SUMMARY Many bacteria produce antimicrobial peptides, which are also referred to as peptide bacteriocins. The class IIa bacteriocins, often designated pediocin-like bacteriocins, constitute the most dominant group of antimicrobial peptides produced by lactic acid bacteria. The bacteriocins that belong to this class are structurally related and kill target cells by membrane permeabilization. Despite their structural similarity, class IIa bacteriocins display different target cell specificities. In the search for new antibiotic substances, the class IIa bacteriocins have been identified as promising new candidates and have thus received much attention. They kill some pathogenic bacteria (e.g., Listeria) with high efficiency, and they constitute a good model system for structure-function analyses of antimicrobial peptides in general. This review focuses on class IIa bacteriocins, especially on their structure, function, mode of action, biosynthesis, bacteriocin immunity, and current food applications. The genetics and biosynthesis of class IIa bacteriocins are well understood. The bacteriocins are ribosomally synthesized with an N-terminal leader sequence, which is cleaved off upon secretion. After externalization, the class IIa bacteriocins attach to potential target cells and, through electrostatic and hydrophobic interactions, subsequently permeabilize the cell membrane of sensitive cells. Recent observations suggest that a chiral interaction and possibly the presence of a mannose permease protein on the target cell surface are required for a bacteria to be sensitive to class IIa bacteriocins. There is also substantial evidence that the C-terminal half penetrates into the target cell membrane, and it plays an important role in determining the target cell specificity of these bacteriocins. Immunity proteins protect the bacteriocin producer from the bacteriocin it secretes. The three-dimensional structures of two class IIa immunity proteins have been determined, and it has been shown that the C-terminal halves of these cytosolic four-helix bundle proteins specify which class IIa bacteriocin they protect against.

In lactococci, the study of chromosomal genes and their regulation is limited by the lack of an efficient transposon mutagenesis system. We associated the insertion sequence ISS1 with the thermosensitive replicon pG ؉ host to generate a mutagenic tool that can be used even in poorly transformable strains. ISS1 transposition is random in different lactococcal strains as well as in Enterococcus faecalis and Streptococcus thermophilus. High-frequency random insertion (of about 1%) obtained with this system in Lactococcus lactis allows efficient mutagenesis, with typically one insertion per cell. After ISS1 replicative transposition, the chromosome contains duplicated ISS1 sequences flanking pG ؉ host. This structure allows cloning of the interrupted gene. In addition, efficient excision of the plasmid leaves a single ISS1 copy at the mutated site, thus generating a stable mutant strain with no foreign markers. Mutants obtained by this transposition system are food grade and can thus be used in fermentation processes.Lactic acid bacteria are important industrial microorganisms because of their role in food fermentations. Lactococcus lactis is widely used in dairy fermentations and also serves as a model organism for biological studies of lactic acid bacteria. Most genes thus far identified in L. lactis have been cloned by (i) complementation (2,10,19,35), (ii) immunoscreening of DNA libraries (11,34,48), (iii) PCR amplification of conserved genes (1,14,15,17,30), and (iv) DNA sequencing of regions adjacent to genes of interest (3,26,36). The chromosome and its genetic regulatory networks, however, remain for the most part unknown.In many bacteria, transposition has been a valuable genetic tool to study chromosomal genes, their functions, and their regulators (4,46,50). In L. lactis, transposition of the conjugative elements Tn916 (41) and Tn919 (22, 23) have been reported. However, their use is limited by a requirement for high-efficiency conjugal transfer and site-specific transposition in certain strains. The transposition of Tn917 has recently been demonstrated in L. lactis MG1614 (28). The vector used in this system is pE194, whose replication is strain specific among lactococci (7), and transposition frequencies appear to be low; also, one-third of the candidates correspond to plasmid integrants. The use of heterologous transposons can be of interest for genetic analyses, but resultant strains containing antibiotic (Ab) resistance markers would be restricted from industrial use, particularly in fermentation.Another class of transposable elements are bacterial insertion sequences (IS). IS elements are small (between 800 and 2,500 bp) and flanked by inverted repeats and generally encode their own transposition functions (16). Three families of IS elements have been defined in lactococci (44), and their host ranges, positions, and frequencies on the chromosome have been shown to vary widely among strains (39, 45). In lactococci, iso-ISS1 elements have been thoroughly characterized (8,18,21,27,39,40). With nonrepli...

The genus Carnobacterium contains nine species, but only C. divergens and C. maltaromaticum are frequently isolated from natural environments and foods. They are tolerant to freezing/thawing and high pressure and able to grow at low temperatures, anaerobically and with increased CO2 concentrations. They metabolize arginine and various carbohydrates, including chitin, and this may improve their survival in the environment. Carnobacterium divergens and C. maltaromaticum have been extensively studied as protective cultures in order to inhibit growth of Listeria monocytogenes in fish and meat products. Several carnobacterial bacteriocins are known, and parameters that affect their production have been described. Currently, however, no isolates are commercially applied as protective cultures. Carnobacteria can spoil chilled foods, but spoilage activity shows intraspecies and interspecies variation. The responsible spoilage metabolites are not well characterized, but branched alcohols and aldehydes play a partial role. Their production of tyramine in foods is critical for susceptible individuals, but carnobacteria are not otherwise human pathogens. Carnobacterium maltaromaticum can be a fish pathogen, although carnobacteria are also suggested as probiotic cultures for use in aquaculture. Representative genome sequences are not yet available, but would be valuable to answer questions associated with fundamental and applied aspects of this important genus.