Conformational changes in bacterial polysomes induced by amino acid starvation

Behavior of sulfate reducing bacteria under oligotrophic conditions and oxygen stress in particle-free systems related to drinking water

The response of sulfate reducing bacteria (SRB) to oxygen stress under oligotrophic conditions in particle-free systems was studied in (i) sterile Berlin drinking water; (ii) mineral medium; and (iii) in coculture experiments with aerobic bacteria. Using a polyphasic approach including anaerobic cultivation, fluorescent in situ hybridization (FISH) and digital image analysis, the behavior of the strains zt3l and zt10e, isolated from Berlin groundwater and affiliated to the family Desulfovibrionaceae, was compared to the type strains Desulfomicrobium baculatum and Desulfovibrio desulfuricans. Anaerobic deep agar dilution series were performed for the determination of cell culturability. FISH and subsequent digital image analysis of probe-conferred fluorescence intensities were used for the assessment of metabolic activity. For the in situ identification of both isolates in coculture tests, two strain-specific oligonucleotides were developed and evaluated. The total cell counts of stressed SRB in drinking water decreased during the course of the assay dependent on the strain. Both environmental isolates could be cultured for a longer period than cells of D. baculatum and D. desulfuricans, respectively. The FISH intensities showed a strain-specific behavior. When exposed to simultaneous oxygen stress and carbon limitation in mineral medium, total cell counts of all four strains remained constant throughout a period of 72 days. The rate of culturability differed between the investigated strains. The decrease of metabolic activity as assessed by FISH was a strain-specific property. Exposure of SRB to oxygen stress and carbon starvation in coculture experiments with Aquabacterium commune resulted in strain dependent prolonged culturability and a delayed decrease of the metabolic activity compared to pure culture tests for all strains tested. Total cell counts of SRB were constant throughout the whole experiment.

Changes in ribosomal activity of Escherichia coli cells during prolonged culture in sea salts medium

The activity of ribosomes from a clinical isolate of Escherichia coli, exposed to starvation for 7 days in sea salts medium, was investigated by measuring the kinetic parameters of ribosomal peptidyltransferase, by using the puromycin reaction as a model reaction. No alterations in the extent of peptide bond formation were observed during starvation. In contrast, a 50% reduction in the kmax/Ks ratio could be seen after 24 h of starvation; an additional 6 days of starvation resulted in a progressive but less abrupt decline in the kmax/Ks value. (kmax is the apparent catalytic rate constant of peptidyl transferase, and Ks is the dissociation constant of the encounter complex between acetyl (Ac)[3H]Phe-tRNA-poly(U)-ribosome and puromycin.) Although the distribution of ribosomal particles remained constant, a substantial decrease in the number of ribosomes per starved cell and a clear decline in the ability of ribosomes to bind AcPhe-tRNA were observed, particularly during the first day of starvation. Further analysis indicated that rRNA in general, but especially 23S rRNA, was rapidly degraded during the starvation period. In addition, the L12/L7 molar ratio decreased from 1.5 to 1 during the initial phase of starvation (up to 24 h) but remained constant during the subsequent starvation period. Ribosomes isolated from 24-h-starved cells, when artificially depleted of L7/L12 protein and reconstituted with L7/L12 protein from mid-logarithmic-phase cells, regenerated an L12/L7 molar ratio of 1.5 and restored the peptidyltransferase activity to a substantial level. An analogous effect of reconstitution on the efficiency of ribosomes in binding AcPhe-tRNA was evident not only during the initial phase but throughout the starvation period.