New insight into the relationship between Salmonella Typhimurium and the German cockroach suggests active mechanisms of vector-borne transmission

Invasive indoor pests under the microbiological lens: bacterial and viral diversity from local to global scales in bed bugs and cockroaches

Essentially, all animal life interacts closely with an array of microorganisms, such as bacteria and viruses, which can have both beneficial and harmful effects. The advancement of high-throughput molecular biology approaches (DNA and RNA sequencing) has led to an ongoing boom in investigating the composition and functions of microbial communities (microbiota) associated with a wide range of animal taxa, including insects. As this area of investigation has blossomed, such research on indoor urban insect pests has lagged more widely studied species. However, over the last several years, significant strides have been made in understanding the diversity and biological roles of microbes associated with such insects. This review highlights and discusses recent key findings, focusing on bed bugs and cockroaches, two of the most prolific globally invasive indoor insect pests. Advances in this area of research have long-term implications for public health and for the development of novel pest control approaches.

Laboratory and in-home evaluations of consumer- and professional-grade cockroach baits

German cockroaches (Blattella germanica (L.)) are a persistent pest in affordable housing and studies indicate that residents implement control on their own to deal with cockroaches within their homes. While many do-it-yourself (DIY) control options have proven ineffective, baits are widely considered to be a viable DIY solution for residents who do not have access to professional pest control services. To evaluate their efficacy, we tested consumer-use baits (Combat gel bait, Combat bait stations, and Hot Shot liquid bait stations) in both laboratory two-choice assays and in-home assays, comparing them with professional gel baits (Vendetta Nitro, Advion Evolution, and Maxforce FC Magnum). All baits (consumer- and professional-grade) caused > 80% mortality within 14 d in laboratory assays, including against home-collected German cockroach populations. However, the in-home efficacy of consumer-grade baits in comparison to their performance in laboratory assays was inconsistent, with some baits resulting in significant declines in trap catch in 1 month (Hot Shot liquid bait stations), some taking longer (Combat gel bait), and some never showing a decline (Combat bait stations). Discrepancies between product performance in laboratory and in-home studies are concerning and suggest that laboratory assays alone might not be indicative of the potential for control, especially in the context of the more complex home environment. Failures of consumer-grade baits may require re-evaluation of current recommendations for the use of consumer-grade cockroach baits as a viable control option for residents struggling with cockroach infestations without access to professional pest management.

The gut microbiota confers resistance against Salmonella Typhimurium in cockroaches by modulating innate immunity

Cockroaches exhibit unexplained intra- and interpopulation variation in susceptibility to Salmonella enterica serovar Typhimurium (S. Typhimurium) infection. Here, we show that the gut microbiota has a protective effect against colonization by ingested S. Typhimurium in cockroaches. We further examine two potential mechanisms for this effect, showing that commensal bacteria present in the gut do not compete with S. Typhimurium during growth in cockroach feces, but rather prime expression of host antimicrobial peptide genes that suppress S. Typhimurium infection. Lastly, we determine that neither absolute abundance of the microbiota nor its overall diversity is linked to infection susceptibility. Instead, we identify several minority bacterial taxa that exhibit interindividual variation in abundance as key indicators of infection susceptibility among genetically similar individuals. These findings illuminate the potential of cockroaches as an invertebrate model for interspecies microbial interactions and provide insight into vector-borne Salmonella transmission, suggesting that the microbiota of cockroaches could be targeted to reduce pathogen transmission.

Differences in Salmonella Typhimurium infection and excretion among laboratory and field strains of the German cockroach suggest a genomic basis for vector competence

