Strains of Bacillus cereus vary in the phenotypic adaptation of their membrane lipid composition in response to low water activity, reduced temperature and growth in rice starch

Polyextremophile engineering: a review of organisms that push the limits of life

Nature exhibits an enormous diversity of organisms that thrive in extreme environments. From snow algae that reproduce at sub-zero temperatures to radiotrophic fungi that thrive in nuclear radiation at Chernobyl, extreme organisms raise many questions about the limits of life. Is there any environment where life could not "find a way"? Although many individual extremophilic organisms have been identified and studied, there remain outstanding questions about the limits of life and the extent to which extreme properties can be enhanced, combined or transferred to new organisms. In this review, we compile the current knowledge on the bioengineering of extremophile microbes. We summarize what is known about the basic mechanisms of extreme adaptations, compile synthetic biology's efforts to engineer extremophile organisms beyond what is found in nature, and highlight which adaptations can be combined. The basic science of extremophiles can be applied to engineered organisms tailored to specific biomanufacturing needs, such as growth in high temperatures or in the presence of unusual solvents.

Comparative genomics, pangenomics, and phenomic studies of Pectobacterium betavasculorum strains isolated from sugar beet, potato, sunflower, and artichoke: insights into pathogenicity, virulence determinants, and adaptation to the host plant

Introduction

Bacteria of genus Pectobacterium, encompassing economically significant pathogens affecting various plants, includes the species P. betavasculorum, initially associated with beetroot infection. However, its host range is much broader. It causes diseases of sunflower, potato, tomato, carrots, sweet potato, radish, squash, cucumber, and chrysanthemum. To explain this phenomenon, a comprehensive pathogenomic and phenomic characterisation of P. betavasculorum species was performed.

Methods

Genomes of P. betavasculorum strains isolated from potato, sunflower, and artichoke were sequenced and compared with those from sugar beet isolates. Metabolic profiling and pathogenomic analyses were conducted to assess virulence determinants and adaptation potential. Pathogenicity assays were performed on potato tubers and chicory leaves to confirm in silico predictions of disease symptoms. Phenotypic assays were also conducted to assess the strains ability to synthesise homoserine lactones and siderophores.

Results

The genome size ranged from 4.675 to 4.931 kbp, and GC % was between 51.0% and 51.2%. The pangenome of P. betavasculorum is open and comprises, on average, 4,220 gene families. Of these, 83% of genes are the core genome, and 2% of the entire pangenome are unique genes. Strains isolated from sugar beet have a smaller pangenome size and a higher number of unique genes than those from other plants. Interestingly, genomes of strains from artichoke and sunflower share 391 common CDS that are not present in the genomes of other strains from sugar beet or potato. Those strains have only one unique gene. All strains could use numerous sugars as building materials and energy sources and possessed a high repertoire of virulence determinants in the genomes. P. betavasculorum strains were able to cause disease symptoms on potato tubers and chicory leaves. They were also able to synthesise homoserine lactones and siderophores.

Discussion

The findings underscore the adaptability of P. betavasculorum to diverse hosts and environments. Strains adapted to plants with high sugar content in tissues have a different composition of fatty acids in membranes and a different mechanism of replenishing nitrogen in case of deficiency of this compound than strains derived from other plant species. Extensive phenomics and genomic analyses performed in this study have shown that P. betavasculorum species is an agronomically relevant pathogen.

Biological activity of pectic polysaccharides investigated through biomembrane models formed at the air-water interface

Pectin, a polysaccharide with potential bioactivity, was inserted in the aqueous subphase of monolayers of the selected lipids DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and DPPE (1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine), representing mammalian and bacterial membranes, respectively. Pectin condensed both monolayers but made the DPPC monolayer more fluid, while for DPPE, it made its monolayer more rigid, as detected with dynamic interfacial rheology. Complementary data using surface potential, infrared spectroscopy, and Brewster angle microscopy also showed distinctive effects of pectin on DPPE and DPPC. We believe these data can be correlated with the action of this polysaccharide with biological lipidic surfaces with different polar heads, which may be relevant, generally speaking, to understanding the molecular mechanism of this bioactive compound for pharmaceutical purposes.

A bactericide peptide changing the static and dilatational surface elasticity properties of zwitterionic lipids at the air-water interface: Relationship with the thermodynamic, structural and morphological properties

In this paper, we studied how different hydrophilicity degrees of the polar groups of the lipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) influence the interaction of the antibiotic peptide vancomycin (VC), affecting the physicochemical properties of the monolayers, including thermodynamic, rheological, structural and morphological ones. Lipid Langmuir monolayers were prepared at air-water interfaces with VC aqueous solution as subphase and characterized with tensiometry, Brewster angle microscopy, infrared spectroscopy, dilatational, and interfacial shear rheology. The presence of PC or PE groups as polar head groups of the phospholipid monolayers modulated the interaction of VC adsorbing from the aqueous subphase since for DPPC, vancomycin condenses the monolayer, making it less stable, fluid, and more disordered. In contrast, for DPPE, vancomycin expands the monolayer, making it more stable, keeping the compressibility, and leading to the formation of interfacial aggregates, which are not observed for DPPC. We concluded thatelectrostatic interactions induced the insertion of the peptide into the polar heads of the monolayers (DPPE), while hydrophobic interactions, in addition to ion-dipole interactions, induced the adsorption of the peptide onto the polar head of the monolayers (DPPC).

