Salt-induced Contraction of Bacterial Cell Walls

Abstract

Intact Bacillus megaterium cells were found to contract as much as 26% in terms of dextran-impermeable volume when transferred from water to unbuffered, nonplasmolyzing NaCl solutions. This shrinkage appeared to be primarily due to electrostatic wall contraction rather than to any osmotic response of the cells. A variety of salts (but not sucrose) added to water suspensions of isolated cell walls caused protons to be released from the walls with resultant lowering of suspension pH and contraction of the structures. In effect, B. megaterium walls behaved as flexible, amphoteric polyelectrolytes, and their compactness in aqueous suspensions was affected by changes in environmental ionic strength and pH. Isolated walls were most compact in low ionic strength media with a pH of about 4, a value close to the apparent isoelectric pH of wall peptidoglycan. Electrostatic attractions appeared to play a major role in determining the compactness of highly contracted walls, and the walls responded to increased environmental ionic strength by expanding. In contrast, electrostatic repulsions were dominant in highly expanded walls, and increased environmental ionic strength induced wall contraction. Walls of whole bacteria also shrank when the cells were plasmolyzed. This second type of contraction seemed to result from relief of wall tension during plasmolysis, and it could be induced with nonionic solutes. Thus, cell wall tone in B. megaterium appeared to be set both by mechanical tension and by electrostatic interactions among wall ions.Many investigations have shown shrinkage of whole bacterial cells after transfer from dilute to concentrated salt solutions. For example, Johnson and Harvey (13) concluded from measurements of packed cell volume and light scattering that gram-negative Achromobacter (Photobacterium) fischeri cells shrank as much as 27% in volume when transferred from seawater to 2.5 times concentrated seawater. Salt-induced contraction of many other gram-negative bacteria has been reported (3,4,12,15), and there appeared to be a close association between whole cell shrinkage and observable plasmolysis.Gram-positive bacteria also can be made to shrink in salt solutions, even though they are not readily plasmolyzed. Dutky (unpublished data, cited in reference 14) found that Bacillus megaterium cells contracted from 15 to 25% when transferred from normal broth to broth containing 2 M NaCl. Mitchell and Moyle (18) indicated that Staphylococcus aureus cells contracted from 2 to 3% in linear dimensions, without becoming visibly plasmolyzed, when transferred from 0.1 to 1.0 M NaCl solution.Salt-induced shrinkage of bacteria has been interpreted as an osmotic response of the cells mainly because the effect can be produced in easily plasmolyzed, gram-negative organisms with either electrolytes or nonelectrolytes (3,4,13,15). Our results, however, indicate that salt-induced contraction of gram-positive B. megaterium cells does not primarily involve osmotic plasmolysis, but that it is caused mainly by electrostatic contr...

Osmoregulation and compatible solutes in eubacteria

“…Thus the situation in Gram-negative bacteria appears similar to that in Gram-positive bacteria, where molecular sieving effects have been observed in the cell wall [14], and contraction of the dextran-impermeable space of as much as 26% was found in non-plasmolysing NaC1 solutions [15]. This effect was attributed to the ionic nature of peptidoglycan and teichoic and teichuronic acids [15]. These findings not only have implications for the whole concept of the osmoregulation of these bacteria, but also are of great concern for determinations of cellular volume spaces and for discussions about function and location of cell bound cations.…”

Detergent-resistant variants of Bacillus subtilis with reduced cell diameter

“…The cell circumference data reported here must differ from the circumferences of living cells. In vivo, the cell walls will be hydrated, stretched by osmotic pressure (24), and subject to effects of inorganic ions (24,29). Indeed, the values of dry weight per calculated cell volume (Table 3) indicate that shrinkage of cell circumference occurred during specimen preparation.…”

Stability of Cells Fixed With Glutaraldehyde and Acrolein

“…Electrostatic repulsion of charges on the macromolecules of the cell would tend to cause swelling of the cells at low ionic strengths, exerting what we refer to as "electrostatic stress ." This phenomenon is observed in ion exchange resins, in erythrocytes fixed in osmium at high pH (Carstensen and Smearing, 1967), and in bacterial cell walls (Carstensen and Marquis, 1968 ;Marquis, 1968) . When cells are suspended in low ionic strength media their internal conductivities are determined primarily by the counterions of fixed charges on the macromolecular solutes in the cytoplasm (Carstensen and Smearing, 1967) .…”