Sublethal injury and recovery of Escherichia coli O157:H7 by high pressure carbon dioxide

Changes in stresses sensitivity of ohmic heating-induced sublethally injured Staphylococcus aureus during repair: Potential mechanisms at the cellular and molecular levels

Ohmic heating (OH), an emerging food processing technology employed in the food processing industry, raises potential food safety concerns due to the recovery of sublethally injured pathogens such as Staphylococcus aureus (S. aureus). In the present study, sensitivity to various stress conditions and the changes in cellular-related factors of OH-injured S. aureus during repair were investigated. The results indicated that liquid media differences (nutrient broth (NB), phosphate-buffered saline (PBS), milk, and cucumber juice) affected the recovery process of injured cells. Nutrient enrichment determines the bacterial repair rate, and the rates of repair for these media were milk > NB > cucumber juice > PBS. The sensitivity of injured cells to various stressors, including different acids, temperature, nisin, simulated gastric fluid, and bile salt, increased during the injury phase and subsequently diminished upon repair. Additionally, the intracellular ATP content, enzyme activities (Na+/K+-ATPase, Ca2+/Mg2+-ATPase, and T-ATPase) and ion concentrations (Mg2+, K+, and Ca2+) gradually increased during repair. After 5 h of repair, the intracellular substances content of cell's was significantly higher than that of the injured bacteria without repair, while some indicators (e.g., Na+/K+-ATPase, K+, and Ca2+) were not restored to the untreated level. The results of this study indicated that OH-injured S. aureus exhibited strengthened resistance post-recovery, potentially due to the restoration of cellular structures. These findings have implications for optimizing food storage conditions and advancing OH processes in the food industry.

Sublethally injured microorganisms in food processing and preservation: Quantification, formation, detection, resuscitation and adaption

Sublethally injured state has been recognized as a survival strategy for microorganisms suffering from stressful environments. Injured cells fail to grow on selective media but can normally grow on nonselective media. Numerous microorganism species can form sublethal injury in various food matrices during processing and preservation with different techniques. Injury rate was commonly used to evaluate sublethal injury, but mathematical models for the quantification and interpretation of sublethally injured microbial cells still require further study. Injured cells can repair themselves and regain viability on selective media under favorable conditions when stress is removed. Conventional culture methods might underestimate microbial counts or present a false negative result due to the presence of injured cells. Although the structural and functional components may be affected, the injured cells pose a great threat to food safety. This work comprehensively reviewed the quantification, formation, detection, resuscitation and adaption of sublethally injured microbial cells. Food processing techniques, microbial species, strains and food matrix all significantly affect the formation of sublethally injured cells. Culture-based methods, molecular biological methods, fluorescent staining and infrared spectroscopy have been developed to detect the injured cells. Cell membrane is often repaired first during resuscitation of injured cells, meanwhile, temperature, pH, media and additives remarkably influence the resuscitation. The adaption of injured cells negatively affects the microbial inactivation during food processing.

Integrative Physiological and Transcriptome Analysis Reveals the Mechanism for the Repair of Sub-Lethally Injured Escherichia coli O157:H7 Induced by High Hydrostatic Pressure

The application of high hydrostatic pressure (HHP) technology in the food industry has generated potential safety hazards due to sub-lethally injured (SI) pathogenic bacteria in food products. To address these problems, this study explored the repair mechanisms of HHP-induced SI Escherichia coli O157:H7. First, the repair state of SI E. coli O157:H7 (400 MPa for 5 min) was identified, which was cultured for 2 h (37 °C) in a tryptose soya broth culture medium. We found that the intracellular protein content, adenosine triphosphate (ATP) content, and enzyme activities (superoxide dismutase, catalase, and ATPase) increased, and the morphology was repaired. The transcriptome was analyzed to investigate the molecular mechanisms of SI repair. Using cluster analysis, we identified 437 genes enriched in profile 1 (first down-regulated and then tending to be stable) and 731 genes in profile 2 (up-regulated after an initial down-regulation). KEGG analysis revealed that genes involved in cell membrane biosynthesis, oxidative phosphorylation, ribosome, and aminoacyl-tRNA biosynthesis pathways were enriched in profile 2, whereas cell-wall biosynthesis was enriched in profile 1. These findings provide insights into the repair process of SI E. coli O157:H7 induced by HHP.