Short-Chain and Unsaturated Fatty Acids Increase Sequentially From the Lag Phase During Cold Growth of Bacillus cereus

Fatty acids of two mesophilic and one psychrotrophic strains of the foodborne pathogen Bacillus cereus were analyzed by gas chromatography coupled to mass spectrometry during growth at cold (10 and 12°C) vs. optimal (30°C) temperatures and during the whole growth process (6–7 sampling times) from lag to stationary phase. In all these strains, a sequential change of fatty acids during cold growth was observed. Fatty acids were modified as soon as the end of lag, with an increase of the short-chain fatty acids (less than 15 carbons), particularly i13. These short-chain fatty acids then reached a maximum at the beginning of growth and eventually decreased to their initial level, suggesting their importance as a rapid cold adaptation mechanism for B. cereus. In a second step, an increase in Δ^5,10 di-saturated fatty acids and in monounsaturated fatty acids in Δ⁵ position, at the expense of unsaturation in Δ¹⁰, started during exponential phase and continued until the end of stationary phase, suggesting a role in growth consolidation and survival at cold temperatures. Among these unsaturated fatty acids, those produced by unsaturation of n16 increased in the three strains, whereas other unsaturated fatty acids increased in some strains only. This study highlights the importance of kinetic analysis of fatty acids during cold adaptation.

Morphological Features and Cold-Response Gene Expression in Mesophilic Bacillus cereus Group and Psychrotolerant Bacillus cereus Group under Low Temperature

At low temperatures, psychrotolerant B. cereus group strains exhibit a higher growth rate than mesophilic strains do. However, the different survival responses of the psychrotolerant strain (BCG³⁴) and the mesophilic strain (BCG^T) at low temperatures are unclear. We investigated the morphological and genomic features of BCG^T and BCG³⁴ to characterize their growth strategies at low temperatures. At low temperatures, morphological changes were observed only in BCG^T. These morphological changes included the elongation of rod-shaped cells, whereas the cell shape in BCG³⁴ was unchanged at the low temperature. A transcriptomic analysis revealed that both species exhibited different growth-related traits during low-temperature growth. The BCG^T strain induces fatty acid biosynthesis, sulfur assimilation, and methionine and cysteine biosynthesis as a survival mechanism in cold systems. Increases in energy metabolism and fatty acid biosynthesis in the mesophilic B. cereus group strain might explain its ability to grow at low temperatures. Several pathways involved in carbohydrate mechanisms were downregulated to conserve the energy required for growth. Peptidoglycan biosynthesis was upregulated, implying that a change of gene expression in both RNA-Seq and RT-qPCR contributed to sustaining its growth and rod shape at low temperatures. These results improve our understanding of the growth response of the B. cereus group, including psychrotolerant B. cereus group strains, at low temperatures and provide information for improving bacterial inhibition strategies in the food industry.

Bactericidal urea crown ethers target phosphatidylethanolamine membrane lipids

An increasing number of people are infected with antibiotic-resistant bacteria each year, sometimes with fatal consequences. In this manuscript, we report a novel urea-functionalized crown ether that can bind to the bacterial lipid phosphatidylethanolamine (PE), facilitate PE flip-flop and displays antibacterial activity against the Gram-positive bacterium Bacillus cereus with a minimum inhibitory concentration comparable to that of the known PE-targeting lantibiotic duramycin.

Bacillus toyonensis COPE52 Modifies Lipid and Fatty Acid Composition, Exhibits Antifungal Activity, and Stimulates Growth of Tomato Plants Under Saline Conditions

Salinity is one of the most important factors that limit the productivity of agricultural soils. Certain plant growth-promoting bacteria (PGPB) have the ability to stimulate the growth of crop plants even under salt stress. In the present study, we analysed the potential of PGPB Bacillus toyonensis COPE52 to improve the growth of tomato plants and its capacity to modify its membrane lipid and fatty acid composition under salt stress. Thus, strain COPE52 increased the relative amount of branched chain fatty acids (15:0i and 16:1∆9) and accumulation of an unknown membrane lipid, while phosphatidylethanolamine (PE) levels decreased during growth with 100 and 200 mM NaCl. Importantly, direct and indirect plant growth-promoting (PGP) mechanisms of B. toyonensis COPE52, such as indole-3-acetic acid (IAA), protease activity, biofilm formation, and antifungal activity against Botrytis cinerea, remained unchanged in the presence of NaCl in vitro, compared to controls without salt. In a greenhouse experiment, tomato plants (Lycopersicon esculentum 'Saladette') showed increased shoot and root length, higher dry biomass, and chlorophyll content when inoculated with B. toyonensis COPE52 at 0 and 100 mM NaCl. In summary, these results indicate that Bacillus toyonensis COPE52 can modify cell membrane lipid components as a potential protecting mechanism to maintain PGP traits under saline-soil conditions.

