Capacity of the outer membrane of a gram-negative marine bacterium in the presence of cations to prevent lysis by Triton X-100

Improved in vitro assay for determining the mucin adherence of bacteria sensitive to Triton X-100 treatment

Mucin-associated microbiota are in relatively close contact with the intestinal epithelium and may thus have a more pronounced effect on host health. We have previously developed a simple mucin agar assay to simulate initial mucus colonization by intestinal microbial communities. Adherence of microbiota was estimated using flow cytometry after detachment with Triton X-100. In this study, the effect of this detergent on the cultivability of both virulent and commensal strains was investigated. Mucin attachment of selected strains was evaluated using the mucin adhesion assay. Bacteria were dislodged from the mucin surface by incubation with Triton or from the whole mucin agar layer using a stomacher. Mechanical extraction resulted in 1.24 ± 0.42, 2.69 ± 0.44, and 1.56 ± 0.85 log CFU/mL higher plate counts of Lactobacillus rhamnosus, Bacillus cereus, and Escherichia coli strains, respectively, than the chemical method. The sensitivity of bacteria to Triton varied among microbial species and strains. Among others, Triton inhibited the growth of Salmonella enterica LMG 10396 and Pseudomonas aeruginosa LMG 8029 on laboratory media, although these bacteria maintained their viability during this treatment. Only Gram-positive strains, Enterococcus hirae LMG 6399 and L. rhamnosus GG, were not affected by this detergent. Therefore, the mechanical method is recommended for the extraction of mucin-adhered bacteria that are sensitive to Triton, especially when followed by traditional cultivation techniques. However, this approach can also be recommended for strains that are not affected by this detergent, because it resulted in higher recovery of adhered L. rhamnosus GG compared to the chemical extraction.

Polymyxin B induces lysis of marine pseudoalteromonads

Polymyxin B (PMB) is a cationic antibiotic that interacts with the envelopes of gram-negative bacterial cells. The therapeutic use of PMB was abandoned for a long time due to its undesirable side effects; however, the spread of resistance to currently used antibiotics has forced the reevaluation of PMB for clinical use. Previous studies have used enteric bacteria to examine the mode of PMB action, resulting in a somewhat limited understanding of this process. This study examined the effects of PMB on marine pseudoalteromonads and demonstrates that the frequently accepted view that "what is true for Escherichia coli is true for all bacteria" does not hold true. We show here that in contrast to the growth inhibition observed for enteric bacteria, PMB induces lysis of pseudoalteromonads, which is not prevented by high concentrations of divalent cations. Furthermore, we demonstrate that a high membrane voltage is required for the interaction of PMB with the cytoplasmic membranes of pseudoalteromonads, further elucidating the mechanisms by which PMB interacts with the cell envelopes of those gram-negative bacteria.

In vivo 23Na nuclear magnetic resonance study of maintenance of a sodium gradient in the ruminal bacterium Fibrobacter succinogenes S85

Sodium gradients (DeltapNa) were measured in resting cells of Fibrobacter succinogenes by in vivo 23Na nuclear magnetic resonance using Tm(DOTP)5- [thulium(III) 1,4,7,10-tetraazacyclododecane-N',N",N"'-tetramethylenephosphonate] as the shift reagent. This bacterium was able to maintain a DeltapNa of -55 to -40 mV for extracellular sodium concentrations ranging from 30 to 200 mM. Depletion of Na+ ions during the washing steps led to irreversible damage (modification of glucose metabolism and inability to maintain a sodium gradient).

Effect of defined lipopolysaccharide core defects on resistance of Salmonella typhimurium to freezing and thawing and other stresses

A family of mutants of Salmonella typhimurium with altered lipopolysaccharide (LPS) core chain lengths were assessed for sensitivity to freeze-thaw and other stresses. Deep rough strains with decreased chain length in the LPS core were more susceptible to novobiocin, polymyxin B, bacitracin, and sodium lauryl sulfate during growth, to ethylenediaminetetraacetic acid and sodium lauryl sulfate in resting suspension, and to slow and rapid freeze-thaw in water and saline, and these strains exhibited more outer membrane damage than the wild type or less rough strains. Variations in the LPS chain length did not dramatically affect the sensitivity of the strains to tetracycline, neomycin, or NaCl in growth conditions or the degree of freeze-thaw-induced cytoplasmic membrane damage. The deeper rough isogenic strains incorporated larger quantities of less-stable LPS and less protein into the outer membrane than did the wild type or less rough mutants, indicating that the mutations affected outer membrane synthesis or organization or both. Nikaido's model of the role of LPS and protein in determining the resistance of gram-negative bacteria to low-molecular-weight hydrophobic antibiotics is discussed in relation to the stress of freeze-thaw.