The effect of manganese and iron on mediating resuscitation of lactic acid-injured Escherichia coli

Lactic acid can induce sublethal injury of E. coli through oxidative stress. In this study, we investigated changes in SOD activity, CAT activity, GSH production and ROS production during sublethal injury and resuscitation of E. coli. Then, the effect of manganese and iron during resuscitation were studied. Both cations (≥1 mmol l^-1 ) significantly promoted the resuscitation of sublethally injured E. coli induced by lactic acid and shortened the repair time (P < 0·05). Conversely, addition of N,N,N',N'-tetrakis (2-pyridylmethyl) which is a metal chelator extended the repair time. Compared with minA, manganese and iron significantly improved SOD activity at 40, 80 and 120 min and decreased ROS production at 40 and 80 min, thereby recovering injured E. coli quickly (P < 0·05). The deletion of sodA encoding Mn-SOD, sodB encoding Fe-SOD or gshA/gshB encoding GSH significantly strengthened sublethal injury and extended the repair time (P < 0·05). It meant these genes-related oxidative stress played important roles in the acid resistance of E. coli and recovery of sublethal injury. Therefore, manganese and iron can promote the recovery of lactic-injured E. coli by the way of increasing SOD activity, scavenging ROS, and relieving oxidative stress.

Investigation of the potential direct and cross protection effects of sublethal injured Salmonella Typhimurium induced by radio frequency heating stress

Many studies demonstrated that radio frequency (RF) was an effective pasteurization method for low-moisture foods (LMFs), and our previous study confirmed RF heating stress generated sublethal injured cells (SICs) of Salmonella enterica serovar Typhimurium (S. Typhimurium) in red pepper powder with initial a_w ≥ 0.53. So this study investigated the potential direct protection and cross protection effects of the SICs of S. Typhimurium to multiple stresses, and analyzed fatty acid composition and cell morphology. Results showed that the SICs were repaired after incubating for 5 h, and there were no obvious direct and cross protection effects by exposing to different external stresses (heat, 15% ethanol, pH 3.0 acid buffer solution, 10% salt). According to the fatty acid composition analysis, no significant difference (p > 0.05) between the ratio of unsaturated to saturated fatty acids (UFA/SFA) was observed for SICs of S. Typhimurium and control cells, indicating the same membrane fluidity which can support the experimental results. This study investigated and confirmed there are no direct and cross protection effects for the SICs of S. Typhimurium induced by RF heating stress, and it would be helpful for deeply understand the response of pathogens under RF heating stress.

New insights into the formation and recovery of sublethally injured Escherichia coli O157:H7 induced by lactic acid

Escherichia coli O157:H7 can be injured by the action of lactic acid (LA) and injured cells can be recovered under suitable condition. In this study, RNA sequencing analysis revealed the overall genes change of sublethally injured (4 mM LA, 60 min; SI) and initial recovered (minA, 20 min; R) cells. Compared with untreated samples, 53 up-regulated and 98 down-regulated differentially expressed genes (DEGs; P_adj < 0.05, change fold ≥2) were found in SI. Meanwhile, Genes related to carbohydrate transport and metabolic were up-regulated and the addition of carbohydrate increased cells resistance to LA. Genes involved in osmotic stress response and cell membrane integrity were down-regulated and E. coli O157:H7 cells were sensitive to osmotic stress during sublethal injury. Genes related to iron stress response and cation transport were changed and cation may affect sublethal injury formation by influencing production of ROS and cellular processes. In R, 1370 up-regulated and 1110 down-regulated DEGs were subdivided into various GO terms and membrane, biological adhesion, cell projection, oxidation-reduction process and catalytic activity, etc., showed significant enrichment (corrected P < 0.05). Particularly, genes related to fimbrial, flagellum and type III secretion system were up-regulated, which may improve infection ability and virulence property during recovery of injured cells. These findings provide novel insights into formation and recovery of sublethally injured E. coli O157:H7 induced by LA.

