History, Taxonomy, Biochemical Characteristics, and Antibiotic Susceptibility Testing of Enterococci
The enterococci are a complex, diverse, and important group of bacteria in terms of their interaction with humans. This chapter presents a variety of techniques and procedures that can be used for isolation and identification of these important microorganisms. Early documentation of microorganisms that are now included in the genus Enterococcus is mainly related to the “streptococci of fecal origin”. Enterococci are gram-positive cocci that occur singly, in pairs, or as short chains. Most enterococci, apart from Enterococcus cecorum, Enterococcus columbae, Enterococcus pollens, and Enterococcus saccharolyticus, hydrolyze pyrrolidonyl-β- naphthylamide (PYR); all strains produce leucine aminopeptidase. The tables depicting the phenotypic characteristics of the Enterococcus species in the chapter are based on correlations between the whole-cell protein (WCP) profiles and the phenotypic tests and, on some occasions, in conjunction with DNA-DNA reassociation experiments. Techniques for isolation of enterococci from inanimate hospital surfaces have been developed. For several years, controversies and confusion surrounded antimicrobial susceptibility testing of enterococcal isolates, particularly with regard to the reliability of phenotypic methods for detection of high-level resistance (HLR) to aminoglycosides and resistance to vancomycin. Some detailing of the specific standard recommendations for detecting HLR to aminoglycosides, as well as resistance to β-lactams and vancomycin, is presented in the chapter as they are considered very helpful for routine detection of these important resistance markers in enterococci. Molecular methods can rapidly detect specific antimicrobial-drug resistance genes and have substantially contributed to the understanding of the spread and genetics of acquired enterococcal resistance.