Temporospatial fate of bacteria and immune effector expression in house flies fed GFP-Escherichia coli O157:H7

House Flies Are Underappreciated Yet Important Reservoirs and Vectors of Microbial Threats to Animal and Human Health

House flies are well recognized as filth-associated organisms and public nuisances. House flies create sanitation issues when they bridge the gap between microbe-rich breeding environments and animal/human habitations. Numerous scientific surveys have demonstrated that house flies harbor bacterial pathogens that pose a threat to humans and animals. More extensive and informative surveys incorporating next-generation sequencing technologies have shown that house fly carriage of pathogens and harmful genetic elements, such as antimicrobial resistance genes, is more widespread and dangerous than previously thought. Further, there is a strong body of research confirming that flies not only harbor but also transmit viable, and presumably infectious, bacterial pathogens. Some pathogens replicate and persist in the fly, permitting prolonged shedding and dissemination. Finally, although the drivers still have yet to be firmly determined, the potential range of dissemination of flies and their associated pathogens can be extensive. Despite this evidence, the house flies’ role as reservoirs, disseminators, and true, yet facultative, vectors for pathogens have been greatly underestimated and underappreciated. In this review, we present key studies that bolster the house fly’s role both an important player in microbial ecology and population biology and as transmitters of microbial threats to animal and human health.

Effects of habitat and sampling time on bacterial community composition and diversity in the gut of the female house fly, Musca domestica Linnaeus (Diptera: Muscidae)

Adult house flies feed and breed in a variety of microbe-rich habitats and serve as vectors for human and animal pathogens. To better understand their role in harbouring and disseminating bacteria, we characterized the composition and diversity of bacterial communities in the gut of female house flies collected from three different habitats in Kansas: agricultural (dairy farm), urban (business area dumpsters) and mixed (business located between residential and animal agriculture areas). Bacterial community composition and diversity were influenced more by the house flies' habitat than by sampling time. The most abundant taxa were also highly prevalent in the house flies collected from all three habitats, potentially representing a 'core microbiome' attributable to the fly's trophic and reproductive associations with substrates and food sources comprised of decaying matter and/or animal waste. Bacterial taxa associated with vertebrate guts/faeces and potential pathogens were highly abundant in agricultural fly microbial communities. Interestingly, taxa of potential pathogens were highly abundant in flies from the mixed and urban sites. House flies harboured diverse bacterial communities influenced by the habitat in which they reside, including potential human and animal pathogens, further bolstering their role in the dissemination of pathogens, and indicating their utility for pathogen surveillance.

Constitutively-expressed and induced immune effectors in the house fly (Musca domestica) and the transcription factors that may regulate them

Insects possess both infection-induced and constitutively expressed innate immune defences. Some effectors, such as lysozymes and antimicrobial peptides (AMPs), are constitutively expressed in flies, but expression patterns vary across tissues and species. The house fly (Musca domestica L.) has an impressive immune repertoire, with more effector genes than any other flies. We used RNA-seq to explore both constitutive and induced expression of immune effectors in flies. House flies were fed either Pseudomonas aeruginosa or Escherichia coli, or sterile control broth, and gene expression in the gut and carcass was analysed 4 h post-feeding. Flies fed either bacterium did not induce AMP expression, but some lysozyme and AMP genes were constitutively expressed. Prior transcriptome data from flies injected with bacteria also were analysed, and these constitutively expressed genes differed from those induced by bacterial injection. Binding sites for the transcription factor Myc were enriched upstream of constitutively expressed AMP genes, while upstream regions of induced AMPs were enriched for NF-κB binding sites resembling those of the Imd-responsive transcription factor Relish. Therefore, we identified at least two expression repertoires for AMPs in the house fly: constitutively expressed genes that may be regulated by Myc, and induced AMPs likely regulated by Relish.

Flies as a potential vector of selected alert pathogens in a hospital environment

Multi-drug resistant pathogens are a global problem. Flies are a potential vector of multi-drug resistant pathogens, which can be particularly dangerous in the hospital environment. This study aimed to evaluate flies as vectors of alert pathogens. The research material consisted of 100 flies (Musca domestica (46.0%), Lucilia sericata (28.0%), and Calliphora vicina (26.0%)) collected at the University Hospital No. 1 dr. A. Jurasz in Bydgoszcz (Poland) in 2018-2019 (summer months). The presence of bacteria of the genera: Enterococcus, Staphylococcus, Escherichia, Leclercia, Citrobacter, Hafnia, Providencia, Proteus, Enterobacter, Klebsiella, Raoultella, Morganella, Moellerella, Bordetella, Pantoea, Serratia, Plesiomonas, Wohlfahrimonas, and Lelliottia was confirmed. The most frequently isolated species included: Enterococcus faecalis (n = 64), Escherichia coli (n = 43) and Moellerella wisconsensis (n = 24). The infection rate and antibiotic resistance of bacteria were assessed. One strain of Proteus mirabilis (isolated from Calliphora vicina) produced ESBLs (extended-spectrum beta-lactamases). The infection rate was 0.38%, 0.26%, and 0.20% for Musca domestica, Lucilia sericata, and Calliphora vicina, respectively. The flies from a hospital area were not a vector of alert pathogens. Monitoring flies as potential vectors of pathogens is an important aspect of public health, especially for hospitalized patients.

Antimicrobial peptide expression in the cockroach gut during enterobacterial infection is specific and influenced by type III secretion

Omnivorous synanthropic cockroaches, such as the German cockroach (Blattella germanica), are reservoirs and vectors of enteric bacterial pathogens. A lifestyle conducive to frequent encounters with high loads of diverse bacteria may have led to the evolution of unique innate immune systems in these insects. The innate immune response of insects relies largely on generalized mechanisms to sense and eliminate foreign microbes. However, analyses of the genomes of common synanthropic cockroaches previously revealed a repertoire of pathogen associated molecular pattern (PAMP) receptors and antimicrobial peptides (AMPs) that is significantly expanded relative to most holometabolous insect models and vectors, supporting the intriguing possibility that cockroaches may encode enhanced recognition within their immune system and may possess an enhanced capacity to fine tune innate immune responses. Investigating how cockroaches respond to infection with enterobacteria provides the opportunity to expand our fundamental knowledge of the regulation of insect innate immunity in a context that is biologically and medically relevant. German cockroaches can harbor both Salmonella enterica serovar Typhimurium and Escherichia coli in their gut without experiencing pathogenesis. The former colonizes the gut and replicates while the latter persists only transiently. We hypothesized that differences in the innate immune response may contribute to or result from the difference in infection dynamics between the two enterobacteria. To test this hypothesis, we used qRT-PCR to analyze expression of five genes encoding representative AMPs (Attacins, Blattellicin, Defensins) in the gut of German cockroaches 1 and 24 h after ingestion of live or heat-killed enterobacteria. We found that robust AMP expression was induced in response to ingestion of a live wild-type strain of S. Typhimurium, but not in response to live E. coli, heat-killed S. Typhimurium, or a live mutant strain of S. Typhimurium lacking type III secretion systems. These results indicate that the cockroach immune system does not respond to stimulation with high levels of ingested bacterial PAMPs such as peptidoglycan. Rather, AMP expression in the gut appears to be induced by active bacterial colonization involving type III secretion. We speculate that this form of regulation may have evolved to prevent over activation of the immune system from frequent ingestion of innocuous, non-colonizing, or non-viable bacteria. While additional work is needed to delineate the molecular mechanisms underlying our observations, our findings provide significant novel insight into the immunological adaptation of cockroaches to life in septic environments as well as the factors that regulate bacterial pathogen transmission by these insects.

Regulatory mechanisms of microbial homeostasis in insect gut

Insects live in incredibly complex environments. The intestinal epithelium of insects is in constant contact with microorganisms, some of which are beneficial and some harmful to the host. Insect gut health and function are maintained through multidimensional mechanisms that can proficiently remove foreign pathogenic microorganisms while effectively maintaining local symbiotic microbial homeostasis. The basic immune mechanisms of the insect gut, such as the dual oxidase-reactive oxygen species (Duox-ROS) system and the immune deficiency (Imd)-signaling pathway, are involved in the maintenance of microbial homeostasis. This paper reviews the role of physical defenses, the Duox-ROS and Imd signaling pathways, the Janus kinase/signal transducers and activators of transcription signaling pathway, and intestinal symbiotic flora in the homeostatic maintenance of the insect gut microbiome.

Cantaloupe Facilitates Salmonella Typhimurium Survival Within and Transmission Among Adult House Flies (Musca domestica L.)

Salmonella enterica serovar Typhimurium is a pathogen harbored by livestock and shed in their feces, which serves as an acquisition source for adult house flies. This study used a green fluorescent protein (GFP) expressing strain of Salmonella Typhimurium to assess its acquisition by and survival within house flies, and transmission from and between flies in the presence or absence of cantaloupe. Female house flies were exposed to manure inoculated with either sterile phosphate-buffered saline or GFP-Salmonella Typhimurium for 12 h, then used in four experiments each performed over 24 h. Experiment 1 assessed the survival of GFP-Salmonella Typhimurium within inoculated flies. Experiment 2 determined transmission of GFP-Salmonella Typhimurium from inoculated flies to cantaloupe. Experiment 3 assessed fly acquisition of GFP-Salmonella Typhimurium from inoculated cantaloupe. Experiment 4 evaluated transmission of GFP-Salmonella Typhimurium between inoculated flies and uninoculated flies in the presence and absence of cantaloupe. GFP-Salmonella Typhimurium survived in inoculated flies but bacterial abundance decreased between 0 and 6 h without cantaloupe present and between 0 and 6 h and 6 and 24 h with cantaloupe present. Uninoculated flies acquired GFP-Salmonella Typhimurium from inoculated cantaloupe and bacterial abundance increased in cantaloupe and flies from 6 to 24 h. More uninoculated flies exposed to inoculated flies acquired GFP-Salmonella Typhimurium when cantaloupe was present than when absent. We infer that the presence of a shared food source facilitated the transfer of GFP-Salmonella Typhimurium from inoculated to uninoculated flies. Our study demonstrated that house flies acquired, harbored, and excreted viable GFP-Salmonella Typhimurium and transferred bacteria to food and each other. Understanding the dynamics of bacterial acquisition and transmission of bacteria between flies and food helps in assessing the risk flies pose to food safety and human health.

The seaweed fly (Coelopidae) can facilitate environmental survival and transmission of E. coli O157 at sandy beaches

The sustainable management of recreational beaches is essential for minimising risk of human exposure to microbial pathogens whilst simultaneously maintaining valuable ecosystem services. Decaying seaweed on public beaches is gaining recognition as a substrate for microbial contamination, and is a potentially significant reservoir for human pathogens in close proximity to beach users. Closely associated with beds of decaying seaweed are dense populations of the seaweed fly (Coelopidae), which could influence the spatio-temporal fate of seaweed-associated human pathogens within beach environments. Replicated mesocosms containing seaweed inoculated with a bioluminescent strain of the zoonotic pathogen E. coli O157:H7, were used to determine the effects of two seaweed flies, Coelopa frigida and C. pilipes, on E. coli O157:H7 survival dynamics. Multiple generations of seaweed flies and their larvae significantly enhanced persistence of E. coli O157:H7 in simulated wrack habitats, demonstrating that both female and male C. frigida flies are capable of transferring E. coli O157:H7 between individual wrack beds and into the sand. Adult fly faeces can contain significant concentrations of E. coli O157:H7, which suggests they are capable of acting as biological vectors and bridge hosts between wrack habitats and other seaweed fly populations, and facilitate the persistence and dispersal of E. coli O157:H7 in sandy beach environments. This study provides the first evidence that seaweed fly populations inhabiting natural wrack beds contaminated with the human pathogen E. coli O157:H7 have the capacity to amplify the hazard source, and therefore potential transmission risk, to beach users exposed to seaweed and sand in the intertidal zone. The risk to public health from seaweed flies and decaying wrack beds is usually limited by human avoidance behaviour; however, seaweed fly migration and nuisance inland plagues in urban areas could increase human exposure routes beyond the beach environment.

Effects of Bacterial Dose and Fly Sex on Persistence and Excretion of Salmonella enterica serovar Typhimurium From Adult House Flies (Musca domestica L.; Diptera: Muscidae)

Salmonella Typhimurium (Le Minor and Popoff 1987; Enterobacteriales: Enterobacteriaceae) is a pathogen that causes gastroenteritis in humans and can be harbored by house flies. Factors influencing excretion of S. Typhimurium from infected flies have not been elucidated but are essential for assessing transmission potential. We determined the persistence and excretion of a green fluorescent protein (GFP) expressing strain of S. Typhimurium from house flies. Individual male and female flies were fed either sterile Luria-Bertani (LB) broth (controls) or cultures of "high" (~105 colony forming units [CFU]) or "low" (~104 CFU) doses of bacteria (treatments). Bacterial persistence was determined over 16 h by culturing whole-fly homogenate. Both sex and dose affected persistence between 6 and 12 h postingestion. In a separate experiment, fly excretion events were monitored during this time interval and excreta droplets were individually cultured for bacteria. Female flies had more excretion events than males across treatments. We observed interactions of fly sex and bacterial abundance (dose), both on the proportion of Salmonella-positive droplets and the CFU shed per droplet (CFU/droplet). In the low-dose treatment, males excreted a greater proportion of positive droplets than females. In the high-dose treatment, males excreted more CFU/droplet than females. High-dose male flies excreted more CFU/droplet than low-dose males, but low-dose females excreted more CFU/droplet than high-dose females. Irrespective of sex, low-dose flies excreted a greater dose-adjusted CFU (CFU droplet/CFU fed) than high-dose flies. This study demonstrates that both bacterial abundance and fly sex may influence excretion of bacteria from flies, and should be considered when assessing the risk of house fly transmission of pathogens.

A systematic review of human pathogens carried by the housefly (Musca domestica L.)

BACKGROUND

The synanthropic house fly, Musca domestica (Diptera: Muscidae), is a mechanical vector of pathogens (bacteria, fungi, viruses, and parasites), some of which cause serious diseases in humans and domestic animals. In the present study, a systematic review was done on the types and prevalence of human pathogens carried by the house fly.

METHODS

Major health-related electronic databases including PubMed, PubMed Central, Google Scholar, and Science Direct were searched (Last update 31/11/2017) for relevant literature on pathogens that have been isolated from the house fly.

RESULTS

Of the 1718 titles produced by bibliographic search, 99 were included in the review. Among the titles included, 69, 15, 3, 4, 1 and 7 described bacterial, fungi, bacteria+fungi, parasites, parasite+bacteria, and viral pathogens, respectively. Most of the house flies were captured in/around human habitation and animal farms. Pathogens were frequently isolated from body surfaces of the flies. Over 130 pathogens, predominantly bacteria (including some serious and life-threatening species) were identified from the house flies. Numerous publications also reported antimicrobial resistant bacteria and fungi isolated from house flies.

CONCLUSIONS

This review showed that house flies carry a large number of pathogens which can cause serious infections in humans and animals. More studies are needed to identify new pathogens carried by the house fly.

Dose-Dependent Effects on Replication and Persistence of Salmonella enterica serovar Typhimurium in House Flies (Diptera: Muscidae)

Adult house flies (Musca domestica L.) ingest variable numbers of bacteria when they encounter microbe-rich substrates. Bacterial abundance may affect replication within the fly gut, which subsequently impacts vector potential. This study investigated the dose-dependent replication of GFP-expressing Salmonella enterica serovar Typhimurium (ex Kauffmann and Edwards1952) Le Minor and Popoff 1987, (Enterobacteriales: Enterobacteriaceae) (GFP S. Typhimurium) within the fly alimentary canal. Adult house flies were fed two doses (colony forming units, CFU) of GFP S. Typhimurium (high, ~105 CFU and low, ~104 CFU). Bacteria were examined at 2-, 4-, 6-, 12-, and 24-h postingestion (PI) in situ in the gut via epifluorescence microscopy and enumerated by culture on selective media. In both treatment groups, GFP S. Typhimurium proliferated and persisted in flies for 24 h. In the high-dose group, proliferation peaked at 6 h PI (>500% increase). In the low-dose group, proliferation peaked at both 4 and 6 h PI (>900% increase). Dose significantly affected bacterial replication within the house fly alimentary canal, particularly at 4-, 6-, and 12-h PI. The ability of S. Typhimurium to proliferate and persist in the alimentary canal demonstrates that house flies may serve as significant reservoirs and probable disseminators of this pathogen. Our results show that bacterial abundance should be considered when assessing the potential of house flies to harbor and transmit pathogens.

Campylobacter jejuni in Musca domestica: An examination of survival and transmission potential in light of the innate immune responses of the house flies

The house fly, Musca domestica, has been implicated as a vector of Campylobacter spp., a major cause of human disease. Little is known whether house flies serve as biological amplifying hosts or mechanical vectors for Campylobacter jejuni. We investigated the period after C. jejuni had been ingested by house flies in which viable C. jejuni colonies could be isolated from whole bodies, the vomitus and the excreta of adult M. domestica and evaluated the activation of innate immune responses of house flies to ingested C. jejuni over time. C. jejuni could be cultured from infected houseflies soon after ingestion but no countable C. jejuni colonies were observed > 24 h postingestion. We detected viable C. jejuni in house fly vomitus and excreta up to 4 h after ingestion, but no viable bacteria were detected ≥ 8 h. Suppression subtractive hybridization identified pathogen-induced gene expression in the intestinal tracts of adult house flies 4-24 h after ingesting C. jejuni. We measured the expression of immune regulatory (thor, JNK, and spheroide) and effector (cecropin, diptericin, attacin, defensing, and lysozyme) genes in C. jejuni-infected and -uninfected house flies using quantitative real time PCR. Some house fly factor, or combination of factors, eliminates C. jejuni within 24 h postingestion. Because C. jejuni is not amplified within the body of the housefly, this insect likely serves as a mechanical vector rather than as a true biological, amplifying vector for C. jejuni, and adds to our understanding of insect-pathogen interactions.