Recovery and virulence factors of sublethally injured Staphylococcus aureus after ohmic heating

Ohmic heating (OH) is an alternative thermal processing technique, which is widely used to pasteurize or sterilize food. However, sublethally injured Staphylococcus aureus induced by OH is a great concern to food safety. The recovery of injured S. aureus by OH and virulence factor changes during recovery were investigated in this study. The liquid media (phosphate-buffered saline, buffered peptone water and nutrient broth (NB)), temperature (4, 25 and 37 °C) and pH (6.0, 7.2 and 8.0) influenced the recovery rate and the injured cells completely repaired in NB at 37 °C, pH 7.2 with the shortest time of 2 h. The biofilm formation ability, mannitol fermentation, hemolysis, and coagulase activities decreased in injured S. aureus and recovered during repair process. Quantitative real-time PCR showed the expression of sek, clfB and lukH involved in virulence factors increased during recovery. The results indicated that the virulence factors of injured S. aureus recovered after repair.

A Population Balance Model to Describe the Evolution of Sublethal Injury

The detection and quantification of sublethal injury (SI) of pathogenic microorganisms has become a common procedure when assessing the efficiency of microbial inactivation treatments. However, while a plethora of studies investigates SI in function of time, no suitable modelling procedure for SI data has been proposed thus far. In this study, a new SI model structure was developed that relies on existing microbial inactivation models. This model is based on the description of inactivation kinetics between the subpopulations of healthy, sublethally injured and dead cells. The model was validated by means of case studies on previously published results, modelled by different inactivation models, i.e., (i) log-linear inactivation; (ii) biphasic inactivation; and (iii) log-linear inactivation with tailing. Results were compared to those obtained by the traditional method that relies on calculating SI from independent inactivation models on non-selective and selective media. The log-linear inactivation case study demonstrated that the SI model is equivalent to the use of independent models when there can be no mistake in calculating SI. The biphasic inactivation case study illustrated how the SI model avoids unrealistic calculations of SI that would otherwise occur. The final case study on log-linear inactivation with tailing clarified that the SI model provides a more mechanistic description than the independent models, in this case allowing the reduction of the number of model parameters. As such, this paper provides a comprehensive overview of the potential and applications for the newly presented SI model.

Zinc mediates resuscitation of lactic acid-injured Escherichia coli by relieving oxidative stress

AIMS

Lactic acid is a natural antimicrobial in food industry, and also exists in fermented food. It was reported that sublethally injured Escherichia coli could survive in acidic conditions. When conditions become advantageous, injured E. coli can restore physiological function, which is a potential threat in food industry. Recovery is a necessary step for discriminating injured bacteria, but the resuscitation mechanism of injured bacteria is still unknown.

METHODS AND RESULTS

In our study, sublethal lactic acid treatment (pH 4·2, 60 min) posed oxidative stress on E. coli by decrease of superoxide dismutase (SOD) activity and overproduction of reactive oxygen species (ROS). Zinc with low concentration (1·0 mmol l^-1 ) significantly increased the recovery ratio of injured E.coli induced by lactic acid. The recovery ratios of injured cell in minimal A medium (minA) with 1·0 mmol l^-1 zinc reached to that with 3·0 mmol l^-1 catalase (CAT). Conversely, the addition of zinc chelator N, N, N', N'-tetrakis (2-pyridylmethyl) decreased the recovery ratio. Zinc accelerated resuscitation of injured E. coli by improving SOD activity, and decreasing ROS production. Deletion of sodC encoding Cu/ZnSOD, katE/katG encoding CAT or regulating gene rpoS significantly decreased the recovery ratio. Among all of the mutants in this study, ΔrpoS and ΔsodC showed the lowest recovery ratio, which means they played significant roles in the process of resuscitation.

CONCLUSION

We provided direct evidence that zinc mediated resuscitation of lactic acid-injured E. coli by relieving oxidative stress. Zinc can be used as a low-cost and effective agent to improve recovery ratio and detection efficiency of injured bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

Antibacterial agents are a challenge for bacteria, but bacteria can survive as a sublethally injured state under stresses. Using injured E. coli induced by lactic acid as a model organism, we validated the significant role of zinc on resuscitation of injured cells.