Bacillus anthracis Responds to Targocil-Induced Envelope Damage through EdsRS Activation of Cardiolipin Synthesis

Bacillus anthracis is a spore-forming bacterium that causes devastating infections and has been used as a bioterror agent. This pathogen can survive hostile environments through the signaling activity of two-component systems, which couple environmental sensing with transcriptional activation to initiate a coordinated response to stress. In this work, we describe the identification of a two-component system, EdsRS, which mediates the B. anthracis response to the antimicrobial compound targocil. Targocil is a cell envelope-targeting compound that is toxic to B. anthracis at high concentrations. Exposure to targocil causes damage to the cellular barrier and activates EdsRS to induce expression of a previously uncharacterized cardiolipin synthase, which we have named ClsT. Both EdsRS and ClsT are required for protection against targocil-dependent damage. Induction of clsT by EdsRS during targocil treatment results in an increase in cardiolipin levels, which protects B. anthracis from envelope damage. Together, these results reveal that a two-component system signaling response to an envelope-targeting antimicrobial induces production of a phospholipid associated with stabilization of the membrane. Cardiolipin is then used to repair envelope damage and promote B. anthracis viability.IMPORTANCE Compromising the integrity of the bacterial cell barrier is a common action of antimicrobials. Targocil is an antimicrobial that is active against the bacterial envelope. We hypothesized that Bacillus anthracis, a potential weapon of bioterror, senses and responds to targocil to alleviate targocil-dependent cell damage. Here, we show that targocil treatment increases the permeability of the cellular envelope and is particularly toxic to B. anthracis spores during outgrowth. In vegetative cells, two-component system signaling through EdsRS is activated by targocil. This results in an increase in the production of cardiolipin via a cardiolipin synthase, ClsT, which restores the loss of barrier function, thereby reducing the effectiveness of targocil. By elucidating the B. anthracis response to targocil, we have uncovered an intrinsic mechanism that this pathogen employs to resist toxicity and have revealed therapeutic targets that are important for bacterial defense against structural damage.

Bacillus cereus cshA Is Expressed during the Lag Phase of Growth and Serves as a Potential Marker of Early Adaptation to Low Temperature and pH

The spore-forming bacterium B. cereus is a major cause of foodborne outbreaks in Europe. Some B. cereus strains can grow at low temperatures and low pH in many processed foods. Modeling of the bacterial lag time is hampered by a lack of knowledge of the timing of events occurring during this phase. In this context, the identification of lag phase markers, not currently available, could be a real advance for the better prediction of lag time duration. Currently, no molecular markers of this phase are available. By determining that cshA was always expressed early during the lag phase, we provide a molecular marker of the early adaptation process of B. cereus cells when exposed to low temperature and pH.

Bacterial adaptation is characterized by a lag phase during which cells do not multiply or modify their physiology to cope with the constraints of their environment. Our aim was to determine a sequence of events during the lag phase of growth at low temperature and pH for three Bacillus cereus strains. The onsets of expression of two genes, one of which is essential for stress adaptation (cshA, coding for a RNA helicase) and one of which is involved in the transition between lag phase and exponential phase (abrB, coding for a transition regulator), were determined using fluorescent transcriptional reporter systems. Regardless of the stressing conditions and the tested strains, the cshA promoter was active very early, while the biomass increased and always did so before the first cell division. At 12°C and pH 7.0, the onset of cshA promoter activity occurred at between 3 h and 7 h, while the bacterial counts started to increase at between 12 h and 13 h. At pH 5.0 and at 20°C or 30°C, the onset of cshA promoter activity occurred before 1 h and earlier than at pH 7.0. In contrast, the onset of abrB promoter activity depended on the strain and the stressing conditions. In the ATCC 14579 strain, the onset of abrB promoter activity always started at between 30 min and 3 h, before biomass increased and cell division occurred. For the other strains, it took place along with the first cell division at 12°C but did so much later during growth under the other tested conditions.

IMPORTANCE The spore-forming bacterium B. cereus is a major cause of foodborne outbreaks in Europe. Some B. cereus strains can grow at low temperatures and low pH in many processed foods. Modeling of the bacterial lag time is hampered by a lack of knowledge of the timing of events occurring during this phase. In this context, the identification of lag phase markers, not currently available, could be a real advance for the better prediction of lag time duration. Currently, no molecular markers of this phase are available. By determining that cshA was always expressed early during the lag phase, we provide a molecular marker of the early adaptation process of B. cereus cells when exposed to low temperature and pH.

Effect of Polyvalence on the Antibacterial Activity of a Synthetic Peptide Derived from Bovine Lactoferricin against Healthcare-Associated Infectious Pathogens

Antimicrobial peptides (AMPs) are gaining interest as potential therapeutic agents. Peptides derived from bovine lactoferricin B (LfcinB) have been reported to exhibit antimicrobial activity, and the LfcinB RRWQWR sequence is the smallest known motif that exhibits antibacterial and cytotoxic activity. Our goal was to examine the effect of multicopy arrangements of the RRWQWR motif, on its antibacterial activity against healthcare-associated infections (HCAIs). Linear and branched peptides containing the RRWQWR motif were generated using solid phase peptide synthesis-Fmoc/tBu methodology, purified, and characterized using reverse phase-high performance liquid chromatography and matrix-assisted laser desorption/ionization time of flight mass spectrometry. For each peptide, the antibacterial activity against Staphylococcus aureus (ATCC 25923 and 33591 strains) and Klebsiella pneumoniae (ATCC 13883 and 700603 strains) was assessed by measuring the minimum inhibitory and the minimum bactericidal concentrations, in the exponential phase. Cells were observed by scanning electron microscopy, and the hemolytic activity of the peptides was assessed. The overall results demonstrate that, compared to linear analogues, polyvalent presentation of the RRWQWR motif enhances its antibacterial activity against both Gram-negative and Gram-positive bacteria even on resistant strain.

