Enterococcal Bacteriocins and Antimicrobial Proteins that Contribute to Niche Control

Excerpt

Enterococci, which belong to the group of lactic acid bacteria (LAB), have received increased attention in recent years for various reasons (Fisher & Phillips, 2009; Franz, Huch, Abriouel, Holzapfel, & Gálvez, 2011; Leavis, Bonten, & Willems, 2006). While lactobacilli, another group of LAB, have been shown to confer numerous benefits and are often regarded as health-bringing organisms, enterococci have become more recognized as emerging human pathogens despite the fact that they are as numerous as the lactobacilli in our gastrointestinal tractgastrointestinal tract. Enterococcus faecalis is the dominant Enterocuccus in the gastrointestinal tract, followed by E. faecium; however, E. avium and E. hirae, as well as other enterococcal species, are frequently found in human stool samples. The commensal/probiotic role of enterococci in humans and animals has evolved through thousands of years in mutual coexistence—but the ability of the enterococci to behave in a way that causes problems to human health is only beginning to be understood. Virulence, which may evolved as an adaptation to the “modern lifestyle,” including the profligate use of antibiotics in medical practice and animal husbandry, needs to be understood and limited where possible. On the other side, enterococci have many positive traits that have been appreciated in food fermentation and preservation, and may also serve as probiotics to promote health.

Bacteriocin-producing bacteria are found in all environments. In many LAB isolates, bacteriocin production has been examined by biochemical and genetic studies, and the bacteriocins produced by enterococci are often similar to those produced by other lactic acid bacteria. A classification scheme has been developed for bacteriocins produced by Gram-positive bacteria, and most of this information is based on findings from LAB. Although classification is still a disputed issue, two major classes of heat-stable, ribosomally synthesized antimicrobial peptides are well defined. Class I constitutes the lantibiotics, while Class II constitutes the unmodified non-lantibiotics.

Bacteriocins within different classes and subclasses also have been isolated and characterized in enterococci (Cotter, Hill, & Ross, 2005; Nes, Diep, Håvarstein, Brurberg, Eijsink, & Holo, 1996; Nes, Yoon, & Diep, 2007). One of the most striking findings so far is the almost complete absence of lantibiotics among enteroccci, with the only exceptions being cytolysin and enterocin W (Coburn & Gilmore, 2003; Cox, Coburn, & Gilmore, 2005; Sawa, et al., 2012). Most of the characterized enterocins belong to the Class II bacteriocins and a few are heat-labile lytic enzymes. The latter were previously classified as bacteriocins, but are now included in a distinct class of antimicrobials (Cotter, Hill, & Ross, 2005).

The hemolytic bacteriocin (cytolysin), the circular AS-48, and bacteriocin 21 have been known as E. faecalis bacteriocins for a long time, and they have been genetically and biochemically well characterized (Clewell, 1981; Gilmore, 1991; Gilmore, Coburn, Nallapareddy, & Murray, 2002; Gilmore, Segarra, Booth, Bogie, Hall, & Clewell, 1994; Haas, Shepard, & Gilmore, 2002; Ike, Clewell, Segarra, & Gilmore, 1990). Many bacteriocin producers have been identified and investigated from infection-derived E. faecalis or E. faecium isolates. From E. faecalis isolates, for example, bacteriocin 31(Tomita, Fujimoto, Tanimoto, & Ike, 1996) and bacteriocin 41 (Tomita, Kamei, & Ike, 2008) have been studied; and from E. faecium, there are bacteriocin 43 (Todokoro, Tomita, Inoue, & Ike, 2006), bacteriocin 32 (Nes, Diep, Håvarstein, Brurberg, Eijsink, & Holo, 1996), and bacteriocin 51(Yamashita, Tomita, Inoue, & Ike, 2011). Besides enterococcal bacteriocins of clinical origin, bacteriocins from enterococci of food origins have been studied, and several bacteriocins from E.faecium isolates have been identified and characterized. These include enterocin L50A/L50B (Cintas, Casaus, Holo, Hernandez, Nes, & Håvarstein, 1998), enterocin Q (Cintas L. M., et al., 2000; Criado, et al., 2006), enterocin A (Aymerich, Holo, Håvarstein, Hugas, Garriga, & Nes, 1996; Nilsen, Nes, & Holo, 1998), enterocin P (Cintas L. M., Casaus, Håvarstein, Hernández, & Nes, 1997; Kang & Lee, 2005), enterocin B (Casaus, Nilsen, Cintas, Nes, Hernández, & Holo, 1997) and others.

Many enterocins have also been characterized from various enterococcal species and from many environments, and the most thoroughly characterized enterocins are summarized in Table 1. Most of the characterized enterocins are from E. faecium and E. faecalis, but enterocins have also been isolated from E. muntii, E. avium, E. hirae, and E. durans (see Table 1). The bacteriocin-producing enterococci are by and large isolated from food, waste, and the feces and gastrointestinal tract of humans and animals, but may also be isolated from other sources. Fermented food, specimens from human infections, and feces from healthy babies seem to be particularly good niches for isolating bacteriocin-producing enterococci (see Figure 1). It seems likely that most enterococci originate from the digestive tract of humans and animals, a notion which is in line with the finding that the same bacteriocins are identified in enterococci isolated from many environments, which most often include those of human origin.