Salmonella Typhimurium Level in Mealworms (Tenebrio molitor) After Exposure to Contaminated Substrate

Abstract

Findings of viable Salmonella spp., which are important foodborne pathogens, are seemingly not reported in mealworms (Tenebrio molitor) for feed and food. Still, the bacterial load of mealworms is naturally high and includes members of the Enterobacteriaceae family to which Salmonella belong. This indicates that Salmonella may be able to thrive in mealworms if introduced into the production. Therefore, this study aimed to assess the quantitative level of Salmonella enterica serovar Typhimurium (ST) in mealworms over a 14-day course after exposure to substrate contaminated with ST levels from 1.7 to 7.4 log CFU/g at start (i.e., day 0). The level of ST found in larvae was below the quantitative detection level (1 or 2 log CFU/g) on day 1 in larvae exposed to contamination levels of 1.7, 3.4, and 3.6 log CFU/g opposed to contamination levels of 5.4, 5.6, and 7.4 log CFU/g, respectively. The maximum level of ST detected in individual 1-g larvae samples was 5.8 log CFU/g, but the level varied among the triplicate samples from each sampling, and the highest average value was 5.3 ± 0.3. Beyond day 7, only larvae exposed to the contamination level of 7.4 log CFU/g were >1.0 log CFU/g in the triplicate samples. However, qualitative testing (10 g) showed the presence of ST in larvae until the end of the experiment on day 14 except for the lowest contamination level of 1.7 log CFU/g. Parallel testing of surface disinfected larvae indicated that some larvae may be ST-positive due to Salmonella residing on the surface only. Still, any detection of Salmonella is of concern from a food safety perspective. In substrate with contamination levels below 3.6 log CFU/g, the level of ST was below the quantitative detection limit within a few days. Still, ST was detected until the end of experiment on day 14 except for the lowest contamination level of 1.7 log CFU/g. This study indicates the importance of avoiding introduction of Salmonella into the production, e.g., via contaminated substrate in order to avoid Salmonella-positive larvae as they remained positive for at least 14 days (except at the lowest contamination level).

Impact and Persistence of Serratia marcescens in Tenebrio molitor Larvae and Feed under Optimal and Stressed Mass Rearing Conditions

“…Analyses of the dynamics and survival of human pathogens during entomoconversion of various substrates are reported for Tm and especially for Hermetia illucens [ 27 ]. Tm studies on the persistence of Salmonella indicated that the survival was clearly dose-dependent, and no multiplication was observed [ 7 , 13 ]. Our objective was to evaluate the capacity of a non- Tm adapted Sm strain to contaminate and persist in Tm mass rearing, therefore, we used an environmental strain with no information on virulence.…”

“…These stress factors challenge mealworms’ health by reducing their defense and immunity or by changing their environment and behavior. In such context, pathogen prevalence could increase and create sanitary risk for the livestock as for the final insect product [ 7 ]. Many insect pathogens, belonging to various groups such as viruses, bacteria, fungi, protists and nematodes, can infect insects produced for food and feed, as described by Eilenberg et al (2015) [ 8 ].…”

“…Otherwise, with respect to sanitary issues, food-borne and environmental pathogens like Listeria monocytogenes , Staphylococcus aureus , Escherichia coli and Clostridium perfringens have so far not been recorded as being present in the gut microbiome of Tm [ 10 , 11 ]. Meanwhile, two studies have reported on the ability of Salmonella enterica to persist in Tm gut and feces, following artificial contamination of the feed substrate [ 7 , 13 ]. In these studies, it was found that Salmonella could persist in the insect, largely dependent on the contamination load, but there was no clear mention of pathogen development in the insect.…”

Yellow Mealworm and Black Soldier Fly Larvae for Feed and Food Production in Europe, with Emphasis on Iceland

“…Insect producers should carry out regular checks of incoming substrate materials and substrates must be stored in dry, temperature appropriate and hygienic conditions [115]. Substrate control is an important part of safety management regarding insect breeding, because the substrates ingested can have a strong influence on insects' microbiota [90][91][92]116].…”

“…For mealworms, blanching is found to be the most successful heating method as it considerably reduces the bacterial content and fungi [85,94,117,119]; however, blanching is not sufficient to kill bacterial endospores [85,87,94,118]. As there is high endospore content in soil [87], which can be used as a substrate, and substrate influences the insects microbiota [90][91][92]116], management of the substrate is important, but the use of classical feed additives or fermentation has been shown to reduce spore forming [87]. Drying and acidifying techniques of insects are also promising to reduce endospores [118].…”