Expression of the genes encoding the CasK/R two-component system and the DesA desaturase during Bacillus cereus cold adaptation

Two-component systems (TCS) allow a cell to elaborate a variety of adaptive responses to environment changes. The recently discovered CasK/R TCS plays a role in the optimal unsaturation of fatty acids necessary for cold adaptation of the foodborne-pathogen Bacillus cereus Here, we showed that the promoter activity of the operon encoding this TCS was repressed during growth at low temperature in the stationary phase in the parental strain when compared to the casK/R mutant, suggesting that CasR negatively regulates the activity of its own promoter in these conditions. The promoter activity of the desA gene encoding the Δ5 fatty acid desaturase, providing unsaturated fatty acids (UFAs) required for low temperature adaptation, was repressed in the casK/R mutant grown at 12°C versus 37°C. This result suggests that CasK/R activates desA expression during B. cereus growth at low temperature, allowing an optimal unsaturation of the fatty acids. In contrast, desA expression was repressed during the lag phase at low temperature in presence of UFAs, in a CasK/R-independent manner. Our findings confirm that the involvement of this major TCS in B. cereus cold adaptation is linked to the upregulation of a fatty acid desaturase.

Extraordinary solute-stress tolerance contributes to the environmental tenacity of mycobacteria

Mycobacteria are associated with a number of well-characterized diseases, yet we know little about their stress biology in natural ecosystems. This study focuses on the isolation and characterization of strains from Yellowstone National Park (YNP) and Glacier National Park (GNP; USA), the majority of those identified were Mycobacterium parascrofulaceum, Mycobacterium avium (YNP) or Mycobacterium gordonae (GNP). Generally, their windows for growth spanned a temperature range of > 60 °C; selected isolates grew at super-saturated concentrations of hydrophobic stressors and at levels of osmotic stress and chaotropic activity (up to 13.4 kJ kg(-1) ) similar to, or exceeding, those for the xerophilic fungus Aspergillus wentii and solvent-tolerant bacterium Pseudomonas putida. For example, mycobacteria grew down to 0.800 water activity indicating that they are, with the sole exception of halophiles, more xerotolerant than other bacteria (or any Archaea). Furthermore, the fatty-acid composition of Mycobacterium cells grown over a range of salt concentrations changed less than that of other bacteria, indicating a high level of resilience, regardless of the stress load. Cells of M. parascrofulaceum, M. smegmatis and M. avium resisted the acute, potentially lethal challenges from extremes of pH (< 1; > 13), and saturated MgCl2 solutions (5 M; 212 kJ kg(-1) chaotropicity). Collectively, these findings challenge the paradigm that bacteria have solute tolerances inferior to those of eukaryotes.

Role of fatty acids in Bacillus environmental adaptation

The large bacterial genus Bacillus is widely distributed in the environment and is able to colonize highly diverse niches. Some Bacillus species harbor pathogenic characteristics. The fatty acid (FA) composition is among the essential criteria used to define Bacillus species. Some elements of the FA pattern composition are common to Bacillus species, whereas others are specific and can be categorized in relation to the ecological niches of the species. Bacillus species are able to modify their FA patterns to adapt to a wide range of environmental changes, including changes in the growth medium, temperature, food processing conditions, and pH. Like many other Gram-positive bacteria, Bacillus strains display a well-defined FA synthesis II system that is equilibrated with a FA degradation pathway and regulated to efficiently respond to the needs of the cell. Like endogenous FAs, exogenous FAs may positively or negatively affect the survival of Bacillus vegetative cells and the spore germination ability in a given environment. Some of these exogenous FAs may provide a powerful strategy for preserving food against contamination by the Bacillus pathogenic strains responsible for foodborne illness.

Involvement of the CasK/R two-component system in optimal unsaturation of the Bacillus cereus fatty acids during low-temperature growth

Bacillus cereus sensu lato is composed of a set of ubiquitous strains including human pathogens that can survive a range of food processing conditions, grow in refrigerated food, and sometimes cause food poisoning. We previously identified the two-component system CasK/R that plays a key role in cold adaptation. To better understand the CasK/R-controlled mechanisms that support low-temperature adaptation, we performed a transcriptomic analysis on the ATCC 14579 strain and its isogenic ∆casK/R mutant grown at 12°C. Several genes involved in fatty acid (FA) metabolism were downregulated in the mutant, including desA and desB encoding FA acyl-lipid desaturases that catalyze the formation of a double-bond on the FA chain in positions ∆5 and ∆10, respectively. A lower proportion of FAs presumably unsaturated by DesA was observed in the ΔcasK/R strain compared to the parental strain while no difference was found for FAs presumably unsaturated by DesB. Addition of phospholipids from egg yolk lecithin rich in unsaturated FAs, to growth medium, abolished the cold-growth impairment of ΔcasK/R suggesting that exogenous unsaturated FAs can support membrane-level modifications and thus compensate for the decreased production of these FAs in the B. cereus ∆casK/R mutant during growth at low temperature. Our findings indicate that CasK/R is involved in the regulation of FA metabolism, and is necessary for cold adaptation of B. cereus unless an exogenous source of unsaturated FAs is available.