The German cockroach, Blattella germanica, can be a vector of human enteric bacterial pathogens, including Salmonella enterica serovar Typhimurium (S. Typhimurium). Transmission of such pathogens by cockroaches has largely been considered a passive mechanical process, but recent studies have argued against this dogma by demonstrating bacterial proliferation within the cockroach gut and the necessity of specific bacterial genes for successful transmission in the feces, revealing unappreciated biological complexity in the vector-pathogen relationship between cockroaches and S. Typhimurium. However, the influence of naturally occurring variation among cockroach populations on pathogen infection and dissemination has not been investigated. Thus, this study aimed to examine whether distinct strains of B. germanica exhibit differences in their ability to become infected by and disseminate S. Typhimurium. We performed controlled infections of one long-term laboratory strain and three recently field-collected strains reared under identical conditions, then compared bacterial loads in the body and excreta of individual insects. Separately, we also compared rates of necrophagy, a behavior known to contribute to the horizontal spread of S. Typhimurium among cockroaches. Our data show significant differences in infection susceptibility, pathogen shedding in the excreta, and necrophagy between laboratory and field strains as well as between some field strains. These observations represent the first evidence that genomic variation among cockroach populations may influence their ability to become infected by and disseminate pathogens, providing further support for the hypothesis that German cockroaches are active biological vectors rather than passive mechanical vectors of S. Typhimurium. Additional studies are needed to identify the genomic drivers of vector competence for S. Typhimurium in B. germanica.

Risk assessment of the mechanical spread of bacterial pathogens due to Lasius neglectus ants infesting a tertiary hospital

INTRODUCTION

Mechanical spread of microbial pathogens has been investigated in cockroaches, but less well in ants. Considerably less information is available for ants. An investigation into ant-borne mechanical pathogen transmission was triggered by an infestation of a tertiary care hospital with Lasius neglectus ants.

METHODS

The L. neglectus infestation of the orthopaedic surgery department, the ear-nose-throat clinic and the eye clinic as well as of outdoor areas was monitored and correlated with seasonal and weather influences. Microbial colonization on the ants' exoskeleton as well as in homogenates of complete insects and decolonization dynamics of artificial Staphylococcus aureus colonization on the exoskeleton was assessed.

RESULTS

In a low-level infestation setting, L. neglectus activity showed seasonal variations and was positively correlated with temperature (r = 0.7515; P=0.0368) but not with precipitation (r = 0.4699, P=0.2431). Colonization with environmental commensals dominated, while exoskeleton colonization with bacteria with potential aetiological relevance for nosocomial infections was higher for ants from the inpatient setting (6%) than from outdoor areas (0%). Artificial colonization of the exoskeleton with S. aureus vanished to values statistically indistinguishable from baseline within 72 h.

CONCLUSIONS

Low colonization rates with aetiologically relevant bacteria and rapid spontaneous decolonization in the case of contamination make ant-borne transmissions to patients unlikely.

Endosymbiont and gut bacterial communities of the brown-banded cockroach, Supella longipalpa

The brown-banded cockroach (Supella longipalpa) is a widespread nuisance and public health pest. Like the German cockroach (Blattella germanica), this species is adapted to the indoor biome and completes the entirety of its life cycle in human-built structures. Recently, understanding the contributions of commensal and symbiotic microbes to the biology of cockroach pests, as well as the applications of targeting these microbes for pest control, have garnered significant scientific interest. However, relative to B. germanica, the biology of S. longipalpa, including its microbial associations, is understudied. Therefore, the goal of the present study was to quantitatively examine and characterize both the endosymbiont and gut bacterial communities of S. longipalpa for the first time. To do so, bacterial 16S rRNA gene amplicon sequencing was conducted on DNA extracts from whole adult females and males, early instar nymphs, and late instar nymphs. The results demonstrate that the gut microbiome is dominated by two genera of bacteria known to have beneficial probiotic effects in other organisms, namely Lactobacillus and Akkermansia. Furthermore, our data show a significant effect of nymphal development on diversity and variation in the gut microbiome. Lastly, we reveal significant negative correlations between the two intracellular endosymbionts, Blattabacterium and Wolbachia, as well as between Blattabacterium and the gut microbiome, suggesting that Blattabacterium endosymbionts could directly or indirectly influence the composition of other bacterial populations. These findings have implications for understanding the adaptation of S. longipalpa to the indoor biome, its divergence from other indoor cockroach pest species such as B. germanica, the development of novel control approaches that target the microbiome, and fundamental insect-microbe interactions more broadly.

Is the gut microbiome of insects a potential source to meet UN sustainable development goals to eliminate plastic pollution?

As insects such as cockroaches can endure high radiation, flourish in unsanitary circumstances, thrive on germ-infested feed, and can even digest the organic polymer cellulose, the gut microbiota of these species likely produces enzymes contributing to their ability to digest a variety of materials. The use of cockroaches as a bio-resource to eliminate plastic is discussed. We explore whether species such as cockroaches are a potential bio-resource to eliminate plastic pollution and contribute to the sustainable development goals adopted by the United Nations as well as the global community to reduce and/or eliminate plastic pollution.

