Bacteriocin Diversity in Streptococcus and Enterococcus

Abstract

Most bacteriocins in gram-positive bacteria are small and heat stable (peptide bacteriocins), and their antimicrobial activities are directed against a broader spectrum of bacteria than is seen for bacteriocins of gram-negative bacteria. Many excellent bacteriocin reviews have been published in recent years (10, 15, 16, 19, 27, 29, 77, 83). The heat-stable peptide bacteriocins from lactic acid bacteria have so far been grouped into two major classes: class I, the lantibiotics, and class II, the heat-stable nonlantibiotics. In addition, a third class of bacteriocins has been suggested which includes secreted heat-labile cell wall-degrading enzymes (71, 88), but classification of such enzymes as bacteriocins has recently been disputed (19, 49). Lantibiotics contain a number of posttranslational modifications that include dehydration of serine and threonine to form 2,3dehydroalanine (Dha) and 2,3-dehydrobutyrine (Dhb), respectively. Some of the dehydrated residues are covalently bound to the sulfur in neighboring cysteines, creating the characteristic lantionine and methyllantionine residues. It has also been shown that in a few cases the dehydroalanine can be converted to D-alanine (109, 118) and that additional modifications, such as lysinoalanine, 2-oxobutyrate, S-aminovinyl-D-cysteine, and S-aminovinyl-D-methylcysteine, are formed in some lantibiotics (59). Both class I and class II bacteriocins display great diversity with regard to their modes of action, structures, genetics, modes of secretion, choices of target organisms, etc. There is still lack of consensus on how to subdivide class I and II peptide bacteriocins further into subclasses. The lantibiotics have been divided into two subgroups, type A and type B, according to structural features (64). Type A lantibiotics (e.g., nisin, subtilin, and Pep5) are elongated molecules with a flexible structure in solution, while type B lantibiotics adapt a more rigid and globular structure (64). However, this picture is changing, since structural studies of the lantibiotic plantaricin C has been shown to hold structural elements of both type A and B lantibiotics (123). Also, nuclear magnetic resonance spectroscopy has shown that the peptides of the two-peptide lantibiotic lacticin 3247 are structurally different. While the peptide designated lacticin 3147 A1 has a specific lanthionine bridging pattern resembling the globular type B lantibiotic mersacidin, the A2 peptide is a member of the elongated type A lantibiotic subclass (80). In the present review, we refer to the A and B types of lantibiotics as one-peptide lantibiotics and mention specifically when a bacteriocin is a two-peptide lantibiotic. Lack of consensus also exists in the differentiation between subgroups of the nonlantibiotic class II peptide bacteriocins. In this review, we retain the pediocin-like bacteriocin in class IIa, the two-peptide bacteriocins in class IIb, and the leaderless peptide bacteriocins in class IIc, and finally, we define the circular bacteriocins as class IId. This overview will discuss the dissemination of the class I and II peptide bacteriocins in enterococci and streptococci and the possibility of identifying such bacteriocins in genome sequences. The lactic acid bacteria in fermented food have been the focus of bacteriocin research during the last 15 to 20 years. Numerous peptide bacteriocins have been characterized, and many have been used intentionally or unintentionally in food