Due to their very potent antimicrobial activity against diverse food-spoiling bacteria and pathogens and their favourable biochemical properties, peptide bacteriocins from Gram-positive bacteria have long been considered promising for applications in food preservation or medical treatment. To take advantage of bacteriocins in different applications, it is crucial to have detailed knowledge on the molecular mechanisms by which these peptides recognize and kill target cells, how producer cells protect themselves from their own bacteriocin (self-immunity) and how target cells may develop resistance. In this review we discuss some important recent progress in these areas for the non-lantibiotic (class II) bacteriocins. We also discuss some examples of how the current wealth of genome sequences provides an invaluable source in the search for novel class II bacteriocins.

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Enterococci are among the most common human intestinal lactic acid bacteria, and they are known to produce bacteriocins. In this study, fecal enterococci were isolated from infants and screened for bacteriocin production. Bacteriocin-producing Enterococcus avium isolates were obtained, and a new pediocin-like bacteriocin was purified and characterized. This bacteriocin, termed avicin A, was found to be produced by isolates from two healthy infants. It was purified to homogeneity from culture supernatant by ion-exchange and reversed-phase chromatography, and part of its amino acid sequence was obtained. The sequence of a 7-kb DNA fragment of a bacteriocin locus was determined by PCR and DNA sequencing. The bacteriocin locus was organized into four operon-like structures consisting of (i) the structural genes encoding avicin A and its immunity protein, (ii) a divergicin-like bacteriocin (avicin B) gene, (iii) an ABC bacteriocin transporter gene and two regulatory genes (histamine protein kinase-and response regulator-encoding genes), and (iv) induction peptide pheromone-and transport accessory protein-encoding genes. It was shown that the production of avicin A was regulated by the peptide pheromone-inducible regulatory system. Avicin A shows very high levels of similarity to mundticin KS and enterocin CRL35. This bacteriocin showed strong antimicrobial activity against many species of Gram-positive bacteria, including the food-borne pathogen Listeria monocytogenes. The avicin A locus is the first bacteriocin locus identified in E. avium to be characterized at the molecular level.Bacteriocins are ribosomally synthesized antimicrobial peptides and proteins. Production of these compounds is widespread in Gram-negative and Gram-positive bacteria (23). Bacteriocins produced by lactic acid bacteria (LAB) have recently been classified into two major categories: the lantibiotics or lanthionine-containing bacteriocins (class I) and the nonlanthionine-containing bacteriocins (class II) (5). According to this classification, the former class III bacteriocins (large heatlabile bacteriocins) were considered nonbacteriocins and hence designated bacteriolysins. The class II bacteriocins are further subdivided into four subclasses: subclass IIa (pediocinlike bacteriocins), subclass IIb (two-peptide bacteriocins), subclass IIc (cyclic bacteriocins), and subclass IId (nonpediocin linear peptide bacteriocins). Class II bacteriocins are most commonly found in enterococci. The subclass IIa bacteriocins are known for their strong antilisterial activity, and they are distinguished by their N-terminal conserved YYGNG motif and two covalently S-S-linked cysteines separated by four amino acid residues (11).The production of subclass IIa bacteriocins usually requires four genes: a bacteriocin gene (which encodes the bacteriocin precursor), an immunity gene (which protects the producer from its bacteriocin), and the ABC transporter and transport accessory genes (31, 44). Bacteriocins are synthesized as biologically inactive prepeptides (pre...

