Lysis and aberrant morphology of Bacillus subtilis cells caused by surfactants and their relation to autolysin activity

The Alternative Sigma Factor SigL Influences Clostridioides difficile Toxin Production, Sporulation, and Cell Surface Properties

The alternative sigma factor SigL (Sigma-54) facilitates bacterial adaptation to the extracellular environment by modulating the expression of defined gene subsets. A homolog of the gene encoding SigL is conserved in the diarrheagenic pathogen Clostridioides difficile. To explore the contribution of SigL to C. difficile biology, we generated sigL-disruption mutants (sigL::erm) in strains belonging to two phylogenetically distinct lineages-the human-relevant Ribotype 027 (strain BI-1) and the veterinary-relevant Ribotype 078 (strain CDC1). Comparative proteomics analyses of mutants and isogenic parental strains revealed lineage-specific SigL regulons. Concomitantly, loss of SigL resulted in pleiotropic and distinct phenotypic alterations in the two strains. Sporulation kinetics, biofilm formation, and cell surface-associated phenotypes were altered in CDC1 sigL::erm relative to the isogenic parent strain but remained unchanged in BI-1 sigL::erm. In contrast, secreted toxin levels were significantly elevated only in the BI-1 sigL::erm mutant relative to its isogenic parent. We also engineered SigL overexpressing strains and observed enhanced biofilm formation in the CDC1 background, and reduced spore titers as well as dampened sporulation kinetics in both strains. Thus, we contend that SigL is a key, pleiotropic regulator that dynamically influences C. difficile's virulence factor landscape, and thereby, its interactions with host tissues and co-resident microbes.

Erythrocyte haemotoxicity profiling of snake venom toxins after nanofractionation

Snakebite is classified as a priority Neglected Tropical Disease by the World Health Organization. Understanding the pathology of individual snake venom toxins is of great importance when developing more effective snakebite therapies. Snake venoms may induce a range of pathologies, including haemolytic activity. Although snake venom-induced erythrocyte lysis is not the primary cause of mortality, haemolytic activity can greatly debilitate victims and contributes to systemic haemotoxicity. Current assays designed for studying haemolytic activity are not suitable for rapid screening of large numbers of toxic compounds. Consequently, in this study, a high-throughput haemolytic assay was developed that allows profiling of erythrocyte lysis, and was validated using venom from a number of medically important snake species (Calloselasma rhodostoma, Daboia russelii, Naja mossambica, Naja nigricollis and Naja pallida). The assay was developed in a format enabling direct integration into nanofractionation analytics, which involves liquid chromatographic separation of venom followed by high-resolution fractionation and subsequent bioassaying (and optional proteomics analysis), and parallel mass spectrometric detection. Analysis of the five snake venoms via this nanofractionation approach involving haemolytic assaying provided venom-cytotoxicity profiles and enabled identification of the toxins responsible for haemolytic activity. Our results show that the elapid snake venoms (Naja spp.) contained both direct and indirect lytic toxins, while the viperid venoms (C. rhodostoma and D. russelii) only showed indirect lytic activities, which required the addition of phospholipids to exert cytotoxicity on erythrocytes. The haemolytic venom toxins identified were mainly phospholipase A2s and cytotoxic three finger toxins. Finally, the applicability of this new analytical method was demonstrated using a conventional snakebite antivenom treatment and a small-molecule drug candidate to assess neutralisation of venom cytotoxins.

Autolysis-mediated membrane vesicle formation in Bacillus subtilis

It is known that Bacillus subtilis releases membrane vesicles (MVs) during the SOS response, which is associated with cell lysis triggered by the PBSX prophage-encoded cell-lytic enzymes XhlAB and XlyA. In this study, we demonstrate that MVs are released under various stress conditions: sucrose fatty acid ester (SFE; surfactant) treatment, cold shock, starvation, and oxygen deficiency. B. subtilis possesses four major host-encoded cell wall-lytic enzymes (autolysins; LytC, LytD, LytE, and LytF). Deletions of the autolysin genes abolished autolysis and the consequent MV production under these stress conditions. In contrast, deletions of xhlAB and xlyA had no effect on autolysis-triggered MV biogenesis, indicating that autolysis is a novel and prophage-independent pathway for MV production in B. subtilis. Moreover, we found that the cell lysis induced by the surfactant treatment was effectively neutralized by the addition of exogenous purified MVs. This result suggests that the MVs can serve as a decoy for the cellular membrane to protect the living cells in the culture from membrane damage by the surfactant. Our results indicate a positive effect of B. subtilis MVs on cell viability and provide new insight into the biological importance of the autolysis phenomenon in B. subtilis.

Preparation of silver-decorated Soluplus® nanoparticles and antibacterial activity towards S. epidermidis biofilms as characterized by STEM-CL spectroscopy

Biofilm infections present a serious problem because antibacterial drugs are not effective against mature biofilms or biofilms formed by drug-resistant bacteria. To address this issue, we developed a drug delivery system based on metal-decorated polymeric particles. Polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer (Soluplus®) is an amphiphilic polymer used in biomedical formulations, while silver nanoparticles are widely acknowledged to have high antibacterial activity. We prepared silver-decorated Soluplus® micelle nanoparticles with high antibacterial activity using the emulsion solvent diffusion method. Decoration of Soluplus® micelles with silver nanoparticles was found to increase their antibacterial activity. Scanning transmission electron microscopy-cathodoluminescence (STEM-CL) spectroscopy allows imaging of the spatial distribution of labeled targets and the chemical identification of materials. However, STEM-CL spectroscopy of fragile polymer materials is challenging. We optimized the STEM-CL spectroscopy technique to determine the distribution of silver nanoparticles in Soluplus® micelles. Additionally, the surface plasmon properties of the silver nanoparticles were successfully characterized without deactivation. The developed silver-decorated Soluplus® nanoparticles were effective against biofilm infections and have the potential to be applied for other biofilm-related diseases. Additionally, the optimized STEM-CL spectroscopy technique is expected to contribute to the analysis and imaging of fragile polymer materials, as well as other soft materials such as cells and tissues.

A surfactant polymer dressing potentiates antimicrobial efficacy in biofilm disruption

A 100% water-soluble surfactant polymer dressing (SPD) that is bio-compatible and non-ionic has been reported to improve wound closure in preliminary clinical studies. The mechanism of action of SPD in wound healing remains unclear. Biofilm infection is a significant problem that hinders proper wound closure. The objective of this study was to characterize the mechanism of action of SPD inhibition of bacterial biofilm development. Static biofilms (48 h) of the primary wound pathogens Pseudomonas aeruginosa (PA01), Staphylococcus aureus (USA300) were grown on polycarbonate membranes and treated with SPD with and without antibiotics for an additional 24 h. The standard antibiotics - tobramycin (10 μg/ml) for PA01 and rifampicin (10 μg/ml) for USA300, were used in these studies. Following 24 h treatment with and without antibiotics, the biofilms were characterized using scanning electron microscopy (SEM) structural imaging, in vitro imaging system (IVIS) proliferation imaging, colony forming units (CFU), viability assay, quantitative PCR (qPCR) for virulence gene expression. Because SPD is a surfactant based dressing, it potentially has a direct effect on Gram negative bacteria such as Pseudomonas primarily due to the lipid-based outer membrane of the bacteria. SPD is a surfactant based dressing that has potent anti-biofilm properties directly or in synergy with antibiotics.

Imaging of intracellular behavior of polymeric nanoparticles in Staphylococcus epidermidis biofilms by slit-scanning confocal Raman microscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy

In drug delivery systems employing polymeric nanoparticles, accurate delivery of drugs to target sites such as bacterial cells, cell tissues, and organelles is essential. In particular, when designing drug delivery systems for the treatment of the biofilm infections, evaluation of the interaction between polymeric nanoparticles and biofilm or bacterial cells using a simple technique is of significant importance. Here we develop two types of novel techniques for the biological imaging of the intracellular behavior of two types of polymeric nanoparticles, biodegradable chitosan-modified poly (dl-lactide-co-glycolide) (PLGA) nanoparticles and chitosan-modified polyvinyl caprolactam - polyvinyl acetate -polyethylene glycol graft copolymer (Soluplus®, Sol) nanoparticles, within a Staphylococcus epidermidis biofilm. As the first technique, Raman imaging of unstained biological materials using slit-scanning confocal Raman microscopy (unstained Raman imaging) was performed, and as the second, field-emission scanning electron microscopy with energy-dispersive X-ray spectroscopy analysis of biological materials labeled with quantum dots (SEM-QD imaging) was demonstrated. These analyses revealed differing localization of the respective nanoparticles within the biofilm in accordance with the specific interactions of PLGA nanoparticles and Sol nanoparticles with the biofilm. These novel techniques open the door to biological imaging and analyses with high spatial resolution, which will help to understand the efficacy of drug delivery to target materials.

