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Roger G Finch, Philip Hill & Paul Williams
Recently, a group of bacteria has risen to prominence as the main cause of infection of medical implants - the fight is now on to beat this "sticky intruder"

Staphylococci bacteria are responsible for a number of illnesses such as food poisoning, the symptoms of which (profuse vomiting and diarrhoea) can result in rapid dehydration and occasionally prove fatal to the young and elderly. Toxins produced by certain strains of these bacteria (Staphylococcus aureus) are also the main cause of tampon-associated toxic shock syndrome, which led to a public scare when first described in 1979.¹ In addition, although staphylococci are not implicated as the disease agent in influenza, this illness can predispose to S. aureus, which, as a cause of pneumonia, has one of the highest mortality rates.

These extreme manifestations of human diseases associated with S. aureus contrast with the more mundane skin problems it can cause, such as pustules, boils and carbuncles (see Table 1). Lurking in the shadow of S. aureus, and rarely grabbing the headlines, is a group of bacteria known as coagulase-negative staphylococci (CoNS). Over the past two decades, CoNS have increasingly been implicated in the infection of medical implants, the results of which can be mild, devastating or life-threatening.

Microbiology

Staphylococci are Gram-positive bacteria that occur in clusters. The name is derived from the Greek and means 'bunch of grapes', which aptly describes their microscopic appearance. S. aureus was the first species to be identified about a century ago, and was clearly associated with a range of pus-forming infections such as post-operative wounds and skin sepsis, as well as more serious problems like pneumonia, meningitis, and intra-abdominal and blood infections. By contrast, CoNS were distinguished from their virulent relative by their inability to clot blood. They were considered to be harmless organisms living on the skin. As recently as the early 1970s only two species of CoNS were recognised, namely Staphylococcus epidermidis and Staphylococcus saprophyticus. In the past two decades, more than 30 staphylococcal species have been described, over half of which are pathogenic to man.² The remaining species are more closely associated with domestic and farm animals.

S. aureus is biologically highly active, producing many proteins and exoenzymes that have been clearly linked to its ability to cause pus-forming infections. Some strains make one of a range of toxins, such as toxic shock syndrome toxin, enterotoxins and a dermotoxin (exfoliative toxin). These are linked to specific human diseases like toxic shock syndrome, gastroenteritis and skin shedding conditions (for example, scalded skin syndrome and a similar condition in newborn babies known as pemphigus neonatorum).

By contrast, CoNS are relatively devoid of conventional toxins and, until recently, were largely thought of as laboratory contaminants of clinical specimens. This, in part, reflects their habitat - the skin and mucous membranes - where they are the main constituent of the normal microbial flora. Consequently, they can contaminate clinical specimens, either when they are collected or when they are handled by technical staff during laboratory processing. CoNS have now emerged as the main cause of infections associated with implanted medical devices. These devices include artificial joints, heart valves, spinal fluid shunting systems, dialysis support, and catheters inserted into veins and arteries for monitoring or treatment (see Figure 1). CoNS cause infection through a variety of mechanisms, for example, by forming biofilms which consist of surface attached bacterial cells embedded within an amorphous slimy polysaccharide matrix.

Fig 1 A patient with kidney failure being treated with dialysis

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20 March 1995