Abstract
Growth models are predominately used in the food industry to estimate the potential growth of selected microorganisms under environmental conditions. The growth kinetics, cellular morphology, and antibiotic resistance were studied throughout the life cycle of Salmonella Typhimurium. The effect of the previous life cycle phase [late log phase (LLP), early stationary phase (ESP), late stationary phase (LSP), and early death phase (EDP)] of Salmonella after reinoculation in brain heart infusion broth (BHI), ground chicken extract (GCE), and BHI at pH 5, 7, and 9 and salt concentrations 2, 3, and 4% was investigated. The growth media and previous life cycle phase had significant effects on the lag time (λ), specific growth rate (μmax), and maximum population density (Ymax). At 2 and 4% salt concentration, the LLP had the significantly (p < 0.05) fastest μmax (1.07 and 0.69 log CFU/ml/h, respectively). As the cells transitioned from the late log phase (LLP) to the early death phase (EDP), the λ significantly (p < 0.05) increased. At pH 5 and 9, the EDP had a significantly (p < 0.05) lower Ymax than the LLP, ESP, and LSP. As the cells transitioned from a rod shape to a coccoid shape in the EDP, the cells were more susceptible to antibiotics. The cells regained their resistance as they transitioned back to a rod shape from the EDP to the log and stationary phase. Our results revealed that growth kinetics, cell’s length, shape, and antibiotic resistance were significantly affected by the previous life cycle phase. The results of this study also demonstrate that the previous life cycle should be considered when developing growth models of foodborne pathogens to better ensure the safety of poultry and poultry products.
Highlights
Salmonella is a Gram-negative, rod-shaped, and non-spore forming bacterium
As the availability of nutrients dissipated and L. monocytogenes transitioned into the long-term stationary (LTS) phase, the morphology changed from a rod to a coccoid shape
The experimental data obtained at various previous life cycle phases and under environmental conditions and stresses was fitted to the full Baranyi growth model (Figures 2–4)
Summary
Salmonella is a Gram-negative, rod-shaped, and non-spore forming bacterium. Salmonellosis, an infection caused by Salmonella, is one of the most common and widely encountered foodborne diseases, with tens of millions of human cases occurring worldwide annually (WHO, 2013). In the United States, there is an estimated 1 million salmonellosis cases annually causing 19,000 hospitalizations and 380 deaths (Scallan et al, 2011) This has resulted in an increased interest to better understand the growth and changes of Salmonella under antibiotic and environmental stresses. Wen et al (2009) suggested that current food safety studies that base the effectiveness of interventions on the ability to reduce foodborne pathogens in the stationary phase may be overestimated. They found that Listeria monocytogenes in the long-term stationary (LTS) phase has more resistance to thermal and high-pressure processing than in the stationary phase. The coccoid shape increased its survivability and resistance to treatments. Wen et al (2013) found that initial viable cell density (~106 to ~1010 CFU/ml) and pH (5.36–6.85) of L. monocytogenes affected the transition to the LTS phase
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