Recovery of Heat Treated Bacillus cereus Spores Is Affected by Matrix Composition and Factors with Putative Functions in Damage Repair

Regulation of the transcription factor CdnL promotes adaptation to nutrient stress in Caulobacter

In response to nutrient deprivation, bacteria activate a conserved stress response pathway called the stringent response (SR). During SR activation in Caulobacter crescentus, SpoT synthesizes the secondary messengers guanosine 5'-diphosphate 3'-diphosphate and guanosine 5'-triphosphate 3'-diphosphate (collectively known as (p)ppGpp), which affect transcription by binding RNA polymerase (RNAP) to down-regulate anabolic genes. (p)ppGpp also impacts the expression of anabolic genes by controlling the levels and activities of their transcriptional regulators. In Caulobacter, a major regulator of anabolic genes is the transcription factor CdnL. If and how CdnL is controlled during the SR and why that might be functionally important are unclear. In this study, we show that CdnL is down-regulated posttranslationally during starvation in a manner dependent on SpoT and the ClpXP protease. Artificial stabilization of CdnL during starvation causes misregulation of ribosomal and metabolic genes. Functionally, we demonstrate that the combined action of SR transcriptional regulators and CdnL clearance allows for rapid adaptation to nutrient repletion. Moreover, cells that are unable to clear CdnL during starvation are outcompeted by wild-type cells when subjected to nutrient fluctuations. We hypothesize that clearance of CdnL during the SR, in conjunction with direct binding of (p)ppGpp and DksA to RNAP, is critical for altering the transcriptome in order to permit cell survival during nutrient stress.

l-tyrosine modulates biofilm formation of Bacillus cereus ATCC 14579

Bacillus cereus is a food-borne pathogen capable of producing biofilms. Following analysis of biofilm formation by B. cereus ATCC 14579 transposon mutants in defined medium (DM), a deletion mutant of bc2939 (Δbc2939) was constructed that showed decreased crystal violet biofilm staining and biofilm cell counts. In addition, Δbc2939 also produced smaller colony biofilms with lower cell counts and loss of wrinkly morphology. The bc2939 gene encodes for Prephenate dehydrogenase, which converts Prephenate to 4-Hydroxy-phenylpyruvate (4-HPPA) in the l-tyrosine branch of the Shikimate pathway. While growth of the mutant and WT in DM was similar, addition of l-tyrosine was required to restore WT-like (colony) biofilm formation. Comparative proteomics showed reduced expression of Tyrosine-protein kinase/phosphatase regulators and extracellular polysaccharide cluster 1 (EPS1) proteins, aerobic electron transfer chain cytochrome aa3/d quinol oxidases, and iso-chorismate synthase involved in menaquinone synthesis in DM grown mutant biofilm cells, while multiple oxidative stress-related catalases and superoxide dismutases were upregulated. Performance in shaking cultures showed a 100-fold lower concentration of menaquinone-7 and reduction in cell counts of DM grown Δbc2939 indicating increased oxygen sensitivity. Combining all results, points to an important role of Tyrosine-modulated EPS1 production and menaquinone-dependent aerobic respiration in B. cereus ATCC 14579 (colony) biofilm formation.

SigB modulates expression of novel SigB regulon members via Bc1009 in non-stressed and heat-stressed cells revealing its alternative roles in Bacillus cereus

BACKGROUND

The Bacillus cereus Sigma B (SigB) dependent general stress response is activated via the two-component RsbKY system, which involves a phosphate transfer from RsbK to RsbY. It has been hypothesized that the Hpr-like phosphocarrier protein (Bc1009) encoded by bc1009 in the SigB gene cluster may play a role in this transfer, thereby acting as a regulator of SigB activation. Alternatively, Bc1009 may be involved in the activation of a subset of SigB regulon members.

RESULTS

We first investigated the potential role of bc1009 to act as a SigB regulator but ruled out this possibility as the deletion of bc1009 did not affect the expression of sigB and other SigB gene cluster members. The SigB-dependent functions of Bc1009 were further examined in B. cereus ATCC14579 via comparative proteome profiling (backed up by transcriptomics) of wt, Δbc1009 and ΔsigB deletion mutants under heat stress at 42 °C. This revealed 284 proteins displaying SigB-dependent alterations in protein expression levels in heat-stressed cells, including a subgroup of 138 proteins for which alterations were also Bc1009-dependent. Next to proteins with roles in stress defense, newly identified SigB and Bc1009-dependent proteins have roles in cell motility, signal transduction, transcription, cell wall biogenesis, and amino acid transport and metabolism. Analysis of lethal stress survival at 50 °C after pre-adaptation at 42 °C showed intermediate survival efficacy of Δbc1009 cells, highest survival of wt, and lowest survival of ΔsigB cells, respectively. Additional comparative proteome analysis of non-stressed wt and mutant cells at 30 °C revealed 96 proteins with SigB and Bc1009-dependent differences in levels: 51 were also identified under heat stress, and 45 showed significant differential expression at 30 °C. This includes proteins with roles in carbohydrate/ion transport and metabolism. Overlapping functions at 30 °C and 42 °C included proteins involved in motility, and ΔsigB and Δbc1009 cells showed reduced motility compared to wt cells in swimming assays at both temperatures.

