Inactivation of nondesiccated and desiccated Cronobacter Sakazakii and Salmonella spp. at low and high inocula levels in reconstituted infant milk formula by vanillin

Sensitive detection of viable Cronobacter sakazakii by bioluminescent reporter phage emitting stable signals with truncated holin

As outbreaks of foodborne illness caused by the opportunistic pathogen Cronobacter sakazakii (Cs) continue to occur, particularly in infants consuming powdered infant formula (PIF), the need for sensitive, rapid, and easy-to-use detection of Cs from food and food processing environments is increasing. Here, we developed bioluminescent reporter bacteriophages for viable Cs-specific, substrate-free, rapid detection by introducing luciferase and its corresponding substrate-providing enzyme complex into the virulent phage ΦC01. Although the reporter phage ΦC01_lux, constructed by replacing non-essential genes for phage infectivity with a luxCDABE reporter operon, produced bioluminescence upon Cs infection, the emitted signal was quickly decayed due to the superior bacteriolytic activity of ΦC01. By truncating the membrane pore-forming protein holin and thus limiting its function, the bacterial lysis was delayed and the resultant engineered reporter phage ΦC01_lux_Δhol could produce a more stable and reliable bioluminescent signal. Accordingly, ΦC01_lux_Δhol was able to detect at least an average of 2 CFU/ml of Cs artificially contaminated PIF and Sunsik and food contact surface models within a total of 7 h of assays, including 5 h of pre-enrichment for Cs amplification. The sensitive, easy-to-use, and specific detection of live Cs with the developed reporter phage could be applied as a novel complementary tool for monitoring Cs in food and food-related environments for food safety and public health.

Multivariate analysis of adaptive response to ferulic acid and p-coumaric acid after physiological stresses in Cronobacter sakazakii

AIM

The present study demonstrated the antimicrobial activity of ferulic acid and p-coumaric acid against unstressed and stressed (cold stressed, starved and desiccated) Cronobacter sakazakii in laboratory media (37°C) and reconstituted powdered infant formulation (PIF) with mild heat treatment (50°C).

METHODS AND RESULTS

Five phenolics, namely, quercetin, rutin, caffeic acid, ferulic acid and p-coumaric acid, were tested for antimicrobial activities against five strains of C. sakazakii either unstressed or stressed. Strain specific higher resistance to ferulic acid and p-coumaric acid was observed after stress adaptation in laboratory media. The effect of cross protection was validated using reconstituted PIF as delivery vehicle of selected compounds. Both p-coumaric acid and ferulic acid showed inhibition of C. sakazakii in a dose and time dependent manner as revealed by their viable cell counts. Principal component analysis revealed that the desiccated cells were more sensitive to phenolics in reconstituted PIF.

CONCLUSIONS

Only ferulic acid and p-coumaric acid showed marked antibacterial activity with minimum inhibitory concentration in the range of 2·5-5 mg ml^-1 for unstressed C. sakazakii cells in tryptone soy broth. The maximum inhibition was achieved with 20 mg ml^-1 of both the tested polyphenols in reconstituted PIF. Cold stress and starvation stress did not impart any protection nor increased the susceptibility of C. sakazakii, whereas desiccation resulted in increased susceptibility to phenolic compounds.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results obtained in this study helps in understanding the effect of environmental stresses during processing on susceptibility of C. sakazakii to natural antimicrobial agents. Future transcriptomic studies and functional genetic studies are warranted to understand the strain specific stress responses for the development of better control methods possibly by using these natural antagonists.

