Bacteriocins of gram-positive bacteria.

Bacteriocins are bacterially produced antimicrobial peptides with narrow or broad host ranges. Many bacteriocins are produced by food-grade lactic acid bacteria, a phenomenon which offers food scientists the possibility of directing or preventing the development of specific bacterial species in food. This can be particularly useful in preservation or food safety applications, but also has implications for the development of desirable flora in fermented food. In this sense, bacteriocins can be used to confer a rudimentary form of innate immunity to foodstuffs, helping processors extend their control over the food flora long after manufacture.

Bacteriocins: developing innate immunity for food

Lactic acid bacteria produce a variety of bacteriocins that have recently come under detailed investigation. The biochemical and genetic characteristics of these antimicrobial proteins are reviewed and common elements are discussed between the different classes of bacteriocins produced by these Gram-positive bacteria.

Genetics of bacteriocins produced by lactic acid bacteria

The genus Lactobacillus comprises 261 species (at March 2020) that are extremely diverse at phenotypic, ecological and genotypic levels. This study evaluated the taxonomy of Lactobacillaceae and Leuconostocaceae on the basis of whole genome sequences. Parameters that were evaluated included core genome phylogeny, (conserved) pairwise average amino acid identity, clade-specific signature genes, physiological criteria and the ecology of the organisms. Based on this polyphasic approach, we propose reclassification of the genus Lactobacillus into 25 genera including the emended genus Lactobacillus, which includes host-adapted organisms that have been referred to as the Lactobacillus delbrueckii group, Paralactobacillus and 23 novel genera for which the names Holzapfelia, Amylolactobacillus, Bombilactobacillus, Companilactobacillus, Lapidilactobacillus, Agrilactobacillus, Schleiferilactobacillus, Loigolactobacilus, Lacticaseibacillus, Latilactobacillus, Dellaglioa, Liquorilactobacillus, Ligilactobacillus, Lactiplantibacillus, Furfurilactobacillus, Paucilactobacillus, Limosilactobacillus, Fructilactobacillus, Acetilactobacillus, Apilactobacillus, Levilactobacillus, Secundilactobacillus and Lentilactobacillus are proposed. We also propose to emend the description of the family Lactobacillaceae to include all genera that were previously included in families Lactobacillaceae and Leuconostocaceae. The generic term 'lactobacilli' will remain useful to designate all organisms that were classified as Lactobacillaceae until 2020. This reclassification reflects the phylogenetic position of the micro-organisms, and groups lactobacilli into robust clades with shared ecological and metabolic properties, as exemplified for the emended genus Lactobacillus encompassing species adapted to vertebrates (such as Lactobacillus delbrueckii, Lactobacillus iners, Lactobacillus crispatus, Lactobacillus jensensii, Lactobacillus johnsonii and Lactobacillus acidophilus) or invertebrates (such as Lactobacillus apis and Lactobacillus bombicola).

A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae.

Solutions are urgently required for the growing number of infections caused by antibiotic-resistant bacteria. Bacteriocins, which are antimicrobial peptides produced by certain bacteria, might warrant serious consideration as alternatives to traditional antibiotics. These molecules exhibit significant potency against other bacteria (including antibiotic-resistant strains), are stable and can have narrow or broad activity spectra. Bacteriocins can even be produced in situ in the gut by probiotic bacteria to combat intestinal infections. Although the application of specific bacteriocins might be curtailed by the development of resistance, an understanding of the mechanisms by which such resistance could emerge will enable researchers to develop strategies to minimize this potential problem.

Bacteriocins — a viable alternative to antibiotics?

Lactic acid bacteria (LAB) constitute a group of gram-positive bacteria united by a constellation of morphological, metabolic, and physiological characteristics The general description of the bacteria included in the group is gram-positive, nonsporing, nonrespiring cocci or rods, which produce lactic acid as the major end product during the fermentation of carbohydrates The LAB term is intimately associated with bacteria involved in food and feed fermentation, including related bacteria normally associated with the (healthy) mucosal surfaces of humans and animals The boundaries of the group have been subject to some controversy, but historically the genera Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus form the core of the group Taxonomic revisions of these genera and the description of new genera mean that LAB could, in their broad physiological definition, comprise around 20 genera However, from a practical, food-technology point of view, the following genera are considered the principal LAB: Aerococcus, Carnobacterium, Enterococcus, Lactobacillus, Lactococcus, Leuconostoc, Oenococcus, Pediococcus, Streptococcus, Tetragenococcus, Vagococcus, and Weissella The genus Bifidobacterium, often considered in the same context as the genuine lactic acid bacteria and sharing some of their typical features, is phylogenetically unrelated and has a unique mode of sugar fermentation The classification of lactic acid bacteria into different genera is largely based on morphology, mode of glucose fermentation, growth at different temperatures, configuration of the lactic acid produced, ability to grow at high salt concentrations, and acid or alkaline tolerance Chemotaxonomic markers such as fatty acid composition and constituents of the cell wall are also used in classification In addition, the present taxonomy relies partly on true phylogenetic relationships,

Lactic acid bacteria: classification and physiology.

