Enterococcus
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CHAPTER 1.2.6 succocore tnE
Enterococcus DONALD J. LEBLANC
Physiology and Genetics—General Introduction Studies of enterococcal physiology have been conducted, for the most part, on members of two species, Enterococcus faecalis and Enterococcus faecium, with the exception of a few reports on Enterococcus hirae, which in many earlier publications was identified as E. faecalis or E. faecium. Enterococcus faecalis, recognized as a distinct species, is distinguishable from the E. faecium group, to which both E. faecium and E. hirae have been assigned on the basis of 16S rRNA sequences. Physiologically, E. faecalis can be distinguished from members of the E. faecium group in that the former can produce acid from glycerol, can ferment pyruvate, can utilize menaquinones as non-cytochrome electron carriers, and does not require exogenous folic acid for growth. Enterococcus hirae is physiologically distinguishable from E. faecium on the basis of the latter’s ability to produce acid from Larabinose (Devriese et al., 1993). Most enterococcal genetic studies have involved strains of E. faecalis, a small number have involved E. faecium, and less than a handful have involved other enterococcal species, mostly antibiotic-resistant clinical isolates. Several plasmids of enterococcal origin have been studied in detail, including two types of conjugative plasmids: those that transfer via solid surface matings only and those that respond to Enterococcusspecific pheromones. Numerous nontransmissible plasmids, many of which are mobilizable by conjugative plasmids, also have been described, and some have been shown to be composites of two or more smaller plasmids. Several transposons, including Tn916 (Franke and Clewell, 1981), the first conjugative transposable element to be described, have been identified in enterococcal isolates, and many have received considerable attention. Until recently, the physiology and genetics of the enterococci have been totally separate fields. However, because of the recent completion of the nucleotide base sequences of the genomes of
strains of two enterococcal species, E. faecalis (see The Institute for Genomic Research website) and E. faecium (see The Department of Energy Joint Genome Institute Web site), it should soon be possible to identify the genetic basis of all enterococcal physiological traits, including their mechanisms of regulation.
Physiology General Physiological Traits The enterococci, unlike the streptococci (once considered members of the same genus), are not confined to carbohydrates for their energy needs. In addition to 15 to greater than 30 different carbohydrates, depending on the species, a variety of other substrates may serve as energy sources, e.g., glycerol, lactic acid, malic acid, citric acid, α-keto acids, and the diamino acids (arginine and agmatine). As with the lactic acid bacteria in general, substrate-level phosphorylation is the primary mechanism of ATP generation. They lack most of the enzymes of the tricarboxylic acid (TCA) cycle, as well as typical electron transport c
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