSA.