Origin of the arthropod microbiota
Arthropod endogenous microbiota is typically adapted to the host and genetically determined (Smee et al. 2021; Suppa et al.2020). However, environmentally acquired bacteria are also found in both endogenous and exogenous microbiota of arthropods. These bacteria may have specific regulatory responses and, therefore, may be selectively acquired from the environment in each generation (Engel & Moran 2013; Hannula et al. 2019). Typically, the exogenous microbiota is more diverse than the endogenous one (Oliveira Ramalho et al. 2019).
Consistent with previous studies, we found that the captured arthropods had significantly distinct microbiota and that the diversity of these microbial communities varied among morphospecies showing, on average, a lower diversity in endogenous than exogenous microbiota. Arthropods vary in the extent to which microorganisms are essential to their survival and fitness, with extremes represented by insects with little to no gut microbiota but dependent on intracellular symbionts (e.g., sap feeding insects), to insects with large and complex communities (e.g., termites). Most insects fall within this range with most having moderately complex gut microbiota with ca. 20 abundant taxa (Hammeret al. 2017; McFall-Ngai 2007). Less is known about the diversity of the exogenous microbiota associated with arthropods.
We found that human pathogenic bacteria were abundant in both the endogenous and exogenous microbiota of the studied arthropods. However, these pathogens were only present in indoor arthropods ecologically associated with humans (synanthropic arthropods), and absent in the microbiota of outdoor arthropods. This suggests that the microbiota may be influenced by the environment, but that the establishment of environmental microbes is genetically determined.
The culture-based approach showed severe limitations as compared to the unbiased metabarcoding approach. Through metabarcoding, we were able to identify a higher number of bacteria families and to obtain an unbiased characterization of the arthropod’s associated microbiota. The metabarcoding approach was also able to identify a higher number of potentially opportunistic pathogens associated with the arthropods, improving our understanding of the role of non-pest arthropods as carriers of human pathogens. A parallel analysis of opportunistic infections in humans and the opportunistic pathogens in indoor the arthropods is necessary to establish the risk associated with non-pest arthropods as carriers of human diseases. However, our study provides important insights into the role that non-pest arthropods may have as carriers of opportunistic pathogens in households.