Antifungal Activity of Diglycerin Ester of Fatty Acids against Yeasts and Its Comparison with Those of Sucrose Monopalmitate and Sodium Benzoate

The antifungal activities of diglycerin monoester of fatty acids (DGCs), which have been employed as food emulsifiers, were examined against three yeasts, Saccharomyces cerevisiae, Candida albicans and Candida utilis and were compared with those of sucrose monoester of palmitic acid (SC16) as another type of emulsifier and sodium benzoate (SB) as a weak acid food preservative. When the minimum growth inhibitory concentrations (MICs) of diglycerin monolaurate (DGC12) against these yeasts were determined 2 d after incubation in YM broth at pH5.0, they were relatively low, being 0.01% (w/v), for both S. cerevisiae and C. utilis, whereas was high, being 4.0% (w/v), for C. albicans. On the contrary, the MICs of sucrose monopalmitate (SC16) were high, being 3.0 and 4.0% (w/v), for the former two yeasts, respectively, but 0.6% (w/v) for the last yeast. In contrast to these emulsifiers, the MICs of sodium benzoate (SB) were similar independently upon the yeast strain, being in order 0.4, 0.3 and 0.5% (w/v), for the above yeasts, respectively. The anti-yeast activities of DGC12 and SC16 were gradually increased with a decrease in pH, in a manner similar to that of SB, except for the action of SC16 on C. albicans, for which the activity was more effective at pHs 5.0 and 6.0 than at pHs 4.0 and 7.0. Among DGCs tested having different fatty acid moieties in the molecule, lauroyl ester (DGC12) was more effective than myristoyl and palmitoyl esters against S. cerevisiae and C. utilis. The inhibitory effect of DGC12 on the yeast growth depended upon both the cell density and the strength of aeration during the treatment. Further, DGC12 was found to kill S. cerevisiae and C. utilis cells at a rather low concentration of 0.005% (w/v) in 50mM acetate buffer at pH5.0, although, against C. albicans cells, only slight fungicidal activity was demonstrated at a high concentration of 0.5% (w/v). The results obtained support the effectiveness of practical application of DGC12 to acidic foods for the control of growth and survival of general yeasts such as S. cerevisiae and C. utilis.

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.

Antibacterial activities of polymeric poly(dl-lactide-co-glycolide) nanoparticles and Soluplus® micelles against Staphylococcus epidermidis biofilm and their characterization

We have successfully prepared polymeric micelles based on polyvinyl caprolactam–polyvinyl acetate–polyethylene glycol graft copolymer (Soluplus®) for a drug delivery system on a biofilm.

Screen for agents that induce autolysis in Bacillus subtilis

The growing prevalence of antibiotic-resistant infections underscores the need to discover new antibiotics and to use them with maximum effectiveness. In response to these needs, we describe a screening protocol for the discovery of autolysis-inducing agents that uses two Bacillus subtilis reporter strains, SH-536 and BAU-102. To screen chemical libraries, autolysis-inducing agents were first identified with a BAU-102-based screen and then subdivided with SH-536 into two major groups: those that induce autolysis by their direct action on the cell membrane and those that induce autolysis secondary to inhibition of cell wall synthesis. SH-536 distinguishes between the two groups of autolysis-inducing agents by synthesizing and then releasing β-galactosidase (β-Gal) in late stationary phase at a time that cells have nearly stopped growing and are therefore tolerant of cell wall synthesis inhibitors. Four hits, named compound 2, compound 3, compound 5, and compound 24, obtained previously as inducers of autolysis by screening a 10,080-compound discovery library with BAU-102, were probed with SH-536 and found to release β-Gal, indicating that their mode of action was to permeabilize the B. subtilis cell membrane. The four primary hits inhibited growth in Staphylococcus aureus, Enterococcus faecium, Bacillus subtilis, and Bacillus anthracis, with MICs in the 12.5- to 25-μg/ml (20 to 60 μM) range. The four primary hits were further used to probe B. subtilis, and their action was partially characterized with respect to the dependence of induced autolysis on specific autolysins.

All genomic mutations in the antimicrobial surfactant-resistant mutant, Escherichia coli OW66, are involved in cell resistance to surfactant

