Epidemiology of Staphylococcus aureus and MRSA Resistance

Introduction

As the percentage of elderly individuals in the United States continues to increase, it is reasonable to expect more people to become hospitalized for one reason or another in the coming years. One of the more challenging problems associated with hospital stays has been nosocomial infections, which are caused primarily by Staphylococcus aureus. Furthermore, the pathogen appears to be developing specific resistances to the pharmacological treatments developed to date, and S. aureus continues to represent a serious threat for elderly people and those already weakened by other physical conditions. To determine the cause and potential continuing threat represented by this bacterium, this paper provides a review of the epidemiology of Staphylococcus aureus, tracing the development of the current body of knowledge about the disease.

Background and Biology of Staphylococcus aureus

While the world has never been a sterile place, one of the most prevalent and virulent of humanity's microscopic threats has long been Staphylococcus aureus. Staphylococci are ubiquitous, and approximately 30% to 50% of healthy human beings carry staphylococci at any given time (Eidson, Olson & Sewell, 1997). Staphylococcus aureus is a genus of spherical bacteria belonging to the family Micrococcaceae and is universally present in large numbers on the mucous membranes and skin of humans and other warm-blooded animals. The umbrella term "staphylococcus" is commonly used for all species of the family and refers to the cells' tendency to aggregate in grape-like clusters (Staphylococcus, 2005). Staphylococci are microbiologically characterized as gram-positive (in young cultures), non-spore-forming, nonmotile, facultative anaerobes — meaning they do not require oxygen. The cells of S. aureus average 1 micrometer (µm; 1 µm = 10⁻⁶ meter) in diameter and are usually clustered (Staphylococcus, 2005).

The two principal variants of S. aureus that represent the greatest threat to humanity are responsible for wound infections, boils, and other human skin infections, as well as being among the most common causes of food poisoning. In addition, S. aureus causes udder inflammation in domestic animals and breast infections in women. Experts note that S. aureus is a particularly challenging pathogen in hospital settings because of its resistance to antibiotics. By contrast, S. epidermidis is a milder, opportunistic pathogen that primarily affects people with already-lowered resistance (Staphylococcus, 2005).

Etiology and Antibiotic Resistance

Chikami and Murphy (1998) report that the development of antibiotic resistance and the interaction among disease-causing organisms has been well illustrated by the evolution of resistance in Staphylococcus aureus, which is one of the leading causes of hospital-acquired infections such as pneumonia and skin infections. "In the pre-antibiotic era," they note, "staph infections were an important cause of sickness and death. The pathogen was initially susceptible to treatment with penicillin, although resistance to the drug began to develop shortly after penicillin's introduction in the 1940s" (p. 12). Penicillin was highly effective at first against S. aureus and was used extensively with burn victims; because it is an opportunistic pathogen, once the protective skin is damaged by fire, staphylococci can enter the body, circulate, and multiply in the bloodstream, resulting in high fevers, shock, and even death (Levy, 1992). As Levy (1992) observes, "Penicillin was so uniquely active that small amounts of culture fluids could produce this killing effect. And, more importantly, it triumphed over a very common and deadly bacterium, Staphylococcus aureus, which caused skin infections, often leading to blood-borne dangerous septic disease" (p. 37). Over the years, however, S. aureus acquired the ability to produce an enzyme (beta-lactamase) capable of overcoming the active ingredients in penicillin (Chikami & Murphy, 1998).

By the late 1950s, various strains of S. aureus had acquired resistance to penicillin and to virtually all other available systemic antibiotics, including erythromycin, streptomycin, and the tetracyclines. These strains were all associated with outbreaks of nosocomial infections. However, the introduction of semisynthetic penicillins — such as methicillin, which are not inactivated by beta-lactamase — and the cephalosporin antibiotics (e.g., cephalothin, cephaloridine, and cefazolin) during the 1960s represented a significant therapeutic advance in treating infections caused by these antibiotic-resistant pathogens (Chikami & Murphy, 1998).

During the late 1970s and early 1980s, there was also an outbreak of Toxic Shock Syndrome (TSS) caused by S. aureus. The bacterium's toxins were amplified by certain synthetic fibers used in tampons at the time to increase absorbency. As a result of this outbreak, more than 50 women died and more than a thousand suffered serious illness (Perlingieri, 2004).

The organism adapted once again, and methicillin-resistant strains of S. aureus (MRSA) began being identified as early as 1961. At the time, these organisms were associated with infections in Europe, but they have since become widespread around the world. According to Chikami and Murphy (1998), "In the United States, the proportion of S. aureus isolates resistant to methicillin and reported to the National Nosocomial Infections Surveillance system increased from 2% in 1975 to 35% in 1996" (p. 13). Furthermore, new strains of S. aureus have also become resistant to other antibiotics previously relied upon by clinicians (Chikami & Murphy, 1998).

Conclusion

Eidson, M., Olson, R. K., & Sewell, C. M. (1997). Staphylococcal food poisoning from a fundraiser. Journal of Environmental Health, 60(3), 7.

Levy, S. B. (1992). The antibiotic paradox: How miracle drugs are destroying the miracle. Plenum Press.

Perlingieri, I. S. (2004, September–October). The trouble with tampons. E, 15(5), 33.

Staphylococcus. (2005). In Encyclopaedia Britannica [premium service].