Microbial growth under supercritical CO2

Growth of microorganisms in environments containing CO2 above its critical point is unexpected due to a combination of deleterious effects, including cytoplasmic acidification and membrane destabilization. Thus, supercritical CO2 (scCO2) is generally regarded as a sterilizing agent. We report isolation of bacteria from three sites targeted for geologic carbon dioxide sequestration (GCS) that are capable of growth in pressurized bioreactors containing scCO2. Analysis of 16S rRNA genes from scCO2 enrichment cultures revealed microbial assemblages of varied complexity, including representatives of the genus Bacillus. Propagation of enrichment cultures under scCO2 headspace led to isolation of six strains corresponding to Bacillus cereus, Bacillus subterraneus, Bacillus amyloliquefaciens, Bacillus safensis, and Bacillus megaterium. Isolates are spore-forming, facultative anaerobes and capable of germination and growth under an scCO2 headspace. In addition to these isolates, several Bacillus type strains grew under scCO2, suggesting that this may be a shared feature of spore-forming Bacillus spp. Our results provide direct evidence of microbial activity at the interface between scCO2 and an aqueous phase. Since microbial activity can influence the key mechanisms for permanent storage of sequestered CO2 (i.e., structural, residual, solubility, and mineral trapping), our work suggests that during GCS microorganisms may grow and catalyze biological reactions that influence the fate and transport of CO2 in the deep subsurface.

Envelope Structures of Gram-Positive Bacteria

Gram-positive organisms, including the pathogens Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis, have dynamic cell envelopes that mediate interactions with the environment and serve as the first line of defense against toxic molecules. Major components of the cell envelope include peptidoglycan (PG), which is a well-established target for antibiotics, teichoic acids (TAs), capsular polysaccharides (CPS), surface proteins, and phospholipids. These components can undergo modification to promote pathogenesis, decrease susceptibility to antibiotics and host immune defenses, and enhance survival in hostile environments. This chapter will cover the structure, biosynthesis, and important functions of major cell envelope components in gram-positive bacteria. Possible targets for new antimicrobials will be noted.

The effect of cold stress on the proteome of the marine bacterium Pseudomonas fluorescens BA3SM1 and its ability to cope with metal excess

This study examined the effect of cold stress on the proteome and metal tolerance of Pseudomonas fluorescens BA3SM1, a marine strain isolated from tidal flat sediments. When cold stress (+10 °C for 36 h) was applied before moderate metal stress (0.4 mM Cd, 0.6 mM Cd, 1.5 mM Zn, and 1.5 mM Cu), growth disturbances induced by metal, in comparison with respective controls, were reduced for Cd and Zn while they were pronounced for Cu. This marine strain was able to respond to cold stress through a number of changes in protein regulation. Analysis of the predicted differentially expressed protein functions demonstrated that some mechanisms developed under cold stress were similar to those developed in response to Cd, Zn, and Cu. Therefore, pre-cold stress could help this strain to better counteract toxicity of moderate concentrations of some metals. P. fluorescens BA3SM1 was able to remove up to 404.3 mg Cd/g dry weight, 172.5 mg Zn/g dry weight, and 11.3 mg Cu/g dry weight and its metal biosorption ability seemed to be related to the bacterial growth phase. Thus, P. fluorescens BA3SM1 appears as a promising agent for bioremediation processes, even at low temperatures.

The CasKR two-component system is required for the growth of mesophilic and psychrotolerant Bacillus cereus strains at low temperatures

The different strains of Bacillus cereus can grow at temperatures covering a very diverse range. Some B. cereus strains can grow in chilled food and consequently cause food poisoning. We have identified a new sensor/regulator mechanism involved in low-temperature B. cereus growth. Construction of a mutant of this two-component system enabled us to show that this system, called CasKR, is required for growth at the minimal temperature (Tmin). CasKR was also involved in optimal cold growth above Tmin and in cell survival below Tmin. Microscopic observation showed that CasKR plays a key role in cell shape during cold growth. Introducing the casKR genes in a ΔcasKR mutant restored its ability to grow at Tmin. Although it was first identified in the ATCC 14579 model strain, this mechanism has been conserved in most strains of the B. cereus group. We show that the role of CasKR in cold growth is similar in other B. cereus sensu lato strains with different growth temperature ranges, including psychrotolerant strains.

Root exudate-induced alterations in Bacillus cereus cell wall contribute to root colonization and plant growth promotion

The outcome of an interaction between plant growth promoting rhizobacteria and plants may depend on the chemical composition of root exudates (REs). We report the colonization of tobacco, and not groundnut, roots by a non-rhizospheric Bacillus cereus (MTCC 430). There was a differential alteration in the cell wall components of B. cereus in response to the REs from tobacco and groundnut. Attenuated total reflectance infrared spectroscopy revealed a split in amide I region of B. cereus cells exposed to tobacco-root exudates (TRE), compared to those exposed to groundnut-root exudates (GRE). In addition, changes in exopolysaccharides and lipid-packing were observed in B. cereus grown in TRE-amended minimal media that were not detectable in GRE-amended media. Cell-wall proteome analyses revealed upregulation of oxidative stress-related alkyl hydroperoxide reductase, and DNA-protecting protein chain (Dlp-2), in response to GRE and TRE, respectively. Metabolism-related enzymes like 2-amino-3-ketobutyrate coenzyme A ligase and 2-methylcitrate dehydratase and a 60 kDa chaperonin were up-regulated in response to TRE and GRE. In response to B. cereus, the plant roots altered their exudate-chemodiversity with respect to carbohydrates, organic acids, alkanes, and polyols. TRE-induced changes in surface components of B. cereus may contribute to successful root colonization and subsequent plant growth promotion.