The spontaneous antimicrobial surfactant-resistant mutant, Escherichia coli OW66, has been isolated, and its physiological properties have been characterized in our previous paper (Ishikawa et al., J Appl Microbiol 92:261-268, 2002b). This report revealed that strain OW66 had seven mutations in their chromosomal DNA by comparative genomic hybridization microarray, and that their alternative functions were involved in cell resistance to antimicrobial surfactants. These mutations were located in oppB, ydcR, IVR(vacJ-yfdC), rpoN, rpoB, rpoC, and soxR. Furthermore, seven of the single-mutated isogenic strains and seven of the six-mutated isogenic strains were constructed from strains OW6 (NBRC106482) and OW66, respectively, through homologous recombination, and their resistances to an antimicrobial surfactant were measured using the minimum inhibitory concentration method. These results revealed that all six-mutated strains were more sensitive than strain OW66, and that the soxR66 mutation was independently involved in antimicrobial surfactant resistance of E. coli cells. Expression of soxR66 and soxS was increased in both strains OW66 and OW6-soxR66 without the surfactant treatment by the quantitative real time-polymerase chain reaction analysis, compared with strain OW6. Two-dimensional polyacrylamide gel electrophoresis analysis also revealed that some proteins in the soxRS regulon, including Mn-SOD, were overexpressed in both strains OW66 and OW6-soxR66. These results indicate that the soxR66 mutation leads to the constitutive expression of the soxRS regulon, resulting in the acquired resistance of E. coli cells to an antimicrobial surfactant.

Surfactants in the Environment

Surfactants are a diverse group of chemicals that are best known for their wide use in detergents and other cleaning products. After use, residual surfactants are discharged into sewage systems or directly into surface waters, and most of them end up dispersed in different environmental compartments such as soil, water or sediment. The toxic effects of surfactants on various aquatic organisms are well known. In general, surfactants are present in the environment at levels below toxicity and in Croatia below the national limit. Most surfactants are readily biodegradable and their amount is greatly reduced with secondary treatment in wastewater treatment plants. The highest concern is the release of untreated wastewater or wastewater that has undergone primary treatment alone. The discharge of wastewater polluted with massive quantities of surfactants could have serious effects on the ecosystem. Future studies of surfactant toxicities and biodegradation are necessary to withdraw highly toxic and non-biodegradable compounds from commercial use and replace them with more environmentally friendly ones.

Differential assay for high-throughput screening of antibacterial compounds

The previously described Bacillus subtilis reporter strain BAU-102 is capable of detecting cell wall synthesis inhibitors that act at all stages of the cell wall synthesis pathway. In addition, this strain is capable of detecting compounds with hydrophobic/surfactant activity and alternative mechanisms of cell wall disruption. BAU-102 sequesters preformed beta-gal in the periplasm, suggesting leakage of beta-gal as the means by which this assay detects compound activities. A model is proposed according to which beta-gal release by BAU-102 reflects activation of pathways leading to autolysis. The authors also report a simplified high-throughput assay using BAU-102 combined with the fluorogenic substrate N-methylumbelliferyl-beta-D-galactoside as a single reagent. Cell wall inhibitors release beta-gal consistently only after 60 min of incubation, whereas compounds with surfactant activity show an almost immediate release. A high-throughput screen of a 480-compound library of known bioactives yielded 8 compounds that cause beta-gal release. These results validate the BAU-102 assay as an effective tool in antimicrobial drug discovery.

Effect of carbohydrate fatty acid esters on Streptococcus sobrinus and glucosyltransferase activity

Mutans streptococci are oral bacteria with a key role in the initiation of dental caries, because their glucosyltransferases synthesize polysaccharides from sucrose that allow them to colonize the tooth surface. Among the strategies to prevent dental caries that are being investigated are (1) the inhibition of bacterial growth of mutans streptococci or (2) the inhibition of glucosyltransferases involved in polysaccharide formation. Pure fatty acid esters of sucrose, maltose and maltotriose were synthesized by an enzyme-catalyzed process and tested as inhibitors of two glucosyltransferases of great homology, those from Streptococcus sobrinus and Leuconostoc mesenteroides NRRL B-512F. In spite of having their nonreducing end glucose blocked at 6-OH, they did not inhibit dextran synthesis. However, their effect on the growth of S. sobrinus in the solid and liquid phase was notable. 6-O-Lauroylsucrose, 6'-O-lauroylmaltose and 6"-O-lauroylmaltotriose at 100 microg/mL showed complete inhibition of S. sobrinus in agar plates. Consequently, these nontoxic derivatives are very promising for inclusion in oral-hygiene products aimed at disrupting plaque formation and preventing caries.

Characterization of a cationic surfactant-resistant mutant isolated spontaneously from Escherichia coli

AIMS

In order to investigate the mechanism of bacterial resistance to surfactants, a spontaneous mutant of Escherichia coli, OW66, resistant to a cationic surfactant cetyltrimethylammonium bromide (CTAB), was isolated and its physiological properties analysed.

