Evaluation of a conventional routine method for identification of clinical isolates of coagulase-negative Staphylococcus and Micrococcus species. Comparison with API-Staph and API-Staph-Ident

Abstract

A collection of 138 consecutive isolates from blood primarily identified as Gram-positive, cluster-forming, coagulase-negative cocci was examined by a conventional routine method for identification of clinical isolates of coagulase-negative Staphylococcus and Micrococcus species. The method was based on selected reactions from the Kloos & Schleifer scheme, utilizing the conventional media of Statens Seruminstitut. Double determinations for each isolate were performed by the conventional method. The results were compared with speciation by the commercial micromethods API-Staph and API-Staph-Ident. For control, 31 Staphylococcus and 13 Micrococcus reference strains were included. Of the 31 Staphylococcus spp. (reference strains), the conventional system, API-Staph, and API-Staph-Ident correctly identified 87%, 87% and 81%, respectively. Micrococcus spp. were only identified to genus level by the conventional method as well as by API-Staph. API-Staph-Ident is not designed for Micrococcus identification. Of 138 blood isolates, 121 belonged to the genus Staphylococcus while 17 were Micrococcus spp. S. epidermidis dominated with all three methods, constituting approx. 35% of the isolates tested. In only 57% of the isolates identification by all three methods agreed. The three methods were unable to put a name on 7.5% (conventional method), 10.7% (API-Staph) and 2.5% (API-Staph-Ident) of the isolates. Reproducibility was high with the conventional method (100% for the reference strains and 91% for blood culture isolates) as well as with API-Staph and API-Staph-Ident (88%/81% and 81%/81%, respectively). We concluded that our conventional system was able to identify most clinically significant staphylococcal species by means of relatively few tests with a high certainty and a high degree of reproducibility.