Exploring the biosynthesis of unsaturated fatty acids in Bacillus cereus ATCC 14579 and functional characterization of novel acyl-lipid desaturases

At low temperatures, Bacillus cereus synthesizes large amounts of unsaturated fatty acids (UFAs) with double bonds in positions Δ5 and Δ10, as well as Δ5,10 diunsaturated fatty acids. Through sequence homology searches, we identified two open reading frames (ORFs) encoding a putative Δ5 desaturase and a fatty acid acyl-lipid desaturase in the B. cereus ATCC 14579 genome, and these were named BC2983 and BC0400, respectively. Functional characterization of ORFs BC2983 and BC0400 by means of heterologous expression in Bacillus subtilis confirmed that they both encode acyl-lipid desaturases that use phospholipids as the substrates and have Δ5 and Δ10 desaturase activities. Thus, these ORFs were correspondingly named desA (Δ5 desaturase) and desB (Δ10 desaturase). We established that DesA utilizes ferredoxin and flavodoxins (Flds) as electron donors for the desaturation reaction, while DesB preferably employs Flds. In addition, increased amounts of UFAs were found when B. subtilis expressing B. cereus desaturases was subjected to a cold shock treatment, indicating that the activity or the expression of these enzymes is upregulated in response to a decrease in growth temperature. This represents the first work reporting the functional characterization of fatty acid desaturases from B. cereus.

Filament formation by foodborne bacteria under sublethal stress

A number of studies have reported that pathogenic and nonpathogenic foodborne bacteria have the ability to form filaments in microbiological growth media and foods after prolonged exposure to sublethal stress or marginal growth conditions. In many cases, nucleoids are evenly spaced throughout the filamentous cells but septa are not visible, indicating that there is a blockage in the early steps of cell division but the mechanism behind filament formation is not clear. The formation of filamentous cells appears to be a reversible stress response. When filamentous cells are exposed to more favorable growth conditions, filaments divide rapidly into a number of individual cells, which may have major health and regulatory implications for the food industry because the potential numbers of viable bacteria will be underestimated and may exceed tolerated levels in foods when filamentous cells that are subjected to sublethal stress conditions are enumerated. Evidence suggests that filament formation under a number of sublethal stresses may be linked to a reduced energy state of bacterial cells. This review focuses on the conditions and extent of filament formation by foodborne bacteria under conditions that are used to control the growth of microorganisms in foods such as suboptimal pH, high pressure, low water activity, low temperature, elevated CO2 and exposure to antimicrobial substances as well as lack a of nutrients in the food environment and explores the impact of the sublethal stresses on the cell's inability to divide.

Influence of anaerobiosis and low temperature on Bacillus cereus growth, metabolism, and membrane properties

The impact of simultaneous anaerobiosis and low temperature on growth parameters, metabolism, and membrane properties of Bacillus cereus ATCC 14579 was studied. No growth was observed under anaerobiosis at 12°C. In bioreactors, growth rates and biomass production were drastically reduced by simultaneous anaerobiosis and low temperature (15°C). The two conditions had a synergistic effect on biomass reduction. In anaerobic cultures, fermentative metabolism was modified by low temperature, with a marked reduction in ethanol production leading to a lower ability to produce NAD(+). Anaerobiosis reduced unsaturated fatty acids at both low optimal temperatures. In addition, simultaneous anaerobiosis and low temperatures markedly reduced levels of branched-chain fatty acids compared to all other conditions (accounting for 33% of total fatty acids against more 71% for low-temperature aerobiosis, optimal-temperature aerobiosis, and optimal-temperature anaerobiosis). This corresponded to high-melting-temperature lipids and to low-fluidity membranes, as indicated by differential scanning calorimetry, 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence anisotropy, and infrared spectroscopy. This is in contrast to requirements for cold adaptation. A link between modification in the synthesis of metabolites of fermentative metabolism and the reduction of branched-chain fatty acids at low temperature under anaerobiosis, through a modification of the oxidizing capacity, is assumed. This link may partly explain the impact of low temperature and anaerobiosis on membrane properties and growth performance.

