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Sublethally heat-injured cells of Salmonella in food can recover under favorable conditions, leading to foodborne illness. To elucidate the molecular mechanism of recovery from heat injury, the global changes in gene transcription of Salmonella Typhimurium were investigated in previous study. In this study, the functions of genes involved in phage shock response (viz., phage shock protein (psp) genes), the transcription levels of which were found in previous study to be increased during recovery from heat injury, were investigated in recovering cells. The increase in pspABCDEFG transcription levels during the recovery process was confirmed by qRT-PCR. To understand the role of psp genes in heat injury recovery, a pspA deletion mutant (ΔpspA) and a pspA-overexpressing strain (S. Typhimurium pBAD30/pspA (+) ) were constructed. ΔpspA showed slightly lower viable counts and membrane potential than those of the wild-type strain during recovery. On the other hand, there was no significant difference in the viable counts between S. Typhimurium pBAD30/pspA (+) and the control strains S. Typhimurium pBAD30/pspA (-) and S. Typhimurium pBAD30 (+) during recovery. It would seem that a lack of PspA protein alone somewhat affects the recovery of S. Typhimurium from heat injury, but overexpression of PspA alone is not sufficient to overcome this effect.

Tomatoes have recently been implicated as an important vehicle in outbreaks of produce-associated salmonellosis. Traceback reports suggested that pre-harvest contamination of Salmonella enterica might be the main reason for these outbreaks; however, the site of pathogen attachment remains unclear. Therefore, it is important to investigate the mechanisms of Salmonella contamination of fresh produce. To trace the presence of Salmonella in soil and plants, Salmonella Enteritidis transformed with a pEGFP plasmid vector (S. Enteritidis-EGFP) was used. Soil was artificially contaminated with S. Enteritidis-EGFP at 104, s 106 or l0 CFU/g, followed by cultivation of tomato plants in the contaminated soil. Samples of the soil and each organ of the tomato (fruit, stems/leaves, and root) were assayed for Salmonella by plating onto Tryptic Soy Agar and using the MPN method. Salmonella levels in the soil gradually decreased over time, and soil persistence was dependent on the initial inoculation level. Salmonella levels were below the detection limit (< 100 CFU/g) in fruits and stems/leaves, regardless of the level of soil contamination. Moreover, S. Enteritidis-EGFP was not detected in the tomato fruits after root injury. These results indicate that the internalization of Salmonella in tomato fruits might not occur after cultivation in contaminated soil. [ABSTRACT FROM AUTHOR]

Proteins and genes involved in the recovery of heat-injured Salmonella Enteritidis were investigated. Salmonella Enteritidis cells cultured overnight in tryptic soy broth (TSB; nonselective medium) were suspended in citric acid-disodium hydrogen phosphate buffer (pH 6). After heat treatment at 55°C for 15 min, the culturable counts measured by tryptic soy agar (TSA; nonselective medium) decreased from 108 to 107 CFU/ml. On the other hand, culturable counts measured by desoxycholate-hydrogen sulfite-lactose (DHL) agar (selective medium) were decreased from 108 to 104 CFU/ml by the same treatment. The results suggest that 99.9% of Salmonella Enteritidis detected on TSA were injured but recoverable. When injured Salmonella Enteritidis was incubated in TSB, the culturable count measured by TSA did not increase for 2 h, whereas that by DHL agar increased after incubation for 30 min. After incubation for 2 h, the culturable count measured by DHL agar reached a similar level with that by TSA, indicating that Salmonella Enteritidis had recovered. The two-dimensional polyacrylamide gel electrophoresis analysis revealed that elongation factor G (FusA) and pyruvate kinase (PykF) specifically increased in the cells just after heat treatment and in the recovery cells. The levels of transcription of 86 stress-inducible genes were also investigated by reverse transcription PCR. Nineteen heat-inducible (clpB, clpX, degP, dnaJ, fkpA, frsJ, gapA, hflB, hslJ, hslU, hslV, htpG, htrA, lon, mopA, mopB, mreB, rpoE, and ppiD), and 12 oxidative-stress and DNA damage-inducible (ahpC, ahpF, fldB, fur, grxA, dinF, katG, mutM, recA, soxR, trxC, and zwf) genes were transcribed extensively during recovery in TSB. The results obtained in this study will be used to develop the media or culture conditions that will promote recovery for the detection of food poisoning bacteria, including injured cells from food products. [ABSTRACT FROM AUTHOR]

