The Physiology and Metabolism of Enterococci

Excerpt

When possible, the authors have provided open reading frame (ORF) numbers (EF####) from the extensively annotated E. faecalis V583 genome sequence. This information can be easily accessed at enterocyc.broadinstitute.org. If applicable, non-V583 enterococcal sequences with homology to the annotated ORF have been supplied. Additionally, we have included IUBMB nomenclature for most reactions, which are available at http://www.iubmb.org.

Enterococci have been isolated and characterized for more than 113 years (MacCallum & Hastings, 1899). During the past century, the classification of this genus has been refined, with the most significant change occurring in 1984 when most members of the Group D streptococci, including Streptococcus faecalis and Streptococcus faecium, were included in the new genus Enterococcus (Schleifer & Kilpper-Bälz, 1984). This genus currently consists of 37 species that occupy a broad range of habitats that include the gastrointestinal microbiota of nearly every animal phylum (See Enterococcus Diversity, Origins in Nature, and Gut Colonization for details). An ability to widely colonize is due, at least in part, to their metabolic versatility and intrinsic resistance to inhospitable conditions. Despite being unable to form spores, enterococci are highly tolerant to desiccation and can persist for months on dried surfaces. Enterococci also tolerate extremes of pH, ionizing radiation, osmotic and oxidative stresses, high heavy metal concentrations, and antibiotics. Enterococci survive or grow over a wide range of temperatures for mesophilic bacteria, from 10 to 45°C. These bacteria, as highly evolved commensals, have been extensively used in the food industry and as probiotics to prevent or ameliorate disease. Finally, rogue strains of enterococci have emerged on the worldwide stage as multidrug-resistant and hospital-acquired pathogens.

Enterococci are often simply described as lactic-acid–producing bacteria—a designation that understates their vast metabolic potential. The ubiquitous nature of enterococci in our environment implies this potential. Investigations into the remarkable physiology of these bacteria have fluctuated over the past century. Prior to publication of The Enterococci (Huycke M. M., 2002), and now with this volume, the last formal comprehensive review of enterococcal metabolism was written in 1964 (Deibel, 1964). At that time, substantial efforts by Gunsalus, Sokatch, Gale, Niven, and Deibel, among others, focused on the central metabolism of enterococci. Since then, research into enterococcal physiology has increasingly used the tools of molecular biology and has shifted toward understanding antibiotic resistance, pathogenesis, and genomics. With this new information, there has been increasing recognition that many metabolic genes and pathways vary, even within single species, and led investigators to question the concept of a uniform core metabolism for enterococci. To address this perspective and update the available information on enterococcal physiology, this chapter compiles and reviews the most recent findings from laboratories around the world, and integrates those results with the older literature. As will be evident, the rapid growth of genomic databases continues to offer valuable insights into the physiology of enterococci and greatly facilitates experimental designs into their metabolism.