Abstract
BackgroundThe foodborne pathogen Listeria monocytogenes causes the potentially lethal disease listeriosis. Within food-associated environments, L. monocytogenes can persist for long periods and increase the risk of contamination by continued presence in processing facilities or other food-associated environments. Most research on phenotyping of persistent L. monocytogenes’ has explored biofilm formation and sanitizer resistance, with less data examining persistent L. monocytogenes’ phenotypic responses to extrinsic factors, such as variations in osmotic pressure, pH, and energy source availability. It was hypothesized that isolates of persistent strains are able to grow, and grow faster, under a broader range of intrinsic and extrinsic factors compared to closely related isolates of sporadic strains.ResultsTo test this hypothesis, 95 isolates (representing 74 isolates of 20 persistent strains and 21 isolates of sporadic strains) from a series of previous studies in retail delis, were grown at 37 °C, in (i) stress conditions: salt (0, 5, and 10% NaCl), pH (5.2, 7.2, and 9.2), and sanitizer (benzalkonium chloride, 0, 2, and 5 μg/mL) and (ii) energy sources: 25 mM glucose, cellobiose, glycogen, fructose, lactose, and sucrose; the original goal was to follow up with low temperature experiments for treatments where significant differences were observed. Growth rate and the ability to grow of 95 isolates were determined using high-throughput, OD600, growth curves. All stress conditions reduced growth rates in isolates compared to control (p < 0.05). In addition, growth varied by the tested energy sources. In chemically defined, minimal media there was a trend toward more isolates showing growth in all replicates using cellobiose (p = 0.052) compared to the control (glucose) and fewer isolates able to grow in glycogen (p = 0.02), lactose (p = 2.2 × 10− 16), and sucrose (p = 2.2 × 10− 16). Still, at least one isolate was able to consistently grow in every replicate for each energy source.ConclusionsThe central hypothesis was rejected, as there was not a significant difference in growth rate or ability to grow for retail deli isolates of persistent strains compared to sporadic strains for any treatments at 37 °C. Therefore, these data suggest that persistence is likely not determined by a phenotype unique to persistent strains grown at 37 °C and exposed to extrinsic stresses or variation in energy sources.
Highlights
The foodborne pathogen Listeria monocytogenes causes the potentially lethal disease listeriosis
Ninety-five L. monocytogenes isolates, comprised of 74 isolates of persistent strains and sporadic strains, were tested for their growth rates and ability to grow in the presence of extrinsic stress conditions and utilization of energy sources (Table 1)
These strains were collected from a previous, longitudinal study in retail delis [8], where persistent strains were identified based on whole genome sequencing (WGS) core genome Single Nucleotide Polymorphisms (SNP) analysis [32]
Summary
The foodborne pathogen Listeria monocytogenes causes the potentially lethal disease listeriosis. Within food-associated environments, L. monocytogenes can persist for long periods and increase the risk of contamination by continued presence in processing facilities or other food-associated environments. L. monocytogenes is found commonly in natural and agricultural soil, water, and animals, where it can contaminate raw food materials directly and be transferred into processing facilities [2]. L. monocytogenes can be found on ready-to-eat (RTE) foods, such as produce, soft cheeses, and, relevant to this study, deli meats [3]. In 2003, a risk assessment linked 90% of listeriosis cases in the U.S to contaminated RTE deli meats [3]. Biofilms can make it even harder to eliminate the strain as the biofilm physically protects the strain from exposure to bactericidal concentrations of sanitizer [21, 22]
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