Molecular Characterization of ISerratia marcescens/I Strain Isolated from Yellow Mealworms, ITenebrio molitor/I, in The Netherlands

Serratia marcescens is an important pathogenic bacterium associated with human infections, with remarkably important features related to its intrinsic and acquired antimicrobial resistance. In this study, we aimed to molecularly characterize an S. marcescens strain isolated from the skin of rearing-sized yellow mealworms (Tenebrio molitor) during an increase in mortality of up to ~30% at a Dutch mealworm farm. We identified the presence of several antimicrobial-resistance genes, all located in the chromosome, and the presence of several virulence genes associated with bacterial invasion. This case demonstrates that T. molitor can act as a reservoir and as an alternative path for exposing clinically important antibiotic-resistant bacteria that may affect animals and humans. Although the entomopathogenic activity was not confirmed, it underlines the need to monitor and assess the One Health risks of the bacteria present in individual insect farms, before insects and their products may enter the feed and food chain. Insect culture has developed rapidly worldwide; it faces important security and safety control issues, including animal infections and disease development. In the Netherlands, in 2021, a ~30% mortality of mealworms, Tenebrio molitor, occurred at one farm, where over-humid sites in the substrate were observed. Bacterial cultures from both the external and internal partsof fry and larger mealworms were identified by MALDI-TOF to predominantly Serratia marcescens, Staphylococcus xylosus and Staphylococus saprofyticus. Due to the important role of S. marcescens as a potential zoonotic bacterium, we performed a molecular characterization of the isolated strain. Genomic analysis showed a multidrug-resistant S. marcescens isolate carrying a tet (41), aac (6′)-Ic, and blaSST-1 chromosomal class C beta-lactamase-resistantgenes, all located on the chromosome. Additionally, several virulence genes were identified. The phylogenetic tree revealed that the S. marcescens strain from this study was similar to other S. marcescens strains from different ecological niches. Although the entomopathogenic activity was not confirmed, this case demonstrates that T. molitor can act as a reservoir and as an alternative path for exposing clinically important antibiotic-resistant bacteria that can affect animals and humans. It underlines the need to keep management factors optimal, before insects and their products enter the feed and food chain.