Summary auto-generated
This study investigated how the osmoprotectant glycine betaine (GB) affects salt stress adaptation in Enterococcus faecalis. Using a chemically defined medium lacking osmoprotectants, researchers demonstrated that E. faecalis actively accumulates GB from complex media under high salt conditions. GB and its structural analogues (methylated onium compounds) effectively restored bacterial growth at 0.75 M NaCl, while other osmoprotectants like proline, pipecolate, and ectoine were ineffective. GB uptake occurred through a high-affinity, salt-inducible porter with strong structural specificity. However, despite improving growth under salt stress, GB accumulation paradoxically inhibited the bacteria's ability to develop cross-tolerance to lethal bile salt challenges. Cells adapted to 0.5 M NaCl without GB showed 6800-fold increased survival against 0.3% bile salts, but this protection was reduced 680-fold when GB was present during adaptation. Analysis revealed that GB presence suppressed synthesis of 11 of 13 salt-induced stress proteins. These findings suggest that while GB protects cells from osmotic stress, it interferes with the broader stress response mechanisms necessary for surviving other lethal challenges, raising questions about GB's net beneficial effects in natural environments where bacteria face multiple stressors.
Key findings
- E. faecalis actively accumulates glycine betaine (GB) from complex media under salt stress through a high-affinity, salt-inducible transporter with strong structural specificity for betaines and their analogues
- GB and related methylated onium compounds effectively restore growth at high salt concentrations (0.75 M NaCl), but other osmoprotectants (proline, pipecolate, ectoine) are ineffective in E. faecalis
- Presence of GB during salt adaptation dramatically reduces cross-tolerance to lethal bile salt challenge (680-fold reduction in protection factor), despite improving osmotic survival
- GB accumulation suppresses synthesis of 11 of 13 salt-induced stress proteins, suggesting interference with the broader adaptive stress response
- The results indicate that osmoprotectants may have detrimental effects on multi-stress survival in natural environments where bacteria encounter sequential or combined stressors
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Abstract
V Pichereau, S Bourot, S Flahaut, C Blanco, Y Auffray and T Bernard
UPRES-A CNRS 6026, Departement Membranes & Osmoregulation, Universite de Rennes I, France. vpichere@univ-rennes1.fr
The response of Enterococcus faecalis ATCC 19433 to salt stress has been characterized previously in complex media. In this report, it has been demonstrated that this bacterium actively accumulates the osmoprotectant glycine betaine (GB) from salt-enriched complex medium BHI. To further understand the specific effects of GB and other osmoprotective compounds in salt adaptation and salt-induced cross- tolerance to lethal challenges, a chemically defined medium lacking putative osmoprotectants was used. In this medium, bacterial growth was significantly reduced by increasing concentrations of NaCl. At 0.75 M NaCl, 90% inhibition of the growth rate was observed; GB and its structural analogues restored growth to the non-salt-stressed level. In contrast, proline, pipecolate and ectoine did not allow growth recovery of stressed cells. Kinetic studies showed that the uptake of betaines shows strong structural specificity and occurs through a salt-stress- inducible high-affinity porter [Km = 3.3 microM; Vmax = 130 nmol min(- 1) (mg protein)(-1); the uptake activity increased 400-fold in the presence of 0.5 M NaCl]. Moreover, GB and its analogues were accumulated as non-metabolizable cytosolic osmolytes and reached intracellular levels ranging from 1-3 to 1.5 micromol (mg protein)(-1). In contrast to the beneficial effect of GB on the growth of salt- stressed cultures of E. faecalis, its accumulation inhibits the salt- induced cross-tolerance to a heterologous lethal challenge. Indeed, pretreatment of bacterial cells with 0.5 M NaCl induced resistance to 0.3% bile salts (survival of adapted cells increased by a factor of 6800). The presence of GB in the adaptation medium reduced the acquisition of bile salts resistance 680-fold. The synthesis of 11 of the 13 proteins induced during salt adaptation was significantly reduced in the presence of GB. These results raise questions about the actual beneficial effect of GB in natural environments where bacteria are often subjected to various stresses.