Flexible response of Rhodococcus metabolism under en environmental conditions

ALVAREZ, HÉCTOR M; BEQUER URBANO S; HERNANDEZ MA; SILVA RA

Semmering

Conferencia; Microbial Stress: from Molecules to Systems. European Federation of Biotechnology; 2009

European Federation of Biotechnology

Rhodococcus bacteria belong to the non-sporulating and mycolic acid-containing actinomycetes, together with other related genera, including Mycobacterium, Nocardia, and Gordonia. The frequent occurrence of Rhodococcus sp. in arid sites like deserts may reflect their adaptation to environments with extreme conditions. Little is known on the physiological state in which cells of non-sporulating bacteria survive under stress conditions usually found in arid environments. In order to understand better the metabolic responses of these bacteria to such conditions, we explored the metabolism of starved cells and performed a genome-wide bioinformatic analysis of key genes encoding metabolism of diverse storage compounds. We examined the ability of diverse Rhodococcus strains to synthesize and accumulate triacylglycerols (TAG), polyhydroxyalkanoates (PHA) and glycogen. Different strains were used in this study, such as R.opacus PD630, R. jostii RHA1, R.jostii 602, R.erythropolis DSMZ43060 and R.fascians D 188-5. In general, Rhodococcus bacteria seems to have a low energy life style showing a relative slow growth even when nutrients are available. These microorganisms seem to posses the ability to conserve metabolic useful energy during catabolism of substrates, thus, a part of the resulting energy can be used for growth and division, and the surplus is channeled into energy storage pathways. In this context, all Rhodococcus strains analyzed in this study were able to accumulate variable amounts of TAG, PHA and glycogen, which were likely produced in a programmed manner. Glycogen was produced principally during exponential growth phase, whereas storage lipids biosynthesis predominated during stationary phase. All studied strains accumulated TAG as the main storage compounds plus PHA (with 3-hydroxybutyrate and 3-hydroxyvalerate monomers) and glycogen as minor compounds. The experiments with an inhibitor of the fatty acid biosynthesis, such as cerulenin, demonstrated that the biosynthesis routes of TAG, PHA and TAG in cells of R.opacus PD630 compete for the carbon flux during cultivation of cells under nitrogen-limiting conditions. In the presence of the inhibitor, the biosynthesis of TAG decreased drastically, whereas the cellular content of PHA and glycogen increased twice and three times, respectively. All key genes for the biosynthesis and mobilization of these storage compounds were identified in the R. jostii RHA1 genome database. We observed a high redundancy of genes and enzymes involved in storage lipid metabolism. Individual isoforms of enzymes potentially have different substrate specificity, may play distinct functional roles in the pathways of glycerolipid biosynthesis or may be differentially expressed under various environmental conditions.   Starvation experiments demonstrated that R.opacus PD630 and R.jostii 602 possess specialized mechanisms for turning metabolism down when nutrients are in short supply or when cells are subjected to other stress conditions that normally occur in arid soils. Metabolic depression may be a relevant physiological mechanism allowing such bacteria to adapt ecologically to poor environments. During nitrogen starvation, cells reduced their metabolic activity and their ability of mineralize the carbon source, but increased significantly the biosynthesis and accumulation of TAG. Under carbon starvation, profound metabolic suppression allowed a slow utilization of stored lipids. The energy obtained by the slow mobilization of stored TAG may support the necessary biochemical and physiological adaptation mechanisms for large time periods. In general, we observed that when cells were exposed to environmental stress, they produced compact aggregates surrounded by an extracellular polymeric substance (EPS), which probably provide protection and prevent the population from becoming dispersed in the environment. Results of this study suggested that Rhodococcus bacteria developed metabolic strategies to cope with such environments where nutrient-limitation and other stresses are common. Some of these mechanisms may be, (a) the accumulation of storage compounds that can be utilized by cells as endogenous carbon sources and electron donors during periods of nutritional scarcity; (b) the occurrence of metabolic gene and enzyme redundancy in genomes; (c) the reduction of energy requirements in response to starvation and other stress conditions; and (d) the formation of cell aggregates, which promotes a relative isolation from the surrounding environment by the presence of an EPS. This multicellular biological system with the availability of a variety of storage compounds, compatible solutes, pigments and other oxidative protection systems may be advantageous in fluctuating environments. These processes may provide cells of energetic autonomy and a temporal independence from the environment and contribute for cell survival when they do not have access to energy resources in soil.