What Is a MRSA Staph Infection?

The genus Staphylococcus is composed of Gram-positive bacteria that includes species of normal flora (good guy bacteria) and species of pathogens (those able to cause disease). Staphylococcus epidermidis (S. epi), for example, exists naturally on the surface of the skin. It does not typically harm its human host, and is even beneficial, in that it takes up space and resources on the body, making it difficult for invading species of bacteria to become established and cause disease.

Pathogen Staphylococcus aureus

Staph aureus has some special features that make it more virulent, better able to cause disease, than its mild mannered cousin S. epi. The outer coating of S. aureus contains proteins and enzymes that protect it from destruction by the body's immune system. S. aureus also produces toxins that can be harmful to the cells of its host.

MRSA Staphylococcus aureus

MRSA stands for methicillin-resistant Staphylococcus aureus. It is a collection of strains of S. aureus that are invulnerable to a wide range of antibiotics, including, but not limited to, methicillin. 'Strains' are bacteria that exist within the same species, but have certain medically meaningful differences among them. All MRSA are S. aureus, but the variety of strains have unique bacterial "weaponry" that make them even better able to cause infection then the less virulent types of S. aureus.

Staphylococcus, Antibiotics and Resistance

Invented in the 1930s, penicillin was one of the first antibiotics. Penicillin derivatives, cephalosporins, monobactams, and carbapenem, are considered beta-lactam antibiotics, containing a beta-lactam chemical ring in their molecular structure. Beta-lactam antibiotics are mainly effective against Gram+ bacteria.

Over time, as bacteria divide and multiply, genetic mutations arise. Mutations are usually bad for an organism, but one particular bacterial mutation arose for the production of an enzyme, beta-lactamase, that could destroy the beta-lactam ring in these antibiotics. Because this mutation conferred an advantage to the bacteria that had it, the beta-lactamase gene persisted and became more widespread in bacterial populations, providing antibiotic resistance to those bacteria that had the beta-lactamase gene. To illustrate, in 1945 about 90% of staphylococci were susceptible to penicillin, whereas today, only about 5% are.

MRSA Antibiotic Resistance

To battle this problem, a semi-synthetic form of penicillin, called methicillin, was invented in the 1960s. Methicillin is not deactivated by beta-lactamase, so this antibiotic soon became the drug of choice for treating staphylococcal infections. But, eventually, through genetic mutation and overuse of antibiotics, staphylococcal bacteria evolved that were also resistant to methicillin as well as to beta-lactam antibiotics.

Antibiotics that are Effective against MRSA

Currently, most MRSA strains are still vulnerable to the antibiotics vancomycin and teicoplanin (Targocid), but the arms race between bacterial virulence and effective antibiotics is always escalating. Bacteria continue to happen upon mutations that confer resistance to antibiotics currently in use, just as scientists race to formulate new antibiotics that will work against these "superbugs."

Microbiology Information

To learn more about microbiology and infectious disease, see the Virtual Microbiology Classroom or Todar's Online Textbook of Bacteriology. This article was originally published in Suite101 online magazine.

Sources

Bauman, R. (2004) Microbiology. Pearson Benjamin Cummings.

Netdoctor (2007) Methicillin-resistant Staphylococcus aureus (MRSA) infection.