Chicken meats are considered as main sources associated with Salmonella infections in humans. In this study, lytic phages against Salmonella were isolated and examined for their efficacy to control Salmonella . Eighteen lytic phages were isolated from raw chicken skin and gizzard. Five phages belonging to Myoviridae and Siphoviridae families were characterized and selected for bacterial challenge tests. The treatment of raw chicken breast samples contaminated with S. Enteritidis and S. Typhimurium at 8 °C by the cocktail of five phages significantly reduced ( P  < 0.05) viable counts by 1.41 and 1.86 log CFU/piece, respectively. When incubated at 25 °C, the highest reductions of viable counts of S. Enteritidis and S. Typhimurium in the phage-treated samples were 3.06 and 2.21 log CFU/piece, respectively ( P  < 0.05). These data suggested that the phages isolated from raw chicken meats are potential agents for controlling Salmonella in raw meats. [ABSTRACT FROM AUTHOR]

Salmonella spp., one of the most frequently reported bacteria, causes foodborne illness and economic losses. Due to the threat of increasing antibiotic resistant foodborne pathogens, application of bacteriophages as novel antibacterial agents in food matrices has become an emerging strategy. In this study, a novel Salmonella phage PS3-1 with high lytic activity against Salmonella Typhimurium was identified from previously isolated phages. PS3-1 belonged to the class Caudoviricetes with a broad host range, and had relatively short latent period (15 min), large burst size (92 PFU/cell), high pH stability (pH 3.0–11.0) and thermal tolerance (4–60 °C). Genome sequencing analysis showed that PS3-1 genome consisted of 107,110 bp DNA, without antibiotic resistance and virulence related genes. The results of growth curve and time-kill assay showed that PS3-1 not only inhibited the growth of S. Typhimurium , but also effectively decreased the viable cell counts (0.30–4.72 log) after 24-h incubation at 7, 25 and 37 °C (P < 0.05). Moreover, >1.28 log of established biofilm cells were effectively removed after 24-h treatment with PS3-1. Besides, PS3-1 significantly reduced the viability of S. Typhimurium in milk, lettuce, raw pork meat and ready-to-eat steamed-chicken breast at different temperatures (P < 0.05). These results demonstrated that PS3-1 may be an excellent antibacterial agent for controlling S. Typhimurium in food industry. • Siphoviridae phage PS3-1 showed a rapid and strong lytic activity against Salmonella. • PS3-1 showed great pH tolerance and thermal stability. • PS3-1 significantly removed the S. Typhimurium biofilms with a maximum reduction of 1.52 log at 7 °C. • PS3-1 effectively reduced S. Typhimurium counts in milk, lettuce, raw pork meat and RTE steamed-chicken breast. [ABSTRACT FROM AUTHOR]

