Recognition of Coagulase‐negative Staphylococcus Aureus Strains by Primary Polymyxin Susceptibility Testing

A prospective study evaluating the usefulness of polymyxin susceptibility testing in unveiling coagulase-negative S. aureus strains was undertaken. During a six months study period, 14 staphylococcal isolates from four patients were initially found to lack coagulase activity and to be polymyxin resistant; in comparison, approximately 1500 ordinary, coagulase-positive, polymyxin resistant S. aureus isolates were found during the same period. One isolate from each of the four patients together with a previously isolated coagulase-negative S. aureus strain from our own collection were further characterized. Two of the strains turned out to show delayed coagulase activity on retesting, while the other three strains failed to produce clotting in any coagulase assay. Judged by thermostable nuclease activity, phage typing and biochemical profiles these three coagulase-negative strains are bona fide S. aureus strains. The polymyxin test thus appears to be useful in disclosing S. aureus variants not identified as S. aureus by routinely performed coagulase assays.