Staphylococci are gram-positive bacteria which normally inhabit and colonize the skin and mucus membranes of humans. If the skin or mucus membrane becomes damaged during surgery or other trauma, the staphylococci may gain access to internal tissues causing infection to develop. If the staphylococci proliferate locally or enter the lymphatic or blood system, serious infectious complications such as those associated with staphylococcal bacteremia may result. Complications associated with staphylococcal bacteremia include septic shock, endocarditis, arthritis, osteomyelitis, pneumonia, and abscesses in various organs.
Staphylococci include both coagulase positive organisms that produce a free coagulase and coagulase negative organisms that do not produce this free coagulase. Staphylococcus aureus is the most common coagulase-positive form of staphylococci. S. aureus generally causes infection at a local site, either extravascular or intravascular, which ultimately may result in bacteremia. S. aureus is also a leading cause of acute osteomyelitis and causes a small number of staphylococcal pneumonia infections. Additionally, S. aureus is responsible for approximately 1–9% of the cases of bacterial meningitis and 10–15% of brain abscesses. There are at least twenty-one known species of coagulase-negative staphylococci, including S. epidermidis, S. saprophyticus, S. hominis, S. warneri, S. haemolyticus, S. saprophiticus, S. cohnii, S. xylosus, S. simulans, and S. capitis. S. epidermidis is the most frequent infection-causing agent associated with intravenous access devices and the most frequent isolate in primary nosocomial bacteremias. S. epidermidis is also associated with prosthetic valve endocarditis.
Staphylococcus is also a common source of bacterial infection in animals. For instance, staphylococcal mastitis is a common problem in ruminants including cattle, sheep, and goats. The disease is generally treated with antibiotics to reduce the infection but the treatment is a costly procedure and still results in a loss of milk production. The most effective vaccines identified to date are live, intact S. aureus vaccines administered subcutaneously. The administration of live vaccines, however, is associated with the risk of infection. For that reason, many researchers have attempted to produce killed S. aureus vaccines and/or to isolate capsular polysaccharides or cell wall components which will induce immunity to S. aureus. None of these attempts, however, has been successful.
S. aureus includes a cell wall component composed of a peptidoglycan complex which enables the organism to survive under unfavorable osmotic conditions and also includes a unique teichoic acid linked to the peptidoglycan. External to the cell wall a thin polysaccharide capsule coats most isolates of S. aureus. This serologically distinct capsule can be used to serotype various isolates of S. aureus. Many of the clinically significant isolates have been shown to include two capsular types, CP5 and CP8.
Type CP5 has the following chemical structure:→4)-β-D-ManpNAcA3Ac-(1→4)-α-L-FucpNAc-(1→3)-β-D-FucpNAc-(1→
Type CP8 has the following chemical structure:→3)-β-D-ManpNAcA4Ac-(1→3)-α-L-FucpNAc-(1→3)-β-D-FucpNAc-(1→
Studies on over 1600 S. aureus isolates showed that 93% were of either type 5 or type 8 capsular polysaccharides. Additionally, more than 80% of the S. aureus isolated from sheep, goats, and cows with mastitis and chickens with osteomyelitis have been demonstrated to include at least one of these two capsular types. Although CP5 and CP8 are structurally similar, they have been demonstrated to be immunologically distinct.