METHODS AND RESULTS

Strain OW66 grew in M9 medium containing CTAB at 45 micromol l(-1), whereas its parent strain, OW6, did not, even at 15 micromol l(-1). The mutant was also resistant to some other surfactants, antibiotics, heavy metals, organic solvents and oxidants examined. To determine the differences in physiology between strains OW66 and OW6, the compositions of their cell surface structures were analysed. In strain OW66, the relative content of OmpC in particular was higher than that of OmpF, whereas a reverse situation was seen in OW6 strain. The lipopolysaccharide (LPS) profile was different between these strains, and altered LPS in strain OW66 was suggested to be involved in the resistance to CTAB.

CONCLUSIONS

A CTAB-resistant E. coli isolate possesses an altered outer membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

Treatment with a relatively low concentration of CTAB was found to introduce multi-drug resistance into bacterial cells. This acquired resistance should be taken into account with the frequent use of surfactants in industries and various environments.

Genetic evidence that antibacterial activity of lysozyme is independent of its catalytic function

A catalytically inactive mutant of hen egg white lysozyme was constructed by site-directed mutagenesis to elucidate the role of enzymatic activity on its antimicrobial activity against Gram-positive bacteria. The catalytic residue aspartic acid at position 52 of lysozyme was substituted with serine (D52S-Lz) and the mutant cDNA was inserted into a yeast expression vector, pYES-2. Western blot analysis indicated that the mutation did not affect secretion of the D52S-Lz lysozyme into the medium of the expressing Saccharomyces cerevisiae, INVSC1. In addition, circular dichroism and fluorescence spectral analysis revealed no change in the structure of D52S-Lz compared to that of wild-type (Wt-Lz) lysozyme. The mutation (D52S) abolished the catalytic activity of lysozyme. Antimicrobial tests against Staphylococcus aureus and Bacillus subtilis revealed that the catalytically inactive D52S-Lz was as bactericidal as the Wt-Lz lysozyme. Heat treatment leading to enzyme inactivation had no effect on the bactericidal activity of either wild-type or the mutant D52S-Lz lysozyme. The binding affinity of D52S-Lz to the isolated peptidoglycan of S. aureus was unaffected. Our results provide the first demonstration of direct genetic evidence that the antimicrobial activity of lysozyme is operationally independent of its muramidase activity, and strongly suggest the antimicrobial action of lysozyme is due to structural factors.

Phenotypes of Bacillus subtilis mutants lacking multiple class A high-molecular-weight penicillin-binding proteins

Examination of Bacillus subtilis strains containing multiple mutations affecting the class A high-molecular-weight penicillin-binding proteins (PBPs) 1, 2c, and 4 revealed a significant degree of redundancy in the functions of these three proteins. In rich media, loss of PBPs 2c and 4 resulted in no obvious phenotype. The slight growth and cell morphology defects associated with loss of PBP 1 were exacerbated by the additional loss of PBP 4 but not PBP 2c. Loss of all three of these PBPs slowed growth even further. In minimal medium, loss of PBPs 2c and 4 resulted in a slight growth defect. The decrease in growth rate caused by loss of PBP 1 was accentuated slightly by loss of PBP 2c and greatly by loss of PBP 4. Again, a lack of all three of these PBPs resulted in the slowest growth. Loss of PBP 1 resulted in a 22% reduction in the cell radius. Cultures of a strain lacking PBP 1 also contained some cells that were significantly longer than those produced by the wild type, and some of the rod-shaped cells appeared slightly bent. The additional loss of PBP 4 increased the number of longer cells in the culture. Slow growth caused by a mutation in prfA, a gene found in an operon with the gene encoding PBP 1, was unaffected by the additional loss of PBPs 2c and 4, whereas loss of both prfA and PBP 1 resulted in extremely slow growth and the production of highly bent cells.

Evidence that tetracycline analogs whose primary target is not the bacterial ribosome cause lysis of Escherichia coli

The modes of action of atypical tetracyclines that do not directly inhibit bacterial protein synthesis were investigated. The analogs tested, chelocardin, anhydrotetracycline, 6-thiatetracycline, anhydrochlortetracycline, and 4-epi-anhydrochlortetracycline, were bactericidal and caused the lysis of Escherichia coli accompanied by the release of the cytoplasmic enzyme beta-galactosidase into the supernatant. Examination by electron microscopy demonstrated that cells exposed to these analogs underwent marked morphological alterations that included the formation of numerous ghosts and the appearance of cellular debris in the culture medium. Although atypical tetracyclines promoted lysis in intact organisms, they did not cause lysis of E. coli spheroplasts, indicating that the analogs do not directly destroy the cytoplasmic membrane. These agents may promote cell lysis and death by interfering with the membrane's electrochemical gradient, which in turn leads to stimulation of autolytic enzyme activity and cellular lysis. The results support recently published data which indicate that tetracyclines are divisible into two classes on the basis of their modes of action.