Stress Physiology of Lactic Acid Bacteria

Abstract

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.

FIG 1

Schematic representation of selected LAB stress sensory systems. HK, histidine kinase; RR, response regulator.

FIG 2

Schematic representation of changes of the carbohydrate metabolism, glycolysis, and fate of pyruvate in lactic acid bacteria. Colored arrows and enzymes indicate common reactions (black), those mainly induced during fermentation by unstressed cells (blue), and those induced in respiratory and/or environmentally stressed cells (red). LDH, lactate dehydrogenase; ACK, acetate kinase; POX, pyruvate oxidase; PFL, pyruvate formate lyase; PdhABCD, pyruvate dehydrogenase complex; α-ASL, α-acetolactate synthase; AdhE, alcohol dehydrogenase; NOX, NADH oxidase; α-ALD, α-acetolactate decarboxylase; DAR, diacetyl reductase.

FIG 3

Schematic representation of the main free amino acid pathways of lactic acid bacteria induced under acid stress and/or starvation conditions. GABA, γ-aminobutyric acid; ADI, arginine deiminase; cOTC, catabolic ornithine transcarbamoylase; CK, carbamate kinase; AguA, agmatine deiminase; AguB, putrescine carbamoyl transferase; AguC, carbamate kinase; AguD, agmatine/putrescine antiporter; GAD, glutamate decarboxylase; HDC, histidine decarboxylase; AspD, aspartic acid decarboxylase; AspT, aspartate-alanine antiporter; Bcat, α-ketoglutarate-dependent branched-chain aminotransferase; HycDH, hydroxyacid dehydrogenase; KaDH, keto acid dehydrogenase; KDC, 2-keto acid decarboxylase; ADH, alcohol dehydrogenase; AIDH, aldehyde dehydrogenase; PTAC, phosphotransacylase; ACK, acetate kinase.

FIG 4

Schematic representation of the main molecular mechanisms preventing macromolecules from being damaged in LAB. SOD, superoxide dismutase.

FIG 5

Schematic representation of the main molecular mechanisms repairing damaged macromolecules in LAB.