COAGULASE NEGATIVE STAPHYLOCOCCI

The species of Staphylococci that do not give positive coagulase test are considered Coagulase negative Staphylococci (CoNS). Traditionally only S. aureus is considered coagulase positive, however there are few species other than S. aureus that also give positive coagulase test (e.g, S. lugdensis, S. schleiferi, S. hyicus and S. intermedius). More than twenty species of coagulase negative Staphylococci are recognized.

Species of CoNS: S. epidermidis, S. saprophyticus, S. hemolyticus, S. hominis, S. warneri, S. saccharolyticus, S. cohnii, S. capitis etc.

Habitat: Many coagulase negative staphylococci are part of normal bacterial flora on human and animals. Along with diphtheroides, they form a large part of skin flora. The most common species is S. epidermidis, followed by S. hominis and S. hemolyticus. S. capitis colonizes the scalp, S. epidermidis is found on head and trunk while S. hominis is found on arms and legs.

Whenever they were isolated from the clinical specimens, they were considered as contaminants from the skin or mucosal surfaces. Over the last two decades the roles of S. epidermidis and other coagulase-negative staphylococci have been recognized in causing nosocomial infections. The infection rate has been correlated with the increase in the use of prosthetic and indwelling devices and the growing numbers of immunocompromised patients in hospitals. S. epidermidis is the most common cause of both foreign device infection and nosocomial bacteremia. Since S. epidermidis and other coagulase-negative staphylococci are part of the normal microflora of humans they are frequently dismissed as skin contaminants. Repeated isolation of a predominant strain or a strain in pure culture is convincing as the etiologic agent.

Predisposing factors: Prolonged hospital stay, immunosuppression, use of prosthetic joints, CSF shunts, heart valves, central IV lines and catheters predispose to infection by CoNS.

Pathogenicity: Virtually all medical prostheses placed into normally sterile body sites are at risk from infection. For example, where catheters are used, the CoNS can gain entry through the skin during insertion of the tip. Once the skin barrier has been breached, the first step to infection involves the bacterium adhering to the surface of the implanted medical device. CoNS have the unusual property of being able to stick to and colonize any such implants. S. epidermidis is known to produce slime that aid in formation of biofilm on the foreign body. Prosthetic devices, which are inserted deep within the body, such as heart valves or joints, run the risk of becoming infected during the operation. Spinal fluid shunting system inserted into patients with hydrocephalus can become infected and result in meningitis. Following infection of a prosthetic heart valve, vegetations attached to the valve can break loose and settle in other parts of the body.

Infections caused by CoNS: Bacteremia, meningitis, prosthetic valve endocarditis, post-surgical endophthalmitis infections, postoperative wound infections. S. saprophyticus is responsible for urinary tract infection in sexually active young women.

Laboratory diagnosis: Specimen collection is according to the site of infection, which may include pus, urine, blood, CSF, infected tips of catheters etc. The material may be subjected to Gram staining and culture on Blood agar. They are usually non-pigmented and non-hemolytic.

Identification of CoNS: Species of coagulase negative Staphylococci are identified primarily on basis of negative coagulase test. Further identification to the species level is usually not required, but may be performed on the basis of growth characteristics, sugar fermentation, novobiocin resistance and detection of enzyme activity. S. saprophyticus is novobiocin resistant while S. epidermidis is sensitive. 

Treatment: Many CoNS are increasing becoming resistant to several antibiotics making treatment of nosocomially-acquired infections difficult to treat. Isolates of S. haemolyticus and S. epidermidis with decreased susceptibility to glycopeptides (e.g., Vancomycin) have been reported.