The responses of Salmonella enterica serovar Typhimurium to vanillin in apple juice through global transcriptomics

Salmonella enterica serovar Typhimurium can survive some extreme environment in food processing, and vanillin generally recognized as safe is bactericidal to pathogens. Thus, we need to explore the responses of S. Typhimurium to vanillin in order to apply this antimicrobial agent in food processing. In this study, we exposed S. Typhimurium to commercial apple juice with/without vanillin (3.2 mg/mL) at 45 °C for 75 min to determine the survival rate. Subsequently, the 10-min cultures were selected for transcriptomic analysis. Using high-throughput RNA sequencing, genes related to vanillin resistance and their expression changes of S. Typhimurium were identified. The survival curve showed that S. Typhimurium treated with vanillin were inactivated by 5.5 log after 75 min, while the control group only decreased by 2.3 log. Such a discrepancy showed the significant antibacterial effect of vanillin on S. Typhimurium. As a result, 265 differentially expressed genes (DEGs) were found when coping with vanillin, among which, 225 showed up-regulation and 40 DEGs were down-regulated. Treated with vanillin, S. Typhimurium significantly up-regulated genes involved in cell membrane, acid tolerance response (ATR) and oxidative stress response, cold shock cross-protection, DNA repair, virulence factors and some key regulators. Firstly, membrane-related genes, including outer membrane (bamE, mepS, ygdI, lolB), inner membrane (yaiY, yicS) and other proteins (yciC, yjcH), were significantly up-regulated because of the damaged cell membrane. Then, up-regulated proteins associated with arginine synthesis (ArgABCDIG) and inward transportation (ArtI, ArtJ, ArtP and HisP), participated in ATR to pump out the protons inside the cell in this scenario. Next, superoxide stress response triggered by vanillin was found to have a significant up-regulation as well, which was controlled by SoxRS regulon. Besides, NADH-associated (nuoA, nuoB, nuoK, nadE, fre and STM3021), thioredoxin (trxA, trxC, tpx and bcp) and glutaredoxin (grxC and grxD) DEGs led to the increase of the oxidative stress response. Cold shock proteins such as CspA and CspC showed an up-regulation, suggesting it might play a role in cross-protecting S. Typhimurium from vanillin stress. Furthermore, DEGs in DNA repair and virulence factors, including flagellar assembly, adhesins and type III secretion system were up-regulated. Some regulators like fur, rpoE and csrA played a pivotal role in response to the stress caused by vanillin. Therefore, this study sounds an alarm for the risks caused by stress tolerance of S. Typhimurium in food industry.

Natural Antimicrobials Suitable for Combating Desiccation-Resistant Salmonella enterica in Milk Powder

Some Salmonella enterica strains survive well in low-water activity (low-aw) foods and cause frequent salmonellosis outbreaks in these products. Methods are needed to overcome such desiccation-resistant Salmonella and to improve the safety of low-aw foods. Building on a recent finding, we hypothesized that natural antimicrobial food additives, which are active against cytoplasmic membrane, could overcome this desiccation resistance phenomenon, and thus, sensitize the pathogen to drying and mild processing. Food additives were screened for the ability to cause leakage of intracellular potassium ions; retention of these ions is vital for protecting Salmonella against desiccation. Two antimicrobial food additives, carvacrol and thymol, caused considerable potassium leakage from the desiccation-resistant S. enterica serovars, Tennessee and Livingstone. Thus, carvacrol and thymol were investigated for their ability to sensitize the desiccation-adapted S. enterica to heat treatment. The combined use of food additives, at their minimum inhibitory concentrations, with heat treatment at 55 °C for 15 min caused 3.1 ± 0.21 to more than 5.5 log colony forming unit (CFU)/mL reduction in desiccation-adapted S. enterica, compared to 2.4 ± 0.53-3.2 ± 0.11 log CFU/mL reduction by sole heat treatment. Carvacrol was the additive that caused the greatest potassium leakage and sensitization of Salmonella to heat; hence, the application of this compound was investigated in a food model against Salmonella Typhimurium ASD200. Addition of carvacrol at 200 or 500 ppm into liquid milk followed by spray-drying reduced the strain's population by 0.9 ± 0.02 and 1.3 ± 0.1 log CFU/g, respectively, compared to 0.6 ± 0.02 log CFU/g reduction for non-treated spray-dried milk. Additionally, freeze-drying of milk treated with high levels of carvacrol (5000 ppm) reduced the population of Salmonella Typhimurium ASD200 by more than 4.5 log CFU/g, compared to 1.1 ± 0.4 log CFU/g reduction for the freeze-dried untreated milk. These findings suggest that carvacrol can combat desiccation-resistant S. enterica, and thus, potentially improve the safety of low-aw foods.