Dose-dependent fate of GFP-expressing Escherichia coli in the alimentary canal of adult house flies

The adult house fly Musca domestica (L.) (Diptera: Muscidae) can disseminate bacteria from microbe-rich substrates to areas in which humans and domesticated animals reside. Because bacterial abundance fluctuates widely across substrates, flies encounter and ingest varying amounts of bacteria. This study investigated the dose-dependent survival of bacteria in house flies. Flies were fed four different 'doses' of green fluorescent protein (GFP)-expressing Escherichia coli (GFP E. coli) (very low, low, medium, high) and survival was determined at 1, 4, 10 and 22 h post-ingestion by culture and epifluorescent microscopy. Over 22 h, the decline in GFP E. coli was significant in all treatments (P < 0.04) except the very low dose treatment (P = 0.235). Change in survival (ΔS) did not differ between flies fed low and very low doses of bacteria across all time-points, although ΔS in both treatments differed from that in flies fed high and medium doses of bacteria at several time-points. At 4, 10 and 22 h, GFP E. coli ΔS significantly differed between medium and high dose-fed flies. A threshold dose, above which bacteria are detected and destroyed by house flies, may exist and is likely to be immune-mediated. Understanding dose-dependent bacterial survival in flies can help in predicting bacteria transmission potential.

Cues from chewing insects - the intersection of DAMPs, HAMPs, MAMPs and effectors

Chewing herbivores cause massive damage when crushing plant tissues with their mandibles, thus releasing a vast array of cues that may be perceived by the plant to mobilize defenses. Besides releasing damage cues in wounded tissues, herbivores deposit abundant cues from their saliva, regurgitant and feces that trigger herbivore specific responses in plants. Herbivores can manipulate the perception mechanisms and defense signals to suppress plant defenses by secreting effectors and/or by exploiting their associated oral microbes. Recent studies indicate that both the composition of herbivore cues and the plant's ability to recognize them are highly dependent upon the specific plant-herbivore system. There is a growing amount of work on identifying herbivore elicitors and effectors, but the most significant bottleneck in the discipline is the identification and characterization of plant receptors that perceive these herbivore-specific cues.

Ingested Salmonella enterica, Cronobacter sakazakii, Escherichia coli O157:H7, and Listeria monocytogenes: transmission dynamics from adult house flies to their eggs and first filial (F1) generation adults

BACKGROUND

The mechanical transmission of pathogenic bacteria by synanthropic filth flies is widely recognized. While many studies report the fate and the temporospatial distribution of ingested foodborne bacteria by filth flies, there is little evidence about the transmission dynamics of ingested foodborne bacteria by adult house flies (Musca domestica) to their progeny. In this study, we fed parental house fly adults with food contaminated with low, medium, and high concentrations of Salmonella enterica, Cronobacter sakazakii, Escherichia coli O157:H7, and Listeria monocytogenes and evaluated the probability of transmission of these pathogens to house fly eggs and the surface and the alimentary canal of their first filial (F1) generation adults.

RESULTS

All foodborne pathogens were present in samples containing pooled house fly eggs. The probability of transmission was higher after parental house flies ingested food containing medium bacterial loads. Cronobacter sakazakii was 16, 6, and 3 times more likely to be transmitted to house fly eggs than S. enterica, E. coli O157:H7, and L. monocytogenes, respectively. Only S. enterica and C. sakazakii were transmitted to F1 generation adults and their presence was 2.4 times more likely on their body surfaces than in their alimentary canals. The highest probabilities of finding S. enterica (60 %) and C. sakazakii (28 %) on newly emerged F1 adults were observed after parental house flies ingested food containing medium and high levels of these pathogens, respectively.

CONCLUSION

Our study demonstrates that adult house flies that fed from food contaminated with various levels of foodborne bacteria were able to transmit those pathogens to their eggs and some were further transmitted to newly emerged F1 generation adults, enhancing the vector potential of these insects. Understanding the type of associations that synanthropic filth flies establish with foodborne pathogens will help to elucidate transmission mechanisms and possible ways to mitigate the spread of foodborne pathogens.