Abstract Sodium chloride maintains osmotic pressure of living cells including bacteria. Heat treatment is common for decontamination of bacteria in food. In this study, effects of NaCl on heat resistance of Salmonella Typhimurium were investigated. After cultivation in TSB containing 0.5% (TSB), 4% (4SC) and 8% (8SC) NaCl, S. Typhimurium cells were heated at 60 °C for 20 min. Total viable counts including intact cells and injured but recoverable cells determined by the plating method using TSA of S. Typhimurium cultured in 4SC and 8SC were significantly higher than those of the cells cultured in TSB. Meanwhile, changes of gene transcription were analyzed by DNA microarray. Transcription of genes involved in the colanic acid synthesis largely increased after cultivation in 4SC and 8SC. The amount of colanic acid significantly increased in the cells cultured in 4SC compared to that in M9-glucose medium. After recovery culture for 3 h, the genes involved in the phage shock response strongly up-regulated, suggesting contribution of these gene products in recovery of heat injured cells. The outcome of this study contributes to understand the mechanism of cross protection in Salmonella. Highlights • Salmonella cultured in the presence of 4% and 8% NaCl increased heat resistance. • Transcription of 366 and 463 genes changed under 4% and 8% NaCl, respectively. • Colanic acid production was induced with high concentration of NaCl. • Phage shock proteins seem important for recovery from heat injury under 4% NaCl. [ABSTRACT FROM AUTHOR]

As one major foodborne pathogen, Salmonella can cause serious food poisoning outbreaks worldwide. Bacteriophage therapy is increasingly considered as one of the promising antibacterial agents for the biocontrol of foodborne pathogens. In the current study, a lytic phage STG2 capable of infecting S. enteritidis and S. typhimurium was characterized, and its efficacy in reducing these foodborne pathogens in both planktonic and biofilm forms was evaluated on cabbage and various surfaces. Genomic characterization revealed that phage STG2 was Siphoviridae phage (Epseptimavirus genus) with a dsDNA genome comprising of 114,275 bp and its genome does not contain any genes associated to antibiotic resistance, toxins, lysogeny, or virulence factors. Additionally, phage STG2 exhibited great efficacy in reducing (>2 Log) planktonic cells on cabbage as well as the biofilms formed on cabbage, polystyrene, and stainless steel, suggesting that phage STG2 is capable of simultaneously controlling both S. enteritidis and S. typhimurium contaminations on food and food-related surfaces. • Lytic phage STG2 has high stress tolerance, and a genome free of virulence genes. • STG2 genome encodes many lytic proteins and tail fiber proteins containing chaperone of endosialidase at C-terminal ends. • Phage STG2 showed great antimicrobial and antibiofilm efficacy against S. enteritidis and S. typhimurium. • Phage STG2 has the potential to reduce the risk of Salmonella infection in food. [ABSTRACT FROM AUTHOR]

• Salmonella and E. coli O157:H7 phages were isolated from chicken products. • Polyvalent phage PS5 was characterized by host range, morphology, one-step growth, stability and genome sequencing. • PS5 significantly reduced S. Enteritidis, S. Typhimurium and E. coli O157:H7 in in vitro and in foods. Salmonella Enteritidis, Salmonella Typhimurium, and Escherichia coli O157:H7 are the most important foodborne pathogens, causing serious food poisoning outbreaks worldwide. Bacteriophages are increasingly considered as novel antibacterial agents to control foodborne pathogens. In this study, 8 Salmonella phages and 10 E. coli O157:H7 phages were isolated from chicken products. A polyvalent phage PS5 capable of infecting S. Enteritidis, S. Typhimurium, and E. coli O157:H7 was further characterized and its efficacy in reducing these foodborne pathogens was evaluated in in vitro and in foods. Morphology, one-step growth, and stability assay showed that phage PS5 was a myovirus, with relatively short latent periods, large burst sizes, and high stability. Genome sequencing analysis revealed that the genome of PS5 does not contain any genes associated to antibiotic resistance, toxins, lysogeny, and virulence factors. In broth, phage PS5 significantly decreased the viable counts of all the three bacterial hosts by more than 1.3 log CFU/mL compared to controls after 2 h of incubation at 4 °C and 24 °C. In foods, treatment with PS5 also resulted in significant reductions of viable counts of all the three bacterial hosts compared to controls at temperatures tested. This is the first report on single phage capable of simultaneously controlling S. Enteritidis, S. Typhimurium and E. coli O157:H7 in both in vitro and in foods. [ABSTRACT FROM AUTHOR]