The staphylococci are one of the most common bacteria associated with human infections. They are found widely in nature, both in the environment and as normal inhabitants of the skin and other body sites of humans and animals. Because of this, staphylococci can cause infection as the result of a breakdown of the skin barrier such as in trauma or an invasive procedure, or they can be transmitted from person to person. All the staphylococci are gram-positive cocci, which break down hydrogen peroxide to water and oxygen, as in the catalase test. There are dozens of species of staphylococci, but the three more important species to humans are Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus saprophyticus.
Staphylococcus Aureus
According to Elmer Koneman, M.D., professor emeritus, University of Colorado School of Medicine, S. aureus is by far the most important human pathogen among the staphylococci. It can be found not only in the external environment but is found normally on the human body in the nose, armpits, skin and the perineum. S. aureus can cause infection in any part of the body and it has a number of virulence factors, or factors that make it pathogenic. It is the cause of a number of skin infections such as boils, furuncles, carbuncles and impetigo. But it is also frequently seen in more serious systemic infections like pneumonia and bone infections. Some strains of S. aureus also produce toxins that can result in diseases ranging from food poisoning to toxic shock syndrome.
Staphylococcus Epidermidis
S. epidermidis is found normally on the skin and outer surfaces of the human body. Most infections caused by S. epidermidis are the result of health care related procedures like surgery wounds, invasive instruments and indwelling medical devices. Though not nearly as pathogenic as S. aureus, S. epidermidis does have the ability to produce biofilms on things like plastic indwelling devices. This is a result of the organism's ability to produce slime that is used for adhering to devices and catheters. The test used to differentiate S. epidermidis from S. aureus is the ability to clot rabbit plasma, better known as coagulase. Staphyloccus aureus has this virulence factor.
Staphylococcus Saprophyticus
Staphyloccus saprophyticus, like S. epidermidis, is a coagulase-negative staphylococci. S. saprophyticus is an important cause of both upper and lower urinary tract infections, particularly in young, sexually active women. According to Koneman, several studies have found this organism to be the second most common cause of urinary tract infection only behind Escherichia coli, representing up to 32 percent of urinary tract infections in female outpatients.
References
- Todar's Online Textbook of Bacteriology: Staphylococcus Aureus and Staphylococcal Disease
- "Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 6th Ed."; Elmer Koneman, M.D.; 2006
