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

SIMPLE SUMMARY: A few insects are actually considered for mass production as an alternative protein source for animal feed, notably in the fish and poultry sector. Industrial insect rearing, aims at producing high quality insects, following secure sanitary conditions. As for other livestock’s optimal rearing conditions should avoid stress and pathogens. In this study we investigated how abiotic stresses (sieving, starvation and density) and the presence of the bacteria, Serratia marcescens, an opportunistic human and insect pathogen, affect the growth and survival of the yellow mealworm, Tenebrio molitor. We also set up tests to determine if this bacterium could survive in the rearing system by analysing over time its persistence in the feed, in the insect and in the feces. Our result indicate that the studied Serratia strain is not very virulent to the yellow mealworm, that it can persist in the rearing system and can be detected easily by using a selective growth medium. The stress factors showed no impact from sieving but positive importance was found from high larval density while starvation should be avoided. Serratia could be considered as a potential marker, in the assessment of infectious pressure in a rearing system, and should be monitored for sanitary risk issues as well. ABSTRACT: Industrial insect mass rearing aims to produce quality insects under safe sanitary conditions which can be compromised by pathogens and abiotic stressors. Therefore, knowledge on pathogen persistence, virulence and means of detection is of importance. This study focuses on the opportunistic pathogen Serratia marcescens (Sm) as a possible candidate to reveal sanitary issues in Tenebrio molitor (Tm) breeding. A screening test was performed to assess the impact of abiotic stressors (starvation, density and sieving) in presence and absence of Sm. Two Sm detection methods were conducted, and the kinetics of Sm persistence were investigated. Our results show that (i) the presence of Sm had a low but significant effect on Tm mortality, (ii) a short temporary starvation period had a negative impact on larval growth, (iii) the detection of Sm by q-PCR was sensitive but less convenient than a specific Sm growth media, (iv) the kinetics of persistence showed that Sm declined but survived for nine days in the feed and in the feces for three weeks. Both the relatively low virulence and the persistence in the environment suggest that Sm could be used as an indicator for the sanitary status of mealworm production.