Pathogenic enterococci: new developments in the 21st century

Enterococci, traditionally viewed as Gram-positive commensal bacteria inhabiting the alimentary canals of humans and animals, are now acknowledged to be organisms capable of causing life-threatening infections in humans, especially in the nosocomial environment. The existence of enterococci in such a dual role is facilitated, at least in part, by its intrinsic and acquired resistance to virtually all antibiotics currently in use. Beginning with the initial identification of a streptococci of fecal origin in the late 19th century, enterococci have been studied for over a century now. A number of comprehensive reviews during this time have addressed various aspects of enterococci, including classification, biology, virulence, antibiotic resistance and so on. This review specifically addresses the important advances in the field of enterococcal research that have occurred since the beginning of the 21st century. Most notable among these developments have been the insights into enterococcal genomes and pathogenicity.Received 10 April 2003; received after revision 31 May 2003; accepted 3 June 2003     

Modulation of virulence within a pathogenicity island in vancomycin-resistant Enterococcus faecalis

Enterococci are members of the healthy human intestinal flora, but are also leading causes of highly antibiotic-resistant, hospital-acquired infection. We examined the genomes of a strain of Enterococcus faecalis that caused an infectious outbreak in a hospital ward in the mid-1980s (ref. 2), and a strain that was identified as the first vancomycin-resistant isolate in the United States, and found that virulence determinants were clustered on a large pathogenicity island, a genetic element previously unknown in this genus. The pathogenicity island, which varies only subtly between strains, is approximately 150 kilobases in size, has a lower G + C content than the rest of the genome, and is flanked by terminal repeats. Here we show that subtle variations within the structure of the pathogenicity island enable strains harbouring the element to modulate virulence, and that these variations occur at high frequency. Moreover, the enterococcal pathogenicity island, in addition to coding for most known auxiliary traits that enhance virulence of the organism, includes a number of additional, previously unstudied genes that are rare in non-infection-derived isolates, identifying a class of new targets associated with disease which are not essential for the commensal behaviour of the organism.