Evaluation of viability of E. coli O157: H7 on chlorine and lactic acid treated spinach leaves using combined propidium monoazide staining and real-time PCR

A meta-analysis of factors influencing the inactivation of Shiga toxin-producing Escherichia coli O157:H7 in leafy greens

Recent advancements in modeling suggest that microbial inactivation in leafy greens follows a nonlinear pattern, rather than the simple first-order kinetics. In this study, we evaluated 17 inactivation models commonly used to describe microbial decline and established the conditions that govern microbial survival on leafy greens. Through a systematic review of 65 articles, we extracted 530 datasets to model the fate of Shiga toxin-producing Escherichia coli O157:H7 on leafy greens. Various factor analysis methods were employed to evaluate the impact of identified conditions on survival metrics. A two-parameter model (jm2) provided the best fit to most of both natural and antimicrobial-induced persistence datasets, whereas the one-parameter exponential model provided the best fit to less than 20% of the datasets. The jm2 model (adjusted R2 = .89) also outperformed the exponential model (adjusted R2 = .58) in fitting the pooled microbial survival data. In the context of survival metrics, the model averaging approach generated higher values than the exponential model for >4 log reduction times (LRTs), suggesting that the exponential model may be overpredicting inactivation at later time points. The random forest technique revealed that temperature and inoculum size were common factors determining inactivation in both natural and antimicrobial-induced die-offs.. The findings show the limitations of relying on the first-order survival metric of 1 LRT and considering nonlinear inactivation in produce safety decision-making.

The biochemical characteristics of viable but nonculturable state Yersinia enterocolitica induced by lactic acid stress and its presence in food systems

The viable but nonculturable (VBNC) state is adopted by many foodborne pathogenic bacteria to survive in adverse conditions. This study found that lactic acid, a widely used food preservative, can induce Yersinia enterocolitica to enter a VBNC state. Y. enterocolitica treated with 2 mg/mL lactic acid completely lost culturability within 20 min, and 10.137 ± 1.693 % of the cells entered a VBNC state. VBNC state cells could be recovered (resuscitated) in tryptic soy broth (TSB), 5 % (v/v) Tween80-TSB, and 2 mg/mL sodium pyruvate-TSB. In the VBNC state of Y. enterocolitica induced by lactic acid, the intracellular adenosine triphosphate (ATP) concentration and various enzyme activities were decreased, and the reactive oxygen species (ROS) level was elevated, compared with uninduced cells. The VBNC state cells were significantly more resistant to heat and simulated gastric fluid than uninduced cells, but their ability to survive in a high-osmotic-pressure environment was significantly less than that of uninduced cells. The VBNC state cells induced by lactic acid changed from long rod-like to short rod-like, with small vacuoles at the cell edges; the genetic material was loosened and the density of cytoplasm was increased. The VBNC state cells had decreased ability to adhere to and invade Caco-2 (human colorectal adenocarcinoma) cells. The transcription levels of genes related to adhesion, invasion, motility, and resistance to adverse environmental stress were downregulated in VBNC state cells relative to uninduced cells. In meat-based broth, all nine tested strains of Y. enterocolitica entered the VBNC state after lactic acid treatment; among these strains, only VBNC state cells of Y. enterocolitica CMCC 52207 and Isolate 36 could not be recovered. Therefore, this study is a wake-up call for food safety problems caused by VBNC state pathogens induced by lactic acid.

Use of a silver-based sanitizer to accelerate Escherichia coli die-off on fresh-cut lettuce and maintain produce quality during cold storage: Laboratory and pilot-plant scale tests

Outbreaks and product recalls involving romaine and iceberg lettuce are frequently reported in the United States. Novel technologies are needed to inactivate pathogens without compromising product quality and shelf life. In this study, the effects of a process aid composed of silver dihydrogen citrate, glycerin, and lactic acid (SGL) on Escherichia coli and Listeria monocytogenes concentrations on lettuce immediately after washing and during cold storage were evaluated. Sensory and quality attributes of fresh-cut iceberg lettuce were also evaluated. Laboratory results indicated that application of SGL solution for 30 s as a first step in the washing process resulted in a 3.15 log reduction in E. coli O157:H7 immediately after washing. For E. coli O157:H7 a significant difference between SGL treatment and all other treatments was maintained until day 7. On day zero, SGL led to a 2.94 log reduction of L. monocytogenes. However, there was no significant difference between treatments with or without SGL regardless of storage time. Pilot-plant results showed that samples receiving SGL spray followed by chlorinated flume wash exhibited a greater reduction (1.48 log) in nonpathogenic E. coli populations at the end of shelf life than other treatments (p < 0.05). Additional pilot plant tests were conducted to investigate the hypothesis that SGL residues could continue to impact microbial survival on the final washed lettuce. Results show that pathogens introduced subsequent to flume washing of lettuce pretreated with SGL solution were not affected by antimicrobial residues. The final quality and shelf life of flume washed lettuce were also unaffected by pretreatment with SGL. In conclusion, the results of this study demonstrate that this new technology has the potential to accelerate E. coli die-off on fresh-cut lettuce during cold storage and improve product safety, while not affecting quality throughout the shelf life of the finished products.