Comparison of swab transport media for recovery of Listeria monocytogenes from environmental samples

Presence of Listeria at primary production and processing of food of non-animal origin (FNAO) in Bavaria, Germany

Several foodborne outbreaks associated with food of non-animal origin (FNAO) were reported within the last years. In recent years, Listeriamonocytogenes has been associated with such outbreaks. For this reason, different producers of FNAO at the primary production and processing level in Bavaria, Germany, were inspected from July 2020 to June 2021. Environmental and food sampling as well as the sampling of irrigation and processing water was performed to investigate the prevalence of Listeriaspp., including L.monocytogenes at facilities that produce ready-to-eat FNAO. Altogether, 39 producers of soft fruit, vegetables, ready-to-eat raw fruits, and vegetables/fresh cut were inspected. In addition to the on-spot inspections, 407 samples were taken in total, among them, 229 were swab samples from food contact material and the environment, 59 food samples (including soft fruit, vegetables and ready-to-eat vegetables), and 119 samples of irrigation and processing water. Samples were analyzed using methods according to ISO11290-1:2017. Furthermore, the samples of irrigation and processing water were also quantitatively tested for the number of Escherichiacoli (ISO9308-2:2014-06), enterococci (ISO7899-2:2000-11), and Pseudomonasaeruginosa (ISO16266:2008-05). No contamination with E.coli, enterococci, and P.aeruginosa could be detected in most of the samples. Overall, in 12.53% of the samples, Listeriaspp. were detected. L.monocytogenes was identified in 1.72% of the environmental and processing water samples, whereas L.monocytogenes was not detected in food samples. In addition to water sources and quality, this study demonstrates that irrigation regime, cultivation, hygienic handling, and maintenance protocols are highly important to reduce the potential contamination of ready-to-eat soft fruits and vegetables with Listeriaspp.

Effect of Time, Temperature, and Transport Media on the Recovery of Listeria monocytogenes from Environmental Swabs

ABSTRACT

Environmental monitoring for Listeria monocytogenes in food processing environments is key for ensuring the safety of ready-to-eat foods. For sampling, swabs are often hydrated with a wetting or transport medium that may contain neutralizers and other ingredients. After swabbing the environment, the swabs may then be transported or shipped cold to an off-site laboratory for testing, ideally within 48 h. Extended shipping times may subject the pathogen to increased temperatures in the presence of the wetting medium, organics, and other chemicals from the processing facility that could confound detection. This study evaluated growth and detection of L. monocytogenes on stainless steel exposed to either buffer or sodium hypochlorite before drying. Swabs were rehydrated with Butterfield's phosphate buffer, neutralizing buffer, Letheen broth, or Dey-Engley neutralizing broth before swabbing. Swabs were stored in the presence of no added food, cheese whey, or ice cream under both optimal (4°C) and suboptimal (15°C) temperatures for up to 72 h. Overall, there was no growth of L. monocytogenes at 4°C through 72 h of storage, although enrichment from these swabs was dependent on the presence and type of food matrix. Pathogen growth during storage at 15°C was more variable and depended on both the food matrix and transport media used, with Dey-Engley and Letheen broths allowing for the highest population increases. Overall, more enrichments resulting in L. monocytogenes detections were observed when using Letheen broth and neutralizing buffer than Dey-Engley broth, which resulted in fewer detections at 15°C. Logistic regression and Cochran-Mantel-Haenszel analyses determined that storage temperature, transport media, and food matrix all significantly affected detection of L. monocytogenes, whereas storage time did not have a clear effect on recovery from swabs.

HIGHLIGHTS

Comparison of three neutralizing broths for environmental sampling of low levels of Listeria monocytogenes desiccated on stainless steel surfaces and exposed to quaternary ammonium compounds

Background

An effective environmental sampling method involves the use of a transport/neutralizing broth with the ability to neutralize sanitizer residues that are collected during sampling and to maintain viability of stressed Listeria monocytogenes (Lm) cells.

Results

We applied Lm onto stainless steel surfaces and then subjected Lm to desiccation stress for 16–18 h at room temperature (RT, 21–24 °C). This was followed by the subsequent application of Whisper™ V, a quaternary ammonium compound (QAC)-based sanitizer, diluted to 400 ppm and 8000 ppm of active quat, for 6 h. We then sampled Lm with sponges pre-moistened in three transport broths, Dey/Engley (D/E) broth, Letheen broth and HiCap™ broth, to generate environmental samples that contained sanitizer residues and low levels of stressed Lm, which were subsequently analyzed by an enrichment-based method. This scheme conformed with validation guidelines of AOAC International by using 20 environmental test portions per broth that contained low levels of Lm such that not all test portions were positive (i.e., fractional positive). We showed that D/E broth, Letheen broth and HiCap™ broth performed similarly when no quat or 400 ppm of quat was applied to the Lm contaminating stainless steel surfaces. However, when 8000 ppm of quat was applied, Letheen broth did not effectively neutralize the QAC in the samples. These comparisons were performed on samples stored under three conditions after collection to replicate scenarios of sample transport, RT for 2 h, 4 °C for 24 h and 4 °C for 72 h. Comparisons under the three different scenarios generally reached the same conclusions. In addition, we further demonstrated that storing Letheen and HiCap™ broths at RT for two months before sampling did not reduce their capacity to neutralize sanitizers.

Conclusions

We developed a scheme to evaluate the ability of transport broths to neutralize QAC sanitizers. The three transport broths performed similarly with a commonly used concentration of quat, but Letheen broth could not effectively neutralize a very high concentration of QAC. The performance of transport broths was not significantly affected under the assessed pre-sampling and post-sampling storage conditions.