The antimicrobial activity of coenzyme Q0 against planktonic and biofilm forms of Cronobacter sakazakii

Coenzyme Q₀ (CoQ₀) has demonstrated antitumor, anti-inflammatory, and anti-angiogenic activities. Cronobacter sakazakii is an opportunistic foodborne pathogen associated with high mortality in neonates. In this study, the antimicrobial activity and possible antimicrobial mechanism of CoQ₀ against C. sakazakii were investigated. Moreover, the inactivation effect of CoQ₀ on C. sakazakii in biofilms was also evaluated. The minimum inhibitory concentration (MIC) of CoQ₀ against C. sakazakii strains ranged from 0.1 to 0.2 mg/mL. Treatment caused cell membrane dysfunction, as evidenced by cell membrane hyperpolarization, decreased intracellular ATP concentration and cell membrane integrity, and changes in cellular morphology. CoQ₀ combined with mild heat treatment (45, 50, or 55 °C) decreased the number of viable non-desiccated and desiccated C. sakazakii cells in a time- and dose-dependent manner in reconstituted infant milk. Furthermore, CoQ₀ showed effective inactivation activity against C. sakazakii in biofilms on stainless steel, reducing the number of viable cells and damaging the structure of the biofilm. These findings suggest that CoQ₀ has a strong inactivate effect on C. sakazakii and could be used in food production environments to effectively control C. sakazakii and reduce the number of illnesses associated with it.

Microbial community structure and distribution in the air of a powdered infant formula factory based on cultivation and high-throughput sequence methods

The air in a powdered infant formula (PIF) factory is a potential transfer medium for microorganisms. In this study, air samples from 6 main processing areas, almost covering the whole PIF processing line and 1 outdoor location, were collected from a PIF manufacturing plant during the winter and summer periods. A cultivation-based and an Illumina (San Diego, CA) high-throughput 16S rRNA sequencing method was used to investigate the community structures and distributions of bacteria in the air. High microbial diversity (25 genera, 56 species), with a dominant community including Staphylococcus, Bacillus, Acinetobacter, and Kocuria, was found by the cultivation-based method. Moreover, 104 genera were obtained from all samples by high-throughput sequencing methods. Lactococcus (32.3%), Bacillus (29.6%), and Staphylococcus (14.0%) were the preponderant genera. The indices from high-throughput sequencing results indicated that the bacterial community of the air samples was highly diverse. Significant differences in the diversity and distribution at 6 sampling locations were revealed using the 2 methods. In particular, the packaging process contained the highest proportion (39.4%) of isolated strains. The highest diversity in bacterial community structure was found in the outdoor location. More bacterial isolates and higher community diversity were observed in the summer samples compared with the winter samples. In addition, some pathogens, such as Acinetobacter baumannii, Bacillus cereus, and Staphylococcus cohnii, were mainly found in the large bag filling process, can filling, and packaging process areas. The present study provides greater insight into the microbial community and identifies potential sources of air contamination in PIF production environments and can serve as a guide to reduce the risk of microbial contamination in the production of PIF.

Inactivation of Nondesiccated and Desiccated Cronobacter sakazakii in Reconstituted Infant Formula by Combination of Citral and Mild Heat

The objective of this study was to evaluate the combined effect of citral plus mild heat on nondesiccated and desiccated Cronobacter sakazakii in reconstituted infant formula. Various concentrations of citral (0, 0.3, 0.6, and 0.9%) combined with various temperatures (25, 45, 50, and 55°C) were applied to nondesiccated and desiccated cocktails of three C. sakazakii strains (approximately 6.0 log CFU mL^-1) in reconstituted infant formula, and the bacterial populations were assayed periodically. The combined treatments had marked antimicrobial effects on C. sakazakii compared with the control. Desiccated cells were more susceptible to citral than were nondesiccated cells in reconstituted infant formula. These findings suggest there is a potential application of citral in combination with mild heat to control C. sakazakii during preparation of reconstituted infant formula.