Staphylococcus aureus Infections
Staphylococcus aureus Infections
Staphylococcus aureus is a bacterium that colonizes, or normally inhabitants the surface of the skin and, in about 25% of humans, the inside of the nose. In a healthy person, the bacterium is usually not a health concern. But, if a person's skin is damaged by a cut or a burn, or if S. aureus gains access to areas inside the body, infection can result.
S. aureus is the most common cause of the so-called staph infections. (Other species of Staphylococcus can also cause infections.) S. aureus can cause a number of life threatening infections, including toxic shock syndrome. While uncommon now, the marketing of super absorbent tampons in the 1970s caused illness and death of a large number of women. The tampon design encouraged the growth of the bacteria, which subsequently produced a poison (toxin) that entered the blood stream.
Other life-threatening infections can occur in susceptible people. These infections are described as being opportunistic infections, since they normally do not occur in healthy people.
S. aureus is a spherical-shaped bacterium. It is a Grampositive bacterium, meaning that it consists of a membrane layer made up mainly of lipids and proteins, and a thick, strong network called peptidoglycan. (Gramnegative bacteria have two membrane layers and a thin peptidoglycan.) The design of the bacterium makes it quite environmentally hardy, which enables it to live in microscopic depressions on the surface of the skin. The bacterium also thrives in the warm and moist atmosphere inside the nose.
S. aureus typically grows and divides to form microscopic clusters that appear grapelike. When grown on a solid food source that contains blood, the visible mounds of bacteria (colonies) that develop tend to be golden in color; aureus means “gold” in Latin. These characteristics aid in the identification of the organism. Other tests of biochemical activity, such the ability of the bacterium to clot blood, also are used in identification.
If the normal barrier of the skin's surface is breached, by a cut or a burn for example, or if a person is immunocompromised (their immune system is not functioning properly) and so is less capable of fighting off invading microorganisms, S. aureus can rapidly cause infections. These range from skin infections that are relatively minor, such as boils and pimples, to life-threatening infections of the skin in infants (scalded skin syndrome), of the lungs (pneumonia), of the lining of certain nerves (meningitis), of theheartvalves(endocarditis), and, in the case of toxic shock syndrome, of the blood (septicemia). Heart-related infections are associated with the implantation of devices designed to assist proper heart function. The devices can become contaminated before being implanted, if they are handled by bare hands that have not been washed properly. This transfers S. aureus from the skin to the plastic surface of the device, where the bacteria can adhere and grow.
WORDS TO KNOW
ANTIBIOTIC RESISTANCE: The ability of bacteria to resist the actions of antibiotic drugs.
COLONIZE: Colonize refers to the process where a microorganism is able to persist and grow at a given location.
IMMUNOCOMPROMISED: A reduction of the ability of the immune system to recognize and respond to the presence of foreign material.
OPPORTUNISTIC INFECTION: An opportunistic infection is so named because it occurs in people whose immune systems are diminished or are not functioning normally; such infections are opportunistic insofar as the infectious agents take advantage of their hosts’ compromised immune systems and invade to cause disease.
RESISTANT ORGANISM: Resistant organisms are bacteria, viruses, parasites, or other diseasecausing agents that have stopped responding to drugs that once killed them.
The association of S. aureus and infections has been known since 1880, when the bacterium was isolated from wounds. The environmental hardiness of the bacterium is one important factor in its ability to cause infection. If present on a moist surface, such as a towel, the bacteria can remain alive and capable of causing infection for hours. Even more importantly, skin-to-skin contact can easily spread S. aureus from one person to another. The ability of the bacterium to invade host tissue is one cause of infection. Toxins can also be produced when S. aureus gets into a wound or other niche away from the skin's surface, or when the bacterium contaminates food.
S. aureus infections occur virtually anywhere in the world and are very common. For example, even in a developed country like the United States with high-quality medical care, more than 500,000 people are hospitalized with S. aureus infections every year. The bacterium is one of the important causes of what are termed nosocomial (hospital-acquired) infections.
The bacterium is also a concern in agriculture, since it is the cause of a disease in cattle called mastitis.
Despite the antibiotic resistance of some types of S. aureus, infections still usually respond to treatment with antibiotics. Completing the full course of antibiotic treatment is very important. Some patients may stop taking antibiotics before the course of treatment is completed because they begin to feel better. This is very unwise, since the infection may not yet be eliminated. Surviving S. aureus can cause the illness to recur, and these survivors may even become resistant to the anti-biotic(s) being used in the treatment.
The impacts of S. aureus infections are enormous, both in terms of the number of serious illnesses and deaths caused, and also in terms of the financial cost of caring for these patients. In a nationwide analysis of hospitalized patients in 2001, patients with S. aureus were found to average three times the length of hospital stay, three times the total charges, and three times the risk of death while in the hospital than hospitalized patients without S. aureus infection.
Furthermore, the development of antibiotic resistance by the bacterium (strains that are resistant to almost all antibiotics exist) is ominous. At the time of the commercial introduction of the first antibiotic, penicillin, in 1943, antibiotic resistance among S. aureus isolated from infections was unknown. Only seven years later, approximately 40% of all isolates were resistant to penicillin. By 1960, 80% of hospital isolates of S. aureus were penicillin-resistant.
In addition, resistance to a variety of other antibiotics has developed. As of 2007, disease outbreaks due to S. aureus that are almost completely resistant to anti-biotics are becoming much more common. The challenge now is to quickly discover new antibiotics and/or methods of infection control that protect hospitalized patients, who are the most susceptible to grave illness and death from these infections.
Freeman-Cook, Lisa, and Kevin D. Freeman-Cook. Staphylococcus aureus Infections. London: Chelsea House, 2005.
Prescott, Lansing M., John P. Harley, and Donald A. Klein. Microbiology. New York: McGraw Hill, 2004.
Tortora, Gerard J., Berell R. Funke, and Christine L. Case. Microbiology: An Introduction. New York: Benjamin Cummings, 2006.