Investigations on the Possible Role of Aromatic β-Glucoside Metabolism in Self-Defense in Enterobacteriaceae
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Bacteria are ubiquitous in all ecosystems and are often challenged by multiple stresses such as extreme temperatures, high salt concentrations, nutrient limitation, pH variations, radiation, predation and the presence of antibiotics/toxins. The most challenging among them is predation pressure which is one of the major causes of their mortality in different niches. Bacteria have evolved different adaptive measures to counter predation. Some of them include change in shape, size, motility, and unpalatable aggregate formation. Aromatic β-glucosides such as salicin, produced by plants as secondary metabolites, play a significant role in protecting them from herbivores. Members of the family Enterobaceriaceae primarily present in soil, e.g. Erwinia chrysanthemi (a phytopathogen) and Klebsiella aerogenes, can utilize the aromatic β-glucosides salicin and arbutin (likely to be present in soil derived from decomposing plant materials) as a carbon source unlike their fellow members such as Escherichia coli, Shigella sonnei, and Salmonella present in the gut environment. Bacteria can obtain energy by metabolizing β-glucosides in the form of glucose. Whether they can also use these molecules as defense tools in a manner similar to plants is an intriguing possibility. In such an event, Bgl+ bacteria could derive a dual advantage in terms of energy generation and protection from predation. The current study was initiated to investigate a possible link between β-glucoside metabolism and self-defense in Enterobacteriaceae. Different members of Enterobacteriaceae comprising of both laboratory strains and natural isolates were considered as prey. Predators included were laboratory strains and soil isolates of bacteriovorous nematodes of the Rhabditidae family, the amoeba Dictyostelium discoidium and a bacteriovorous Streptomyces sp. The predator-prey interaction was analyzed by performing viability and behavioral assays in the context of β-glucoside metabolism Results presented in Chapter 2 show that active catabolism of aromatic β¬glucosides like salicin, arbutin and esculin by Bgl+ bacteria decreases the viability of their predators. The aglycone products released during β-glucosides metabolism, e.g. saligenin in the case of salicin, are the causative agents of the mortality of the predators. The lethality is reversible up to a specific threshold of exposure. Saligenin acts as a chemo-attractant that lures and kills Caenorhabditis elegans N2. In the case of nematodes that succumb, bacteria can derive nutrition from the dead predators indicating a conversion of prey to predator. Experiments with mutant strains of Caenorhabditis elegans suggest that the dopaminergic receptor dop-1 is involved in mediating saligenin toxicity. Studies mentioned in Chapter 3 revolve around the relevance of the predator-prey interaction discussed in Chapter 2 in the natural environment. Members of Enterobacteriaceae and their predator amoebae (cellular slime molds) and nematodes were isolated from soil. They show coexistence in most of the soil samples analyzed. All the predators isolated from soil and other natural isolates of Caenorhabditis succumb to saligenin as their laboratory counterparts with higher sensitivity in some of the strains. Soil nematodes belonging to genera Oscheius and Mesorhabditis avoid saligenin unlike the members of Caenorhabditis genus which are attracted towards saligenin. This indicates that the soil nematodes are often exposed to saligenin or saligenin-like compounds, resulting in the evolution of a genetic machinery to avoid these toxic compounds. Studies with quasi-natural environments like soil and fruit indicate that β-glucoside metabolism have similar effects on predator prey interaction in these environments, reinforcing the relevance of these observations to the natural ecology of the organisms. The studies reported in Chapter 2 and 3 shed light on a novel defense strategy of otherwise non-pathogenic members of Enterobacteriaceae which comes with a dual advantage. These results have also brought into focus issues such as the benefit derived by bacterial populations that are genetically heterogeneous, consisting of both Bgl+ and Bgl-strains. The broad implications and future directions of the work are discussed in Chapter 4. Work presented in Appendix deals with the investigation of the pattern of cellobiose utilization in Shigella sonnei. As mentioned in Chapter 1, it is known that members of Enterobacteriaceae exhibit diversity in their pattern of β-glucoside utilization. Wild type strains of both E. coli and Shigella sonnei are unable to utilize Arbutin, Salicin and Cellobiose. While E. coli can acquire cellobiose utilizing ability directly from the wild type state (Arb-Sal-Cel-), Shigella sonnei strains, though closely related to E. coli, have to undergo a series of mutations in a specific sequence to become capable of utilizing these sugars. Characterization of a few Shigella sonnei Cel+ mutants showed a different mode of activation of the chb operon (known to be involved in cellobiose utilization in E. coli). Considering the ecological significance of the ability to hydrolyze aromatic β-glucosides, a detailed understanding of the metabolic capability of different strains and the molecular mechanism involved becomes significant.