Lactic acid bacteria (LAB) fight competing Gram-positive microorganisms by secreting anti-microbial peptides called bacteriocins. Peptide bacteriocins are usually divided into lantibiotics (class I) and non-lantibiotics (class II), the latter being the main topic of this review. During the past decade many of these bacteriocins have been isolated and characterized, and elements of the genetic mechanisms behind bacteriocin production have been unravelled. Bacteriocins often have a narrow inhibitory spectrum, and are normally most active towards closely related bacteria likely to occur in the same ecological niche. Lactic acid bacteria seem to compensate for these narrow inhibitory spectra by producing several bacteriocins belonging to different classes and having different inhibitory spectra. The latter may also help in counteracting the possible development of resistance mechanisms in target organisms. In many strains, bacteriocin production is controlled in a cell-density dependent manner, using a secreted peptide-pheromone for quorum-sensing. The sensing of its own growth, which is likely to be comparable to that of related species, enables the producing organism to switch on bacteriocin production at times when competition for nutrients is likely to become more severe. Although today a lot is known about LAB bacteriocins and the regulation of their production, several fundamental questions remain to be solved. These include questions regarding mechanisms of immunity and resistance, as well as the molecular basis of target-cell specificity.

Production of class II bacteriocins by lactic acid bacteria; an example of biological warfare and communication

Lactobacillus reuteri resides in the gastrointestinal tract of humans, swine, poultry and other animals. Resting cells of this species convert glycerol into a potent, broad-spectrum antimicrobial substance termed reuterin. Reuterin is a low molecular weight, neutral, water soluble compound, capable of inhibiting growth of species representing all bacterial genera tested thus far, including: Escherichia, Salmonella, Shigella, Proteus, Pseudomonas, Clostridium and Staphylococcus. Also affected, but to a lesser degree, are lactic acid bacteria belonging to the genera Streptococcus, Pediococcus, Leuconostoc, and Lactobacillus. In this report we describe a method to screen lactobacilli for production of unique antimicrobial substances and the discovery of reuterin.

https://www.tandfonline.com/doi/pdf/10.3109/08910608909140210

Production of a Broad Spectrum Antimicrobial Substance by Lactobacillus reuteri

The effects of process conditions and growth kinetics on the production of the bacteriocin sakacin P by Lactobacillus sakei CCUG 42687 have been studied in pH-controlled fermentations. The fermentations could be divided into phases based on the growth kinetics, phase one being a short period of exponential growth, and three subsequent ones being phases of with decreasing specific growth rate. Sakacin P production was maximal at 20 °C. At higher temperatures (25–30 °C) the production ceased at lower cell masses, when less glucose was consumed, resulting in much lower sakacin P concentrations. With similar media and pH, the maximum sakacin P concentration at 20 °C was seven times higher than that at 30 °C. The growth rate increased with increasing concentrations of yeast extract, and the maximum concentration and specific production rate of sakacin P increased concomitantly. Increasing tryptone concentrations also had a positive influence upon sakacin P production, though the effect was significantly lower than that of yeast extract. The maximum sakacin P concentration obtained in this study was 20.5 mg l−1. On the basis of the growth and production kinetics, possible metabolic regulation of bacteriocin synthesis is discussed, e.g. the effects of availability of essential amino acids, other nutrients, and energy.

Influence of complex nutrients, temperature and pH on bacteriocin production by Lactobacillus sakei CCUG 42687.

Sakacin A is a small, heat-stable, antilisterial bacteriocin produced by Lactobacillus sake Lb706. The nucleotide sequence of a 8,668-bp fragment, shown to contain all information necessary for sakacin A production and immunity, was determined. The sequence revealed the presence of two divergently transcribed operons. The first encompassed the structural gene sapA (previously designated sakA) and saiA, which encoded a putative peptide of 90 amino acid residues. The second encompassed sapK (previously designated sakB), sapR, sapT, and sapE. sapK and sapR presumably encoded a histidine kinase and a response regulator with marked similarities to the AgrB/AgrA type of two-component signal-transducing systems. The putative SapT and SapE proteins shared similarity with the Escherichia coli hemolysin A-like signal sequence-independent transport systems. SapT was the HlyB analog with homology to bacterial ATP-binding cassette exporters implicated in bacteriocin transport. Frameshift mutations and deletion analyses showed that sapK and sapR were necessary for both production and immunity, whereas sapT and sapE were necessary for production but not for immunity. The putative SaiA peptide was shown to be involved in the immunity to sakacin A. The region between the operons contained IS1163, a recently described L. sake insertion element. IS1163 did not appear to be involved in expression of the sap genes. Northern (RNA) blot analysis revealed that the putative SapK/SapR system probably acts as a transcriptional activator on both operons. A 35-bp sequence, present upstream of the putative sapA promoter, and a similar sequence (30 of 35 nucleotides identical) upstream of sapK were shown to be necessary for proper expression and could thus be possible targets for transcriptional activation.

Lars Axelsson

Papers

Lactic acid bacteria: classification and physiology.

Production of class II bacteriocins by lactic acid bacteria; an example of biological warfare and communication

Production of a Broad Spectrum Antimicrobial Substance by Lactobacillus reuteri

Influence of complex nutrients, temperature and pH on bacteriocin production by Lactobacillus sakei CCUG 42687.

The genes involved in production of and immunity to sakacin A, a bacteriocin from Lactobacillus sake Lb706.