INVESTIGADORES
ALVAREZ Hector Manuel
congresos y reuniones científicas
Título:
Flexible response of Rhodococcus metabolism under en environmental conditions
Autor/es:
ALVAREZ, HÉCTOR M; BEQUER URBANO S; HERNANDEZ MA; SILVA RA
Lugar:
Semmering
Reunión:
Conferencia; Microbial Stress: from Molecules to Systems. European Federation of Biotechnology; 2009
Institución organizadora:
European Federation of Biotechnology
Resumen:
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.