Survival of Salmonella Typhimurium in the hemolymph of the German cockroach vector is limited by both humoral immune factors and hemocytes but not by trehalose metabolism

The German cockroach (Blattella germanica) has been linked to transmission of Salmonella enterica serovar Typhimurium (S. Typhimurium), but infection dynamics within this vector are poorly characterized. Our recent work has focused on S. Typhimurium infection in the cockroach gut. However, microbial dissemination to the hemolymph is an essential aspect of many vector-borne pathogen transmission cycles and could potentially contribute to S. Typhimurium colonization of cockroaches. Therefore, the goal of this study was to examine the ability of S. Typhimurium to disseminate, survive, and proliferate in the hemolymph of cockroaches after oral infection. We detected only low numbers of bacteria in the hemolymph of a minority of insects (~26%) after oral infection. Further, S. Typhimurium was unable to survive overnight in cell-free hemolymph. Several hypotheses to explain the inability of S. Typhimurium to colonize hemolymph were tested. First, we investigated the ability of S. Typhimurium to metabolize trehalose, the primary sugar in hemolymph. S. Typhimurium grew efficiently in vitro using trehalose as a sole carbon source and mutant strains lacking trehalose metabolism genes exhibited no growth deficiencies in media mimicking the composition of hemolymph, suggesting that trehalose metabolism ability is not a factor involved in restricting survival in hemolymph. On the other hand, heat-inactivated cell-free hemolymph was permissive of S. Typhimurium growth, demonstrating that survival in hemolymph is limited specifically by heat-labile humoral factors. The involvement of cellular immune responses was also investigated and cockroach hemocytes in culture were observed to internalize S. Typhimurium within 1 h of exposure. Most hemocytes harbored few to no bacteria after 24 h, indicating that hemocyte responses are additionally involved in clearing infection from the hemolymph. However, dense intracellular clusters of S. Typhimurium were observed sporadically, suggesting a small subset of hemocytes may serve as reservoirs for bacterial replication. Together, our results reveal that a minute proportion of ingested S. Typhimurium is able to escape the cockroach gut and enter the hemolymph, but this systemic population is limited by both humoral effectors and hemocytes. Thus, we conclude that invasion of the hemolymph appears minimally important for colonization of the cockroach vector and that colonization of the gut is the main driver of vector-borne transmission. Our insight into the antimicrobial mechanisms of cockroach hemolymph also highlights the strong ability of these prevalent pests/vectors to cope with frequent infectious challenges in septic habitats.

Salmonella enterica Infection of Synanthropic Non-native Geckos in Southern Florida

Wild geckos are a significant source of human salmonellosis. We swabbed the cloacas of 37 non-native synanthropic geckos (Gekko gecko, n = 16; Phelsuma grandis, n = 21) from southern Florida, USA, and assayed swab DNA extracts using quantitative polymerase chain reaction of the invA gene. Salmonella enterica was detected in both species with a pooled prevalence of 13.5% (5/37; 95% CI 5.3-27.1%), indicating the potential for zoonotic transmission. Implications for human health in the region are discussed.

The gut microbiota induces melanin deposits that act as substrates for fimA-mediated aggregation of Salmonella Typhimurium and enhance infection of the German cockroach vector