The importance of bacterial membrane composition in the structure and function of aurein 2.2 and selected variants

For cationic antimicrobial peptides to become useful therapeutic agents, it is important to understand their mechanism of action. To obtain high resolution data, this involves studying the structure and membrane interaction of these peptides in tractable model bacterial membranes rather than directly utilizing more complex bacterial surfaces. A number of lipid mixtures have been used as bacterial mimetics, including a range of lipid headgroups, and different ratios of neutral to negatively charged headgroups. Here we examine how the structure and membrane interaction of aurein 2.2 and some of its variants depend on the choice of lipids, and how these models correlate with activity data in intact bacteria (MICs, membrane depolarization). Specifically, we investigated the structure and membrane interaction of aurein 2.2 and aurein 2.3 in 1:1 cardiolipin/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (CL/POPG) (mol/mol), as an alternative to 1:1 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine(POPC)/POPG and a potential model for Gram positive bacteria such as S. aureus. The structure and membrane interaction of aurein 2.2, aurein 2.3, and five variants of aurein 2.2 were also investigated in 1:1 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE)/POPG (mol/mol) lipids as a possible model for other Gram positive bacteria, such as Bacillus cereus. Solution circular dichroism (CD) results demonstrated that the aurein peptides adopted α-helical structure in all lipid membranes examined, but demonstrated a greater helical content in the presence of POPE/POPG membranes. Oriented CD and ³¹P NMR results showed that the aurein peptides had similar membrane insertion profiles and headgroup disordering effects on POPC/POPG and CL/POPG bilayers, but demonstrated reduced membrane insertion and decreased headgroup disordering on mixing with POPE/POPG bilayers at low peptide concentrations. Since the aurein peptides behaved very differently in POPE/POPG membrane, minimal inhibitory concentrations (MICs) of the aurein peptides in B. cereus strain C737 were determined. The MIC results indicated that all aurein peptides are significantly less active against B. cereus than against S. aureus and S. epidermidis. Overall, the data suggest that it is important to use a relevant model for bacterial membranes to gain insight into the mode of action of a given antimicrobial peptide in specific bacteria.

Spores of Bacillus cereus strain KBAB4 produced at 10 °C and 30 °C display variations in their properties

Spores of the psychrotrophic Bacillus cereus KBAB4 strain were produced at 10 °C and 30 °C in fermentors. Spores produced at 30 °C were more resistant to wet heat at 85 °C, 1% glutaraldehyde, 5% hydrogen peroxide, 1M NaOH and pulsed light at fluences between 0.5 and 1.75 Jcm(-2) and to a lesser extent to monochromatic UV-C at 254 nm. No difference in resistance to 0.25 mM formaldehyde, 1M nitrous acid and 0.025 gl(-1) calcium hypochlorite was observed. Spores produced at 10 °C germinated more efficiently with 10 mM and 100 mM l-alanine than spores produced at 30 °C, while no difference in germination was observed with inosine. Dipicolinic acid (DPA) content in the spore was significantly higher for spores prepared at 30 °C. Composition of certain fatty acids varied significantly between spores produced at 10 °C and 30 °C.

Insertional mutagenesis reveals genes involved in Bacillus cereus ATCC 14579 growth at low temperature

Transposon mutagenesis of Bacillus cereus ATCC 14579 yielded cold-sensitive mutants. Mutants of genes encoding enzymes of the central metabolism were affected by cold, but also by other stresses, such as pH or salt, whereas a mutant with transposon insertion in the promoter region of BC0259 gene, encoding a putative DEAD-box RNA helicase displaying homology with Escherichia coli CsdA and Bacillus subtilis CshA RNA helicases, was only cold-sensitive. Expression of the BC0259 gene at 10 degrees C is reduced in the mutant. Analysis of the 5' untranslated region revealed the transcriptional start and putative cold shock-responsive elements. The role of this RNA helicase in the cold-adaptive response of B. cereus is discussed.

Identification of Bacillus cereus genes specifically expressed during growth at low temperatures

The mechanisms involved in the ability of Bacillus cereus to multiply at low temperatures were investigated. It was assumed that many genes involved in cold acclimation would be upregulated at low temperatures. Recombinase-based in vivo expression technology (IVET) was adapted to the detection of the transient activation of B. cereus promoters during growth at 10 degrees C. Four independent screenings of a promoter library from type strain ATCC 14579 were performed, and 17 clones were isolated. They corresponded to 17 promoter regions that displayed reproducibly elevated expression at 10 degrees C relative to expression at 30 degrees C. This analysis revealed several genes that may be important for B. cereus to grow successfully under the restrictive conditions of cold habitats. Among them, a locus corresponding to open reading frames BC5402 to BC5398, harboring a lipase-encoding gene and a putative transcriptional regulator, was identified three times. While a mutation in the putative regulator-encoding gene did not cause any particular phenotype, a mutant deficient in the lipase-encoding gene showed reduced growth abilities at low temperatures compared with the parental strain. The mutant did not change its fatty acid profiles in the same way as the wild type when grown at 12 degrees C instead of 37 degrees C. This study demonstrates the feasibility of a promoter trap strategy for identifying cold-induced genes. It outlines a first picture of the different processes involved in B. cereus cold acclimation.

