Survival and transcriptomic response of Salmonella enterica on fresh-cut fruits

Understanding the potential of fresh produce as vehicles of Salmonella enterica

This chapter presents an overview of Salmonella enterica as a contaminant in fresh produce, exploring outbreaks and recalls linked to them. It also provides information on potential sources of S. enterica contamination throughout the entire production chain of these products and presents food safety tools and new approaches for controlling this pathogen. S. enterica is recognized worldwide as a pathogen responsible for foodborne outbreaks, and there has been an increase in reported cases of salmonellosis linked to fresh produce. These products are susceptible to contamination throughout various stages of the farm-to-fork process. The potential sources of contamination are present from pre-harvest and harvest stages (e.g., soil, blossoms, seeds, irrigation water and gray/blackwater, wild and domestic animals/organic fertilizers, and distinctive traits of the plant) to post-harvest stages (e.g., processing, packaging, storage/retail, and preparing for consumption). Thus, controlling S. enterica contamination is extremely important for ensuring the safe consumption of fresh produce. However, obtaining practical, efficient, low-cost, and sustainable solutions that ensure the products' sensorial, nutritional, and food quality is still a challenge. As an alternative to conventional methods, recent studies report the use of new technologies, such as neutral, acidic or low chlorine electrolyzed oxidizing water, ultraviolet light, ultrasound, microemulsion of essential oils, cold plasma, irradiation, bacteriophages, and other methods, which can be used alone or in combination with the conventional ones. Therefore, understanding the main sources of S. enterica contamination in fresh produce and the effective approach for controlling this pathogen is crucial to reducing future outbreaks.

Predicting the impact of temperature and relative humidity on Salmonella growth and survival in sliced chard, broccoli and red cabbage

This study assessed the fate of a Salmonella enterica cocktail (S. Typhimurium, S. Enteritidis, S. Newport, S. Agona and S. Anatum; initial counts 3.5 log CFU/g) in minimally processed sliced chard, broccoli and red cabbage at 16 conditions of different temperature (7, 14, 21 and 37 °C) and relative humidity (RH; 15, 35, 65 and 95%) over six days (144 h). Linear regression was used to estimate the rate change of Salmonella in cut vegetables as a function of temperature and relative humidity (RH). R2 value of 0.85, 0.87, and 0.78 were observed for the rates of change in chard, broccoli, and red cabbage, respectively. The interaction between temperature and RH was significant in all sliced vegetables. Higher temperatures and RH values favored Salmonella growth. As temperature or RH decreased, the rate of S. enterica change varied by vegetable. The models developed here can improve risk management of Salmonella in fresh cut vegetables.

Cold-tolerance mechanisms in foodborne pathogens: Escherichia coli and Listeria monocytogenes as examples

The cold chain is an integral part of the modern food industry. Low temperatures can effectively alleviate food loss and the transmission of foodborne diseases caused by microbial reproduction. However, recent reports have highlighted shortcomings in the current cold chain technology's ability to prevent and control cold-tolerant foodborne pathogens. Furthermore, it has been observed that certain cold-chain foods have emerged as new sources of infection for foodborne disease outbreaks. Consequently, there is a pressing need to enhance control measures targeting cold-tolerant pathogens within the existing cold chain system. This paper aims to review the recent advancements in understanding the cold tolerance mechanisms of key model organisms, identify key issues in current research, and explore the potential of utilizing big data and omics technology in future studies.

Development of Shortened Enrichment Methods for Detection of Salmonella Typhimurium Spiked in Milk

Rapid and accurate testing of pathogenic Salmonella enterica in dairy products could reduce the risk of exposure to the bacterial pathogens for consumers. This study aimed to reduce the assessment time needed for enteric bacteria recovery and quantification in food using the natural growth properties of Salmonella enterica Typhimurium (S. Typhimurium) in cow's milk and efficiently using rapid PCR methods. Over 5 h of 37 °C enrichment, culture and PCR methods measured increases in the non-heat-treated S. Typhimurium concentration at similar rates, with an average increase of 2.7 log10 CFU/mL between the start of enrichment and 5 h. In contrast, no bacteria were recovered by culture after S. Typhimurium in milk received heat treatment, and the number of gene copies of heat-treated Salmonella detected by PCR did not increase with the enrichment time. Thus, comparing culture and PCR data over just 5 h of enrichment time can detect and differentiate between replicating bacteria and dead bacteria.

hilD is required for the active internalization of Salmonella Newport into cherry tomatoes

Salmonella enterica is a foodborne pathogen that can be internalized into fresh produce. Most of the Salmonella virulence genes are clustered in regions denominated Salmonella Pathogenicity Islands (SPI). SPI-1 encodes a Type Three Secretion System (T3SS-1) and effector proteins that allow the internalization of Salmonella into animal cells. HilD is a transcriptional regulator that induces expression of SPI-1 genes and other related virulence genes located outside of this island. Here, we assessed the role of hilD in the internalization of Salmonella Newport and Typhimurium into cherry tomatoes, by evaluating either an isolate from an avocado orchard, S. Newport-45, and the laboratory strain S. Typhimurium SL1344 and their isogenic mutants in hilD. The internalization of these bacteria was carried out by using a temperature gradient of 12 °C. The transcription of hilD and invA was tested by qRT-PCR experiments. Our results show that S. Newport-45 hilD mutant viable cells obtained from the interior of the fruit were decreased (2.7-fold), compared with those observed for S. Typhimurium SL1344. Interestingly, at 3 days post-inoculation, the cells recovered from S. Newport-45 hilD mutant were similar to those recovered from all the strains evaluated, suggesting that hilD is required only for the initial internalization of S. Newport.

Diversity and Resistance Profiles of Human Non-typhoidal Salmonella spp. in Greece, 2003-2020

Salmonella spp. is one of the most common foodborne pathogens in humans. Here, we summarize the laboratory surveillance data of human non-typhoidal salmonellosis in Greece for 2003–2020. The total number of samples declined over the study period (p < 0.001). Of the 193 identified serotypes, S. Enteritidis was the most common (52.8%), followed by S. Typhimurium (11.5%), monophasic S. Typhimurium 1,4,[5],12:i:- (4.4%), S. Bovismorbificans (3.4%) and S. Oranienburg (2.4%). The isolation rate of S. Enteritidis declined (p < 0.001), followed by an increase of the less common serotypes. Monophasic S. Typhimurium has been among the five most frequently identified serotypes every year since it was first identified in 2007. Overall, Salmonella isolates were resistant to penicillins (11%); aminoglycosides (15%); tetracyclines (12%); miscellaneous agents (sulphonamides, trimethoprim, chloramphenicol and streptomycin) (12%) and third-generation cephalosporins (2%). No isolate was resistant to carbapenems. In total, 2070 isolates (24%) were resistant to one or two antimicrobial classes and 903 (10%) to three and more. Out of the 1166 isolates resistant to fluoroquinolones (13%), 845 (72%) were S. Enteritidis. S. Enteritidis was also the most frequently identified serotype with a resistance to third-generation cephalosporins (37%, 62/166), followed by S. Typhimurium (12%, 20/166). MDR was most frequently identified for S. Typhimurium and its monophasic variant (resistant phenotype of ampicillin, streptomycin, tetracycline and sulphamethoxazole with or without chloramphenicol or trimethoprim).