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).

Conclusion

Staphylococcus aureus is a particularly challenging pathogen for clinicians seeking to prevent nosocomial infections. Over time, S. aureus has demonstrated a remarkable ability to mutate into various resistant strains, making treatment much like trying to hit a moving target. Furthermore, the incidence of infections caused by S. aureus has been found to be increasing across the country, both in hospital settings and in other public places where skin-to-skin contact occurs. In the final analysis, it is reasonable to conclude that S. aureus is not going anywhere soon, and healthcare practitioners should either begin or continue to apply universal precautions in their day-to-day activities. In addition, those at increased risk of infection by MRSA should take reasonable precautions, such as avoiding skin-to-skin contact when possible and washing hands thoroughly following such contacts when they do occur.

References

Caldwell, J. (2003, April 29). Staph's scary reality: How a little fun on the dance floor — or in the steam room at your gym — can land you in the hospital. The Advocate, 28.

Chikami, G. K., & Murphy, D. (1998). Microbial menace. Forum for Applied Research and Public Policy, 13(4), 11.

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].