CONCLUSION

Our results extend the B. cereus SigB regulon to > 300 members, with a novel role of SigB-dependent Bc1009 in the activation of a subregulon of  > 180 members, conceivably via interactions with other transcriptional regulatory networks.

Post-Cooking Growth and Survival of Bacillus cereus Spores in Rice and Their Enzymatic Activities Leading to Food Spoilage Potential

Bacillus cereus strains vary in their heat resistance, post-processing survival and growth capacity in foods. Hence, this study was carried out to determine the effect of cooking on the survival and growth of eight B. cereus spores in rice at different temperatures in terms of their toxigenic profiles and extracellular enzyme activity. Samples of rice inoculated with different B. cereus spores were cooked and stored at 4 °C, 25 °C and 30 °C for up to 7 days, 48 h and 24 h, respectively. Out of eight B. cereus strains, four and three spore strains were able to grow at 30 °C and 25 °C post-cooking, respectively. Rapid growth was observed after a minimum of 6 h of incubation at 30 °C. All strains possessed proteolytic activity, whereas lipolytic and amylolytic activities were exhibited by 50% and 12.5% of the strains, respectively. The post-cooking survival and growth capacity of the B. cereus strains appeared to be independent of their toxigenic profiles, whereas extracellular enzymatic activities were required for their vegetative growth. Due to the B. cereus spores' abilities to survive cooking and return to their active cellular form, great care should be taken when handling ready-to-eat foods.

Heterogeneity in single-cell outgrowth of Listeria monocytogenes in half Fraser enrichment broth is affected by strain variability and physiological state

The behaviour of pathogens at the single-cell level can be highly variable and can thus affect the detection efficacy of enrichment-based detection methods. The outgrowth of single cells of three Listeria monocytogenes strains was monitored after fluorescence-activated single-cell sorting in non-selective brain heart infusion (BHI) broth and selective half Fraser enrichment broth (HFB) to quantify outgrowth heterogeneity and its effect on the detection probability. Single-cell heterogeneity was higher in HFB compared to non-selective BHI and heterogeneity increased further when cells were heat-stressed. The increase in heterogeneity was also strain-dependent because the fast-recovering strain Scott A showed less outgrowth heterogeneity than the slower-recovering strains EGDe and H7962. Modelling of the outgrowth kinetics during the primary enrichment demonstrated that starting at low cell concentrations could fail detection of L. monocytogenes at least partly due to cell heterogeneity. This highlights that it is important to take single-cell heterogeneity into account when optimizing enrichment formulations and procedures when L. monocytogenes contamination levels are low.

Effect of temperature and soluble solid on Bacillus subtilis and Bacillus licheniformis spore inactivation and quality degradation of pineapple juice

Bacillus subtilis and Bacillus licheniformis spores can survive processing temperatures used in the thermal processes of high-acid foods. Therefore, this study investigated the thermal inactivation of B. subtilis and B. licheniformis spores in pineapple juice at different temperatures (85-100°C) and soluble solids (SS, 11-30°Brix). The quality of juices and microbial loads after the thermal treatments during storage at 4 °C for 35 days was then checked. A linear decrease in D-value was observed with increasing temperature of treatment. Furthermore, the D-values determined in pineapple juice were: D_90°C=13.2 ± 0.5 mins, D_95°C = 6.8 ± 0.9 mins and D_100°C = 2.1 ± 1.7 mins for B. subtilis spores, and D_85°C = 16.6 ± 0.4 mins, D_90°C = 7.6 ± 0.5 mins and D_95°C = 3.6 ± 1.5 min, for B. licheniformis. Generally, the susceptibility of the bacteria to soluble solid change was affected by the interaction between temperature, SS and strain. In addition, pasteurization processes of ≥95°C for ≥33.8 mins was needed to ensure a recommended 5-log reduction of B. subtilis spores and limit vitamin C degradation of pineapple juice within three-week of storage at 4 °C.