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Chapter 18 : General Approaches to Identification of Aerobic Gram-Positive Cocci
Majority of aerobic, or facultatively aerobic, gram-positive cocci isolated from clinical specimens are distributed among the genera Staphylococcus, Streptococcus, and Enterococcus. This chapter provides tables containing organisms with similar cellular morphologies, either "streptococcal," consisting of gram-positive cocci or coccobacilli arranged primarily in pairs and/or chains, or "staphylococcal," signifying that cells appear as cocci arranged in pairs, tetrads, clusters, and irregular groups. The commonly isolated aerobic gram-positive cocci (staphylococci, streptococci, enterococci) can usually be accurately identified by determining a few basic phenotypic traits (cellular morphology, catalase reaction, and production of pyrrolidonyl arylamidase [PYR]). The chapter highlights the fact that it is increasingly difficult to identify some of the less frequently isolated organisms solely on the basis of phenotypic traits, as new genera and species of aerobic gram-positive cocci are described and characterized. Basic phenotypic tests can usually suggest a possible identity for strains of infrequently encountered aerobic gram-positive cocci, but evaluation with a larger battery of phenotypic tests or molecular identification methods is often valuable, if not indispensible, for accurate identification. Nucleic acid probe tests and amplification methods for identification of some of the commonly isolated aerobic gram-positive cocci are commercially available and designed for use in medium to large-volume clinical microbiology laboratories. The chapter concludes by emphasizing that the comparison of 16S rRNA gene sequences is the most useful method for molecular characterization of the aerobic gram-positive cocci of clinical interest, although sequence comparison of other genes may also be helpful for identification.