Effect of membrane composition on antimicrobial peptides aurein 2.2 and 2.3 from Australian southern bell frogs

The effects of hydrophobic thickness and the molar phosphatidylglycerol (PG) content of lipid bilayers on the structure and membrane interaction of three cationic antimicrobial peptides were examined: aurein 2.2, aurein 2.3 (almost identical to aurein 2.2, except for a point mutation at residue 13), and a carboxy C-terminal analog of aurein 2.3. Circular dichroism results indicated that all three peptides adopt an alpha-helical structure in the presence of a 3:1 molar mixture of 1,2-dimyristoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPC/DMPG), and 1:1 and 3:1 molar mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPC/POPG). Oriented circular dichroism data for three different lipid compositions showed that all three peptides were surface-adsorbed at low peptide concentrations, but were inserted into the membrane at higher peptide concentrations. The (31)P solid-state NMR data of the three peptides in the DMPC/DMPG and POPC/POPG bilayers showed that all three peptides significantly perturbed lipid headgroups, in a peptide or lipid composition-dependent manner. Differential scanning calorimetry results demonstrated that both amidated aurein peptides perturbed the overall phase structure of DMPC/DMPG bilayers, but perturbed the POPC/POPG chains less. The nature of the perturbation of DMPC/DMPG bilayers was most likely micellization, and for the POPC/POPG bilayers, distorted toroidal pores or localized membrane aggregate formation. Calcein release assay results showed that aurein peptide-induced membrane leakage was more severe in DMPC/DMPG liposomes than in POPC/POPG liposomes, and that aurein 2.2 induced higher calcein release than aurein 2.3 and aurein 2.3-COOH from 1:1 and 3:1 POPC/POPG liposomes. Finally, DiSC(3)5 assay data further delineated aurein 2.2 from the others by showing that it perturbed the lipid membranes of intact S. aureus C622 most efficiently, whereas aurein 2.3 had the same efficiency as gramicidin S, and aurein 2.3-COOH was the least efficient. Taken together, these data show that the membrane interactions of aurein peptides are affected by the hydrophobic thickness of the lipid bilayers and the PG content.

Osmolality, temperature, and membrane lipid composition modulate the activity of betaine transporter BetP in Corynebacterium glutamicum

The gram-positive soil bacterium Corynebacterium glutamicum, a major amino acid-producing microorganism in biotechnology, is equipped with several osmoregulated uptake systems for compatible solutes, which is relevant for the physiological response to osmotic stress. The most significant carrier, BetP, is instantly activated in response to an increasing cytoplasmic K(+) concentration. Importantly, it is also activated by chill stress independent of osmotic stress. We show that the activation of BetP by both osmotic stress and chill stress is altered in C. glutamicum cells grown at and adapted to low temperatures. BetP from cold-adapted cells is less sensitive to osmotic stress. In order to become susceptible for chill activation, cold-adapted cells in addition needed a certain amount of osmotic stimulation, indicating that there is cross talk of these two types of stimuli at the level of BetP activity. We further correlated the change in BetP regulation properties in cells grown at different temperatures to changes in the lipid composition of the plasma membrane. For this purpose, the glycerophospholipidome of C. glutamicum grown at different temperatures was analyzed by mass spectrometry using quantitative multiple precursor ion scanning. The molecular composition of glycerophospholipids was strongly affected by the growth temperature. The modulating influence of membrane lipid composition on BetP function was further corroborated by studying the influence of artificial modulation of membrane dynamics by local anesthetics and the lack of a possible influence of internally accumulated betaine on BetP activity.

Bacterial lipid composition and the antimicrobial efficacy of cationic steroid compounds (Ceragenins)

Ceragenins are cationic bile salt derivatives having antimicrobial activity. The interactions of several ceragenins with phospholipid bilayers were tested in different systems. The ceragenins are capable of forming specific associations with several phospholipid species that may be involved with their antimicrobial action. Their antimicrobial activity is lower in bacteria that have a high content of phosphatidylethanolamine. Gram negative bacteria with a high content of phosphatidylethanolamine exhibit sensitivity to different ceragenins that corresponds to the extent of interaction of these compounds with phospholipids, including the ability of different ceragenins to induce leakage of aqueous contents from phosphatidylethanolamine-rich liposomes. A second class of bacteria having cell membranes composed largely of anionic lipids and having a low content of phosphatidylethanolamine are very sensitive to the action of the ceragenins but they exhibit similar minimal inhibitory concentrations with most of the ceragenins and for different strains of bacteria. Although Gram negative bacteria generally have a high content of phosphatidylethanolamine, there are a few exceptions. In addition, a mutant strain of Escherichia coli has been made that is essentially devoid of phophatidylethanolamine, although 80% of the lipid of the wild-type strain is phosphatidylethanolamine. Furthermore, certain Gram positive bacteria are also exceptions in that they can have a high content of phosphatidylethanolamine. We find that the antimicrobial action of the ceragenins correlates better with the content of phosphatidylethanolamine in the bacterial membrane than whether or not the bacteria has an outer membrane. Thus, the bacterial lipid composition can be an important factor in determining the sensitivity of bacteria to antimicrobial agents.

Cold hardening and transcriptional change in Drosophila melanogaster

Cold hardening treatment - a brief exposure to low temperatures - can protect certain insects against subsequent exposure to temperatures sufficiently low to cause damage or lethality. Microarray analysis to examine the changes in transcript abundance associated with cold hardening treatment (0 degrees C for 2 h followed by 30 min recovery at 25 degrees C) was undertaken in Drosophila melanogaster in order to gain insight into this phenomenon. Transcripts associated with 36 genes were identified, a subset of which appeared to be also differentially expressed after heat shock treatment. Quantitative RT-PCR was used to independently determine transcript abundance of a subset of these sequences. Taken together, these assays suggest that stress proteins, including Hsp23, Hsp26, Hsp83 and Frost as well as membrane-associated proteins may contribute to the cold hardening response.