The objective of this study was to characterise lactic acid bacteria (LAB) isolated from faecal samples of healthy Ethiopian infants, with emphasis on bacteriocin production and antibiotic susceptibility. One hundred fifty LAB were obtained from 28 healthy Ethiopian infants. The isolates belonged to Lactobacillus (81/150), Enterococcus (54/150) and Streptococcus (15/150) genera. Lactobacillus species were more abundant in the breast-fed infants while Enterococcus dominated the mixed-fed population. Bacteriocin-producing LAB species were isolated from eight of the infants. Many different bacteriocins were identified, including one new bacteriocin from Streptococcus salivarius, avicin A (class IIa) from Enterococcus avium, one class IIa bacteriocin from Enterococcus faecalis strains, one unknown bacteriocin from E. faecalis and two unknown bacteriocins from Lactobacillus fermentum strains and the two-peptide gassericin T from Lactobacillus gasseri isolate. Susceptibility tests performed for nine antibiotics suggest that some lactobacilli might have acquired resistance to erythromycin (3 %) and tetracycline (4 %) only. The streptococci were generally antibiotic sensitive except for penicillin, to which they showed intermediate resistance. All enterococci were susceptible to ampicillin while 13 % showed penicillin resistance. Only one E. faecalis isolate was vancomycin-resistant. Tetracycline (51 %) and erythromycin (26 %) resistance was prevalent among the enterococci, but multidrug resistance was confined to E. faecalis (47 %) and Enterococcus faecium (33 %). Screening of enterococcal virulence traits revealed that 2 % were β-haemolytic. The structural genes of cytolysin were detected in 28 % of the isolates in five enterococcal species, the majority being E. faecalis and Enterococcus raffinosus. This study shows that bacteriocin production and antibiotic resistance is a common trait of faecal LAB of Ethiopian infants while virulence factors occur at low levels.

In this work, we purified and characterized a newly identified lantibiotic (salivaricin D) from Streptococcus salivarius 5M6c. Salivaricin D is a 34-amino-acid-residue peptide (3,467.55 Da); the locus of the gene encoding this peptide is a 16.5-kb DNA segment which contains genes encoding the precursor of two lantibiotics, two modification enzymes (dehydratase and cyclase), an ABC transporter, a serine-like protease, immunity proteins (lipoprotein and ABC transporters), a response regulator, and a sensor histidine kinase. The immunity gene (salI) was heterologously expressed in a sensitive indicator and provided significant protection against salivaricin D, confirming its immunity function. Salivaricin D is a naturally trypsin-resistant lantibiotic that is similar to nisin-like lantibiotics. It is a relatively broad-spectrum bacteriocin that inhibits members of many genera of Gram-positive bacteria, including the important human pathogens Streptococcus pyogenes and Streptococcus pneumoniae. Thus, Streptococcus salivarius 5M6c may be a potential biological agent for the control of oronasopharynx-colonizing streptococcal pathogens or may be used as a probiotic bacterium. Streptococcus salivarius is a member of the lactic acid bacteria (LAB) that forms part of the normal flora of the oral cavity, throat, and upper respiratory tract (23,28,40,44). It has also been observed in the nasopharynx and intestinal tract and isolated from the human feces (25,35,44) and breast milk of healthy women (1, 46). S. salivarius strains produce a number of bacteriocins, most of which are lantibiotics (22,39,(52)(53)(54).Lantibiotics are small, heat-stable, ribosomally synthesized, posttranslationally modified antimicrobial peptides (bacteriocins) produced by Gram-positive bacteria (3). The lantibiotics, unlike other bacteriocins, are characterized by containing the thioether amino acids lanthionine (Lan) and 3-methyllanthionine (MeLan) and the modified amino acids didehydroalanine (Dha) and didehydrobutyrine (Dhb) (55). Lantibiotics are initially synthesized as inactive linear prepeptides that undergo subsequent extensive modifications to be biologically active. The modifications involve mainly dehydration of serine and threonine residues, forming the didehydro amino acids Dha and Dhb, respectively, which react with the nearby C-terminally located cysteine residues (seen among linear lantibiotics) to form a thioether linkage, which results in the formation of Lan and MeLan, respectively. Finally, the modified peptide is exported and cleaved from its leader in order to be active.According to a very recent classification system, lantibiotics and lantipeptides (class Ia), consist of four subclasses (38). Subclass I lantibiotics are modified by two different enzymes, LanB enzyme (dehydratase) and LanC enzyme (cyclase), exported by LanT, and their leader peptides are removed by the LanP enzyme. Subclass II lantibiotics are modified by a single enzyme (LanM) which has both dehydratase and cyclase activity and is exported by LanT, which als...