The genus Staphylococcus is composed of many species of Gram-positive cocci that are distinguished microscopically by their characteristic organization into clusters of organisms as compared to the chain-like arrangement of the genus Streptococcus. Biochemically, a principle difference between the two genera is the ability of staphylococci to produce the enzyme catalase, which breaks down hydrogen peroxide to water and oxygen.
Staphylococci are broadly divided into Staphylococcus aureus, the one species that has the ability to produce the major virulence factor coagulase, an enzyme that causes plasma to clot and Coagulase-negative staphylococci (CNS) a group of many species that are usually not distinguished from one another by clinical laboratories.
This organism causes more serious disease than any other bacterial pathogen in North America. Until quite recently Staphylococcus aureus was considered the only pathogen in the genus. Modern microbiology and medicine has changed this circumstance as will be discussed below however, S. aureus remains, by far, the most important species of staphylococcus and is an organism that all physicians should be very familiar with.
The human body is the normal ecological habitat of S. aureus. It is not plentiful in the inanimate environment except transiently after contact with people. It can be isolated in culture from the skin of all people intermittently and has a predilection for colonizing the anterior nares. The S. aureus strains isolated from infections are very often the same strains that the infected individual is colonized with i.e. one does not often “catch” S. aureus infections from another person. Rather their “own” S. aureus makes them unwell when there is a reduction in immunologic function. However, there is great variation in virulence of S. aureus strains and person-to-person spread by direct contact can be a problem particularly in hospitals where virulent strains are more prevalent and health care personel carry S. aureus from patient to patient on their hands.
S. aureus is the prototypic pyogenic bacterium. That is, infection tends to induce the formation of pus with localization of disease into an abscess. The common clinical manifestations are skin and soft tissue infections including furuncles, carbuncles, impetigo, cellulitis and infection of traumatic or surgical wounds. The word aureus is derived from the Latin word for gold, a reference to the familiar golden colored pus induced by this organism. Any other part of the body can be infected with S. aureus with corresponding wide variety of clinical findings. Most deep infections of organs or other tissue is a result of hematogenous spread of bacteria from an overt skin infection or an unrecognized or trivial skin infection in the recent past. Osteomyelitis is an example of disease caused by S. aureus in this fashion.
S. aureus can also cause disease by means of elaborating exotoxins. Two rare but well characterized entities are Toxic shock syndrome often, but not always, associated with tampon use and Staphylococcal Scalded Skin Syndrome (SSSS) a condition of babies in which toxin causes widespread sloughing of skin. A common, but infrequently definitively diagnosed, toxin mediated disease is “food poisoning” caused by ingestion of enterotoxin in food contaminated with S. aureus. This is manifest by vomiting occurring very shortly (1 – 6 hours) after ingestion and lasting less than 24 hours.
Resistance to antibiotics is a very serious problem. Most clinical strains of S. aureus produce beta-lactamase, which opens the beta-lactam ring of penicillin and renders it ineffective. Synthetic penicillins such as cloxacillin and methicillin have been the mainstays of therapy for many years but now Methicillin Resistant Staphylococcus aureus (MRSA) strains are increasing in frequency in Canada and becoming a large problem in both the hospital and community settings. MRSA strains are resistant to all Beta-lactam antibiotics due to a change in cell wall construction enzymes referred to as “Penicillin binding proteins” or PBPs.
Prevention of Infection
Hand washing and use of skin antisepsis prior to surgical procedures are effective means of limiting infection. Care of traumatic wounds with physical cleansing and debridement is also important.
Coagulase Negative Staphylococci (CNS)
There are at least 29 different species of CNS many of which are normal flora of human beings. Most often clinical laboratories will report all non – S. aureus species as CNS because it is difficult and not clinically relevant to identify them further. One exception is Staphylococcus saprophyticus – a fairly common cause of urinary tract infections in women of childbearing age, which is differentiated and reported routinely in urine cultures.
Clinically, the most important species of CNS is Staphylococcus epidermidis and you will hear clinicians frequently refer to “Staph epi”. Unless the species is identified in the lab, which is uncommon as mentioned above, it is more correct to use the term CNS.
The reason that this group of organisms has gained in importance is that they are very prominent members of the normal flora of skin and devices made of foreign material that are inserted into people often become contaminated with them. This is particularly problematic if the device is permanent as in the case of prosthetic joints or heart valves. Infections of semi-permanent devices such as central intravenous lines are also common. Resistance to antibiotics among CNS is a serious clinical problem with many hospital strains remaining susceptible to a single commonly available antibiotic – vancomycin that is only available in an intravenous preparation.