New Insights Into the Response of Metabolome of Escherichia coli O157:H7 to Ohmic Heating

The objective of this study was to investigate the effects of ohmic heating and water bath heating (WB) on the metabolome of Escherichia coli O157:H7 cells at the same inactivation levels. Compared to low voltage long time ohmic heating (5 V/cm, 8.50 min, LVLT) and WB (5.50 min), the high voltage short time ohmic heating (10 V/cm, 1.75 min, HVST) had much shorter heating time. Compared to the samples of control (CT), there were a total of 213 differential metabolites identified, among them, 73, 78, and 62 were presented in HVST, LVLT, and WB samples, revealing a stronger metabolomic response of E. coli cells to HVST and LVLT than WB. KEGG enrichment analysis indicated that the significantly enriched pathways were biosynthesis and metabolism of amino acids (alanine, arginine, aspartate, and glutamate, etc.), followed by aminoacyl-tRNA biosynthesis among the three treatments. This is the first metabolomic study of E. coli cells in response to ohmic heating and presents an important step toward understanding the mechanism of ohmic heating on microbial inactivation, and can serve as a theoretical basis for better application of ohmic heating in food products.

Decreased resistance of sublethally injured Escherichia coli O157:H7 to salt, mild heat, nisin and acids induced by high pressure carbon dioxide

Resistance of sublethally injured cells (SICs) of Escherichia coli O157:H7 induced by high pressure carbon dioxide (HPCD) to salt, low temperature, mild heat, nisin, acids and low pHs was investigated in this study. The SICs of E. coli were obtained following HPCD at 5 MPa and 25 °C for 40-60 min or 5 MPa and 45 °C for 20 min. The untreated cells could survive normally while the HPCD-treated cells showed 2.87 log₁₀ cycles' reduction on tryptic soy agar (TSA) with 3% NaCl. The counts of the untreated cells were not significantly changed during 5 h incubation at 4 °C or 40 min incubation at 45 °C, and the HPCD-treated cells were also not affected by 5 h incubation at 4 °C but showed 1.75 log₁₀ cycles' reduction at 45 °C for 40 min. The antimicrobial nisin caused an extra 0.25-1.0 log₁₀ cycles' reduction of the HPCD-treated cells while the untreated cells was not inactivated by nisin. Except for oxalic acid (OA), citric acid (CA), malic acid (MA), tartaric acid (TA), lactic acid (LA), acetic acid (AA) and hydrochloric acid (HCl) did not inactivate the untreated cells, but all these seven acids caused a 0.74-1.53 log₁₀ cycles' reduction of HPCD-treated cells. These results indicated that HPCD-induced SICs had a decreased resistance to salt, mild heat, nisin and acids. Moreover, the recovery test was used to investigate the sensitivity of the SICs to different pHs. Results showed that the SICs could not recover below or equal to pH 4.0. These promising results would open up the possibility of exploring the combination of other technologies (eg. mild heat, nisin and acids) with HPCD as hurdle approaches to inactivate target pathogens in foods.

iTRAQ-Based Proteomic Analysis of Sublethally Injured Escherichia coli O157:H7 Cells Induced by High Pressure Carbon Dioxide

High pressure carbon dioxide (HPCD) could cause sublethally injured cells (SICs), which may cause food poisoning and spoilage during food storage and limit its application. Therefore, the formation of SICs of Escherichia coli O157:H7 was investigated by isobaric tag for relative and absolute quantification (iTRAQ) proteomic methods in this study for better controlling the SICs induced by HPCD. A total of 2,446 proteins was identified by iTRAQ, of which 93 and 29 were significantly differentially expressed in the SICs compared with live control cells (CK_L) and dead control cells (CK_D), respectively. Among the 93 differentially expressed proteins (DEP) in the SICs compared with CK_L, 65 proteins showed down-regulation and 28 showed up-regulation. According to the comprehensive proteome coverage analysis, the SICs survived under HPCD by reducing carbohydrate decomposing, lipid transport and metabolism, amino acid transport and metabolism, transcription and translation, DNA replication and repair. Besides, the SICs showed stress response, DNA damage response and an increased carbohydrate transport, peptidoglycan synthesis and disulfide bond formation to HPCD. Among the 29 DEP in the SICs compared with CK_D, 12 proteins showed down-regulation and 17 showed up-regulation. According to the comprehensive proteome coverage analysis, the SICs survived under HPCD by accumulation of cell protective agents like carbohydrates and amino acids, and decreasing transcription and translation activities. Results showed that the formation of the SICs with low metabolic activity and high survival ability was a survival strategy for E. coli O157:H7 against HPCD.