Introduction
As common organisms in indoor environments, arthropods are recognised mechanical and biological vectors of infections in animals, including humans (Doron & Gorbach 2008). Arthropod pests are mechanical carriers of human pathogens, especially of gastrointestinal and other opportunistic pathogens, including Escherichia ,Salmonella , Shigella and Campylobacter (Adamset al. 2015; Gibbons 2016). Klebsiella pneumoniaeand Staphylococcus aureus have been found to be common in the microbiome of cockroaches collected in hospitals, whereas the same arthropod species carries more frequently Escherichia coli, Citrobacter spp., Pseudomonas aeruginosa andStaphylococcus spp. in domestic households (Memona et al.2017; Menasria et al. 2015; Moges et al. 2016). Flies are also known carriers of opportunistic bacteria, such as Klebsiella pneumoniae (Ranjbar et al. 2016), Staphylococcus and Enterobacteriaceae (Barreiro et al. 2013).
Gastrointestinal pathogens carried by flies and cockroaches can cause 2.4 million infections and 180 deaths per year in the UK (Hollandet al. 2020) . Worldwide, pest arthropods are one of the main factors involved in the epidemiology of diarrhoea (Rahmadani & Lee 2020), responsible for the yearly death of 525,000 children under the age of five (Das et al. 2018). Pest arthropods play such an important role in opportunistic infections that the control of e.g. fly populations can reduce the incidence of gastrointestinal bacterial infections by 23% (Das et al. 2018; Knight et al. 1992; Sengupta et al. 1995). However, arthropod pests comprise a minority of the indoor arthropod environment (Barberan et al.2015; Bertone et al. 2016; Leong et al. 2017); studies start to emerge describing the abundance and diversity of non-pest arthropods in these environments (Adams et al. 2015; Barberanet al. 2015; Bertone et al. 2016; Leong et al.2017). The role of these species as potential carriers of opportunistic infections is, to date, understudied. However, human pathogenic bacteria have been isolated from the microbiome of non-pest arthropods, including dark-winged fungus gnats (Diptera: Sciaridae ) (Kwon et al.2016); owl midges (Diptera: Psychodidae ) (Faulde & Spiesberger 2013) , and cellar spiders (Araneae: Pholcidae) (Voloshyn et al.2017). In addition, studies on household surfaces have shown the presence of both human-associated pathogens, includingStaphylococcus , Streptococcus, Corynebacterium ,Lactococcus and Enterobacteriaceae , as well as bacteria associated with the outdoor environment, such as Pseudomonas ,Propionibacterium , Streptophyta and Sphingomonas(Jeon et al. 2013). However, a systematic characterization of non-pest arthropods diversity and their associated microbiota as potential carriers of human pathogens in indoor environments is lacking. Moreover, the screening of pathogenic bacteria associated with arthropods is conducted prevalently with culture-based approaches. Culturable bacteria only constitute 2.5% of existing bacteria, severely limiting the characterization of bacterial communities associated with the indoor environment (Oberauner et al. 2013) .
Here, we study the indoor arthropod community and its associated microbiota, sampled from twenty households located in the West Midlands (UK). The households were evenly split between urban and suburban environments and sampling was conducted by citizen scientists through active captures over a period of 12 months. We classified the arthropods and their relative abundance across the 12-month sampling period using taxonomic cues against entomological references. We applied DNA metabarcoding and culture-based approaches to study the composition and relative abundance of endogenous (gut) and exogenous (exoskeleton) microbiota of the captured arthropod morphospecies. We determined whether the microbiota dynamics and composition were explained by the association with the arthropod morphospecies, the sampling environment (urban/suburban) and the season (month of collection). We then determined how many bacteria families identified in the arthropod microbiota were recognized as human pathogens according to the list of biological agents by the Advisory Committee on Dangerous Pathogens (HSE). Our study provides important insights into the role of non-pest arthropods as potential carriers of opportunistic pathogens for animals and humans, with implications for public health.