Increased cell size and shortened peptidoglycan interpeptide bridge of NaCl-stressed Staphylococcus aureus and their reversal by glycine betaine

Abstract

Staphylococcus aureus cells grown in a defined medium under conditions of high ionic stress (2.5 M NaCl) were significantly larger than cells grown under unstressed conditions, even though the cells grew much more slowly under stressed conditions. Analysis of the structure of peptidoglycan from stressed cells showed a shorter interpeptide bridge than in peptidoglycan from unstressed cells. Glycine betaine inclusion in the high-NaCl medium resulted in cells with sizes and interpeptide bridges similar to those of cells grown under unstressed conditions.The gram-positive coccus Staphylococcus aureus is one of the most halotolerant eubacteria. The organism can grow in medium containing up to 3.5 M NaCl (28). During an investigation of the effects of growth in the presence of NaCl on the production of penicillin-binding protein 2a and autolytic activity (20), it was observed that cells grown in complex medium containing 0.85 M NaCl were larger than cells grown in complex medium alone (21). Similar observations of larger cells in complex medium containing NaCl than in complex medium alone have also been reported by others (13,17).When bacterial cells such as Escherichia coli are exposed to a sudden increase in external osmotic pressure, there is initially a fast efflux of water, resulting in decreased turgor pressure, plasmolysis, and cell shrinkage (1,5,18). Mitchell and Moyle (25) have shown a similar response for S. aureus cells transferred from dilute to concentrated NaCl solutions. Bacteria respond to a decrease in turgor by accumulating high concentrations of osmotically active, compatible solutes in order to restore turgor, either by transport from the medium or by biosynthesis (5). This allows the restoration of growth of the organism, and a new steady state is established.There have been several recent studies of osmoregulation in S. aureus (11,19,24), and choline, glycine betaine, L-proline, and taurine have been shown to act as osmoprotectants. Because complex media contain the osmoprotectants L-proline and glycine betaine (10), cells of S. aureus osmotically stressed by growth in complex media containing high concentrations of NaCl accumulate proline and glycine betaine to high levels (19,24). Thus, complex media are not well suited to study of the effects of NaCl on cell size because of potential modifying effects of osmoprotectants present in such media. In this paper, we report a careful examination of the effects of ionic and osmotic stress on the cell morphology and structure of peptidoglycan of S. aureus grown in defined medium in the presence and absence of the osmoprotectant glycine betaine. MATERIALS AND METHODSBacterial strains and culture conditions. S. aureus RN450 (NCTC 8325-4), a derivative of strain NCTC 8325, was used in this study. All experiments were carried out with cells grown in a defined medium that contained glycerol as the primary carbon source (31). The osmotic strength of this medium is approximately 0.20 osmol (kg of water) Ϫ1 (11). The ionic and/or osmotic strength of the mediu...

Native Cell Wall Organization Shown by Cryo-Electron Microscopy Confirms the Existence of a Periplasmic Space in Staphylococcus aureus

“…Cell walls typically provide a rigid exoskeleton responsible for cell shape in bacteria, but this requirement may not be so strict in these cocci, as a spherical geometry would represent the most energetically favored shape. Moreover, a more flexible wall could aid S. aureus to better respond to variations in environmental osmolarities, a feature very important for this highly osmotolerant pathogen (22,50).…”

The bacA gene, which determines bacitracin susceptibility in Streptococcus pneumoniae and Staphylococcus aureus, is also required for virulence The GenBank accession number for the sequence reported in this paper is AF228662.

“…Single fully conserved residues are in bold and indicated by an asterisk ; residues which are fully conserved within a strongly related group as defined by CLUSTALX are indicated by a double dot ; and residues which are fully conserved within a weakly related group are indicated by a single dot. (Vijaranakul et al, 1997), were also plated on high osmotic strength medium containing 7n5 % NaCl but showed no differences in growth.…”

Culturability and Survival of Marine, Freshwater and Clinical Pseudomonas aeruginosa

“…When a bacterium is subjected to osmotic stress, such as being inoculated into a hyperosmotic medium, it responds by rapidly accumulating compatible solutes (2,9). Staphylococcus aureus cells grown in a defined medium under conditions of high ionic stress (2.5 M NaCl) were significantly larger than cells grown under unstressed conditions, even though the cells grew much more slowly under stressed conditions (52). Similar observations of larger cells in a complex medium containing NaCl rather than in a medium alone have also been reported by others (20).…”