SUMMARY: Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis ), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the “stressome” of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Lactic acid bacteria (LAB) constitute a diverse group of Gram positive obligately fermentative microorganisms which include both beneficial and pathogenic strains. LAB generally have complex nutritional requirements and therefore they are usually associated with nutrient-rich environments such as animal bodies, plants and foodstuffs. Amino acids represent an important resource for LAB and their utilization serves a number of physiological roles such as intracellular pH control, generation of metabolic energy or redox power, and resistance to stress. As a consequence, the regulation of amino acid catabolism involves a wide set of both general and specific regulators and shows significant differences among LAB. Moreover, due to their fermentative metabolism, LAB amino acid catabolic pathways in some cases differ significantly from those described in best studied prokaryotic model organisms such as Escherichia coli or Bacillus subtilis. Thus, LAB amino acid catabolism constitutes an interesting case for the study of metabolic pathways. Furthermore, LAB are involved in the production of a great variety of fermented products so that the products of amino acid catabolism are also relevant for the safety and the quality of fermented products.

The entire genome of Lactobacillus casei BL23, a strain with probiotic properties, has been sequenced. The genomes of BL23 and the industrially used probiotic strain Shirota YIT 9029 (Yakult) seem to be very similar.

Members of the genus Lactobacillus are commonly found at the gastrointestinal tract and other mucosal surfaces of humans. This genus includes various species with a great number of potentially probiotic bacteria. Other often-used probiotic species belong to Bifidobacterium, a genus almost exclusively associated with the gut. As probiotics must survive and be metabolically active at their target sites, namely host mucosal surfaces, consumption of host-produced glycans is a key factor for their survival and activity. The ability to metabolize glycans such as human milk oligosaccharides (HMOs), glycosaminoglycans and the glycan moieties of glycoproteins and glycolipids found at the mucosal surfaces grants a competitive advantage to lactobacilli and bifidobacteria. The analyses of the great number of sequenced genomes from these bacteria have revealed that many of them encode a wide assortment of genes involved in the metabolism and transport of carbohydrates, including several glycoside hydrolases required for metabolizing the carbohydrate moieties of mucins and HMOs. Here, the current knowledge on the genetic mechanisms, known catabolic pathways and biochemical properties of enzymes involved in the utilization of host-produced glycans by lactobacilli and bifidobacteria will be summarized.

Epizootic Rabbit Enteropathy (ERE) is a severe disease of unknown aetiology that mainly affects post-weaning animals. Its incidence can be prevented by antibiotic treatment suggesting that bacterial elements are crucial for the development of the disease. Microbial dynamics and host responses during the disease were studied. Cecal microbiota was characterized in three rabbit groups (ERE-affected, healthy and healthy pretreated with antibiotics), followed by transcriptional analysis of cytokines and mucins in the cecal mucosa and vermix by q-rtPCR. In healthy animals, cecal microbiota with or without antibiotic pretreatment was very similar and dominated by Alistipes and Ruminococcus. Proportions of both genera decreased in ERE rabbits whereas Bacteroides, Akkermansia and Rikenella increased, as well as Clostridium, γ-Proteobacteria and other opportunistic and pathogenic species. The ERE group displayed remarkable dysbiosis and reduced taxonomic diversity. Transcription rate of mucins and inflammatory cytokines was very high in ERE rabbits, except IL-2, and its analysis revealed the existence of two clearly different gene expression patterns corresponding to Inflammatory and (mucin) Secretory Profiles. Furthermore, these profiles were associated to different bacterial species, suggesting that they may correspond to different stages of the disease. Other data obtained in this work reinforced the notion that ERE morbidity and mortality is possibly caused by an overgrowth of different pathogens in the gut of animals whose immune defence mechanisms seem not to be adequately responding.