Bacteria employed in probiotic applications help to maintain or restore a host's natural microbial floral. The ability of probiotic bacteria to successfully outcompete undesired species is often due to, or enhanced by, the production of potent antimicrobial toxins. The most commonly encountered of these are bacteriocins, a large and functionally diverse family of antimicrobials found in all major lineages of Bacteria. Recent studies reveal that these proteinaceous toxins play a critical role in mediating competitive dynamics between bacterial strains and closely related species. The potential use of bacteriocin-producing strains as probiotic and bioprotective agents has recently received increased attention. This review will report on recent efforts involving the use of such strains, with a particular focus on emerging probiotic therapies for humans, livestock, and aquaculture.

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For centuries, biodiversity has spellbound biologists focusing mainly on macroorganism's diversity and almost neglecting the geographic mediated dynamics of microbial communities. We surveyed the diversity of soil bacteria and archaea along a steep precipitation gradient ranging from the Negev Desert in the south of Israel (o100 mm annual rain) to the Mediterranean forests in the north (4900 mm annual rain). Soil samples were retrieved from triplicate plots at five long-term ecological research stations, collected from two types of patches: plant interspaces and underneath the predominant perennial at each site. The molecular fingerprint of each soil sample was taken using terminal restriction length polymorphism of the 16S rRNA gene to evaluate the bacterial and archaeal community composition and diversity within and across sites. The difference in community compositions was not statistically significant within sites (P ¼ 0.33 and 0.77 for bacteria and archaea, respectively), but it differed profoundly by ecosystem type. These differences could largely be explained by the precipitation gradient combined with the vegetation cover: the archaeal and bacterial operational taxonomic units were unique to each climatic region, that is, arid, semiarid and Mediterranean (P ¼ 0.0001, for both domains), as well as patch type (P ¼ 0.009 and 0.02 for bacteria and archaea, respectively). Our results suggest that unlike macroorganisms that are more diverse in the Mediterranean ecosystems compared with the desert sites, archaeal and bacterial diversities are not constrained by precipitation. However, the community composition is unique to the climate and vegetation cover that delineates each ecosystem.

The exploration of spatial patterns of abundance and diversity patterns along precipitation gradients has focused for centuries on plants and animals; microbial profiles along such gradients are largely unknown. We studied the effects of soil pH, nutrient concentration, salinity, and water content on bacterial abundance and diversity in soils collected from Mediterranean, semi-arid, and arid sites receiving approximately 400, 300, and 100 mm annual precipitation, respectively. Bacterial diversity was evaluated by terminal restriction fragment length polymorphism and clone library analyses and the patterns obtained varied with the climatic regions. Over 75% of the sequenced clones were unique to their environment, while ∼2% were shared by all sites, yet, the Mediterranean and semi-arid sites had more common clones (∼9%) than either had with the arid site (4.7% and 6%, respectively). The microbial abundance, estimated by phospholipid fatty acids and real-time quantitative PCR assays, was significantly lower in the arid region. Our results indicate that although soil bacterial abundance decreases with precipitation, bacterial diversity is independent of precipitation gradient. Furthermore, community composition was found to be unique to each ecosystem.

Explaining the coexistence of competing species is a major challenge in community ecology. In bacterial systems, competition is often driven by the production of bacteriocins, which are narrowspectrum proteinaceous toxins that serve to kill closely related species, providing the producer better access to limited resources. Bacteriocin producers have been shown to competitively exclude sensitive, nonproducing strains. However, the dynamics between bacteriocin producers, each lethal to its competitor, are largely unknown. In this study, we used in vitro, in vivo and in silico models to study competitive interactions between bacteriocin producers. Two Escherichia coli strains were generated, each carrying a DNA-degrading bacteriocin (colicins E2 and E7). Using reporter-gene assays, we showed that each DNase bacteriocin is not only lethal to its opponent but, at lower doses, can also induce the expression of its opponent's toxin. In a well-mixed habitat, the E2 producer outcompeted its adversary; however, in structured environments (on plates or in mice colons), the two producers coexisted in a spatially 'frozen' pattern. Coexistence occurred when the producers were initiated with a clumped spatial distribution. This suggests that a 'clump' of each producer can block invasion of the other producer. Agent-based simulation of bacteriocin-mediated competition further showed that mutual exclusion in a structured environment is a relatively robust result. These models imply that colicin-mediated colicin induction enables producers to successfully compete and defend their niche against invaders. This suggests that localized interactions between producers of DNA-degrading toxins can lead to stable coexistence of heterogeneously distributed strains within the bacterial community and to the maintenance of diversity.