Interaction of lipopolysaccharide with detergents and its possible role in the detergent resistance of the outer membrane of Gram-negative bacteria

In the presence of MgCl2, amounts of detergents which disrupted phospholipid vesicles caused lipopolysaccharide I from Proteus mirabilis to aggregate and form vesicular, membrane-like structures. Vesicle formation with P. mirabilis lipopolysaccharide II containing longer O-polysaccharide chains was extremely poor. Lipopolysaccharides of Salmonella minnesota R mutants (chemotypes Ra, Rc and Re) displayed a growing tendency for vesicle formation with increasing deficiency of the R core polysaccharide. Lipopolysaccharides of chemotypes Rc and Re produced vesicles even in the absence of MgCl2 and detergent. Spherical aggregates consisting of P. mirabilis lipopolysaccharide I MgCl2 and detergent were unable to either entrap or retain [14C]-sucrose, [3H=inulin or [3H]dextran. On the other hand, S. minnesota R mutant lipopolysaccharides of chemotypes Rc and Re could entrap all three saccharides and retain them for at least short periods of time. Leakage of [3H]-inulin out of re-lipopolysaccharide vesicles was greatly retarded by addition of MgCl2 to the vesicle system. Incorporation of P. mirabilis lipopolysaccharide I or S. minnesota Rc lipopolysaccharide into phospholipid vesicles protected these model membranes from disruption by detergent. This suggested a similar protective function of lipopolysaccharide in the outer membrane of enteric bacteria against the action of surfactants occurring in their normal intestinal habitat.

Lysis of halophilic Vibrio alginolyticus and Vibrio costicolus induced by chaotropic anions

High concentration (1.0 M) of KSCN, but not of NaSCN, induced lysis of slightly halophilic Vibrio alginolyticus and moderately halophilic Vibrio costicolus, and the decrease in absorbance of the cell suspension was complete after 30 min at 25 degrees C. Replacement of K+ with Na+ effectively prevented the lysis by SCN-.K+ salts of NO3-, Br- and I-, however, induced no significant lysis. In electron micrographs, a prolonged exposure of the cells of V. alginolyticus to 1.0 M KSCN displaced the nucleoplasm to maintain close contact with the cell membranes. After 40 min of interaction, 50% of the cellular protein, 96% of RNA and 94% of DNA were recovered in the lysed cells. In contrast to lysis in hypotonic conditions, the lysis induced by KSCN is due mainly to a partial release of protein from the cells. V. costicolus was more susceptible to SCN- than V. alginolyticus, whereas nonhalophilic Escherichia coli was resistant to 1.0 M KSCN. Thus, lysis by SCN- is characteristic of halophilic bacteria and cell membranes of more halophilic bacteria are more susceptible to chaotropic anions. The protective effect of Na+ observed here was considered to be manifested by specific interactions of Na+ with components of cell membranes, thereby rendering their structures resistant to the action of chaotropic anions.

Growth of heat-injured Vibrio parahaemolyticus in media supplemented with various cations

Mid- to late logarithmic growth phase cells of Vibrio parahaemolyticus grown in tryptic soy broth (TSB) containing 0.5, 3.0, and 7.5% NaCl were heated for 8 min at 45 degrees C in 0.1 M phosphate buffer (pH 7.2) containing 3% NaCl. Colony formation on thiosulfate-citrate-bile salts-sucrose agar (TCBS) containing 2% NaCl was greatest for unheated cells that had been grown in 7.5% NaCl-TSB; cells grown in 0.5% NaCl-TSB formed a greater number of colonies on 1.0% NaCl-TCBS. Thermal injury was evident in heated cells, regardless of the NaCl concentration in TSB growth medium. The effects of Mg2+, K+, and Li+ added as chlorides to 0.5% NaCl-TSB on the growth of nonheated and heated V. parahaemolyticus were studied. Lower levels of Mg2+ and slightly higher levels of K+ were required to replace Na+ in TSB inoculated with thermally injured cells that had been originally grown in 3.0 and 7.5% NaCl-TSB. LiCl had an inhibitory effect on both nonheated and heated cells when present in the recovery medium (0.5% NaCl-TSB) at concentrations as low as 0.5%. Increased numbers of colonies were formed by heated cells plated in MgCl2-supplemented TCBS, regardless of the NaCl concentration in the original growth medium. Potassium had little, if any, effect on colony formation by nonheated V. parahaemolyticus recovered on TCBS and may have had a detrimental effect on heat-injured cells.

Diminution of outer membrane permeability by Mg2+ in a marine pseudomonad

Intact cells of the marine pseudomonad MB-45, in the presence of optimal Mg2+, exhibited little alkaline phosphatase activity as judged by the hydrolysis of p-nitrophenylphosphate. Sonic extracts, in contrast, were rich in this activity. Removal of the loosely bound outer layer did not diminish this crypticity of alkaline phosphatase, but decreasing the concentration of Mg2+ in the suspending medium progressively exposed the alkaline phosphatase. Since MB-45 did not liberate alkaline phosphatase into the surrounding medium even in the absence of Mg2+ and since this enzyme is localized in the periplasmic space, it can be concluded that the crypticity was due to the exclusion of p-nitrophenylphosphate by the outer membrane. Mg2+ is apparently essential for the full expression of this limited permeability.