When Salmonella Typhimurium is ingested by German cockroaches, the bacteria replicate in the gut and persist for at least 7 d, enabling transmission in the feces. However, the mechanisms that facilitate survival and persistence in the cockroach gut remain poorly detailed. We previously reported the formation of biofilm-like aggregate populations of S. Typhimurium in the gut of cockroaches upon ingestion. We also reported that deletion of the type-1 fimbrial subunit of S. Typhimurium, fimA, leads to a reduced bacterial load in the cockroach gut. Here, we link these observations and provide further insight into the mechanism and function of S. Typhimurium aggregation in the gut of the cockroach. We show that S. Typhimurium but not Escherichia coli forms aggregated populations in the cockroach gut, and that aggregate formation requires fimA but not the biofilm formation-related genes csgA and csgD. Furthermore, we show that S. Typhimurium aggregates are formed using small granular deposits present in the cockroach gut, which exhibit properties consistent with melanin, as substrates. These melanin deposits are prevalent in the guts of both immature and adult cockroaches from laboratory colonies and are correlated with increased gut bacterial density while being entirely absent in gnotobiotic cockroaches reared without exposure to environmental bacteria, indicating they are induced as a response to the gut microbiota. When cockroaches lacking melanin deposits in the gut are fed S. Typhimurium, they exhibit lower rates of infection than those harboring melanin deposits, demonstrating that microbiota-induced melanin deposits enhance infection of the gut of the vector. IMPORTANCE Cockroaches, including the German cockroach (Blattella germanica), can be both mechanical and biological vectors of pathogenic bacteria. Together, our data reveal a novel mechanism by which S. Typhimurium interacts with the cockroach gut and its microbiota that promotes infection of the vector. These findings exemplify the emerging but underappreciated complexity of the relationship between cockroaches and S. Typhimurium.

Horizontal transmission of Salmonella Typhimurium among German cockroaches and its possible mechanisms

German cockroaches (Blattella germanica) can be both mechanical and biological (amplifying) vectors of enteric pathogens, including Salmonella enterica serovar Typhimurium (S. Typhimurium), which they acquire by feeding upon contaminated substances. Blattella germanica is also a gregarious species that shelters in groups and partakes in unique feeding behaviors such as conspecific coprophagy, necrophagy, and emetophagy. These properties create an interphase for potential horizontal transmission of pathogens among cockroach populations through the fecal-oral route, which could in turn enhance transmission to humans and other animals. Here, we performed a series of experiments to determine: (1) whether horizontal transmission of S. Typhimurium infection takes place in B. germanica, (2) the prevalence of the phenomenon, and (3) the route(s) through which it may occur. We reveal that true horizontal transmission of S. Typhimurium occurs among B. germanica. That is, uninfected cockroaches acquire infection of the gut when co-housed with orally infected conspecifics, albeit at low frequency. Furthermore, we provide definitive evidence that coprophagy and necrophagy are routes of transmission but could not exclude sharing of food or water as contributing routes. On the contrary, transmission by emetophagy appears less likely as oral regurgitates from infected cockroaches contained S. Typhimurium for less than one day after ingesting the bacteria. Together, our data enhance current understanding of the ecology of vector-borne S. Typhimurium transmission by cockroaches, implicating conspecific horizontal transmission as a phenomenon that contributes to maintaining infected cockroach populations independently of contact with primary sources of the pathogen. Although the relative importance of horizontal transmission of pathogens in cockroaches in the field remains to be determined, these results also highlight the important role that food and water sources in the local environment may play in cockroach-borne pathogen transmission and emphasize the importance of sanitation for not only abating infestations but also mitigating pathogen transmission.

Utility of cockroach as a model organism in the assessment of toxicological impacts of environmental pollutants

Environmental pollution is a global concern because of its associated risks to human health and ecosystem. The bio-monitoring of environmental health has attracted much attention in recent years and efforts to minimize environmental contamination as well as to delineate toxicological mechanisms related to toxic exposure are essential to improve the health conditions of both humans and animals. This review aims to substantiate the need and advantages in utilizing cockroaches as a complementary, non-mammalian model to further understand the noxious impact of environmental contaminants on humans and animals. We discuss recent advances in neurotoxicology, immunotoxicology, reproductive and developmental toxicology, environmental forensic entomotoxicology, and environmental toxicology that corroborate the utility of the cockroach (Periplaneta americana, Blaptica dubia, Blattella germanica and Nauphoeta cinerea) in addressing toxicological mechanisms as well as a sensor of environmental pollution. Indeed, recent improvements in behavioural assessment and the detection of potential biomarkers allow for the recognition of phenotypic alterations in cockroaches following exposure to toxic chemicals namely saxitoxin, methylmercury, polychlorinated biphenyls, electromagnetic fields, pharmaceuticals, polycyclic aromatic hydrocarbon, chemical warfare agents and nanoparticles. The review provides a state-of-the-art update on the current utility of cockroach models in various aspects of toxicology as well as discusses the potential limitations and future perspectives.

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.