Blattella germanica displays a large arsenal of antimicrobial peptide genes

Comparative Transcriptomics of Fat Bodies between Symbiotic and Quasi-Aposymbiotic Adult Females of Blattella germanica with Emphasis on the Metabolic Integration with Its Endosymbiont Blattabacterium and Its Immune System

We explored the metabolic integration of Blattella germanica and its obligate endosymbiont Blattabacterium cuenoti by the transcriptomic analysis of the fat body of quasi-aposymbiotic cockroaches, where the endosymbionts were almost entirely removed with rifampicin. Fat bodies from quasi-aposymbiotic insects displayed large differences in gene expression compared to controls. In quasi-aposymbionts, the metabolism of phenylalanine and tyrosine involved in cuticle sclerotization and pigmentation increased drastically to compensate for the deficiency in the biosynthesis of these amino acids by the endosymbionts. On the other hand, the uricolytic pathway and the biosynthesis of uric acid were severely decreased, probably because the reduced population of endosymbionts was unable to metabolize urea to ammonia. Metabolite transporters that could be involved in the endosymbiosis process were identified. Immune system and antimicrobial peptide (AMP) gene expression was also reduced in quasi-aposymbionts, genes encoding peptidoglycan-recognition proteins, which may provide clues for the maintenance of the symbiotic relationship, as well as three AMP genes whose involvement in the symbiotic relationship will require additional analysis. Finally, a search for AMP-like factors that could be involved in controlling the endosymbiont identified two orphan genes encoding proteins smaller than 200 amino acids underexpressed in quasi-aposymbionts, suggesting a role in the host-endosymbiont relationship.

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.

Comparative Efficacy of a Fungal Entomopathogen with a Broad Host Range against Two Human-Associated Pests

The ability of a fungal entomopathogen to infect an insect depends on a variety of factors, including strain, host, and environmental conditions. Similarly, an insect’s ability to prevent fungal infection is dependent on its biology, environment, and evolutionary history. Synanthropic pests have adapted to thrive in the indoor environment, yet they arose from divergent evolutionary lineages and occupy different feeding guilds. The hematophagous bed bug (Cimex lectularius) and omnivorous German cockroach (Blattella germanica) are highly successful indoors, but have evolved different physiological and behavioral adaptations to cope with the human-built environment, some of which also reduce the efficacy of fungal biopesticides. In order to gain greater insight into the host barriers that prevent or constrain fungal infection in bed bugs and German cockroaches, we tested different doses of Beauveria bassiana GHA through surface contact, topical application, feeding, and injection. Bed bugs were generally more susceptible to infection by B. bassiana with the mode of delivery having a significant impact on infectivity. The German cockroach was highly resilient to infection, requiring high doses of fungal conidia (>8.8 × 104) delivered by injection into the hemocoel to cause mortality. Mortality occurred much faster in both insect species after exposure to surfaces dusted with dry conidia than surfaces treated with conidia suspended in water or oil. These findings highlight the importance of developing innovative delivery techniques to enhance fungal entomopathogens against bed bugs and cockroaches.

Identification of the Gene Repertoire of the IMD Pathway and Expression of Antimicrobial Peptide Genes in Several Tissues and Hemolymph of the Cockroach Blattella germanica

Antimicrobial peptide (AMP) genes, triggered by Toll and IMD pathways, are essential components of the innate immune system in the German cockroach Blattella germanica. Besides their role in killing pathogenic bacteria, AMPs could be involved in controlling its symbiotic systems (endosymbiont and microbiota). We found that the IMD pathway was active in the adult female transcriptomes of six tissues (salivary glands, foregut, midgut, hindgut, Malpighian tubules and fat body) and hemolymph. Total expression of AMP genes was high in hemolymph and salivary glands and much lower in the other sample types. The expression of specific AMP genes was very heterogeneous among sample types. Two genes, defensin_g10 and drosomycin_g5, displayed relevant expression in the seven sample types, although higher in hemolymph. Other genes only displayed high expression in one tissue. Almost no expression of attacin-like and blattellicin genes was observed in any sample type, although some of them were among the genes with the highest expression in adult female whole bodies. The expression of AMP genes in salivary glands could help control pathogens ingested with food and even determine gut microbiota composition. The low expression levels in midgut and hindgut are probably related to the presence of beneficial microbiota. Furthermore, a reduction in the expression of AMP genes in fat body could be the way to prevent damage to the population of the endosymbiont Blattabacterium cuenoti within bacteriocytes.

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.

Nephridiophagids (Chytridiomycota) reduce the fitness of their host insects

Nephridiophagids are unicellular fungi (Chytridiomycota), which infect the Malpighian tubules of insects. While most life cycle features are known, the effects of these endobionts on their hosts remain poorly understood. Here, we present results on the influence of an infection of the cockroach Blattella germanica with Nephridiophaga blattellae (Ni = Nephridiophaga-infected) on physical, physiological, and reproductive fitness parameters. Since the gut nematode Blatticola blattae is a further common parasite of B. germanica, we included double infected cockroaches (N+Ni = nematode plus Ni) in selected experiments. Ni individuals had lower fat reserves and showed reduced mobility. The lifespan of adult hosts was only slightly affected in these individuals but significantly shortened when both Nephridiophaga and nematodes were present. Ni as well as N+Ni females produced considerably less offspring than parasite-free (P-free) females. Immune parameters such as the number of hemocytes and phenoloxidase activity were barely changed by Nephridiophaga and/or nematode infections, while the ability to detoxify pesticides decreased. Quantitative proteomics from hemolymph of P-free, Ni, and N+Ni populations revealed clear differences in the expression profiles. For Ni animals, for example, the down-regulation of fatty acid synthases corroborates our finding of reduced fat reserves. Our study clearly shows that an infection with Nephridiophaga (and nematodes) leads to an overall reduced host fitness.

Of Cockroaches and Symbionts: Recent Advances in the Characterization of the Relationship between Blattella germanica and Its Dual Symbiotic System

Mutualistic stable symbioses are widespread in all groups of eukaryotes, especially in insects, where symbionts have played an essential role in their evolution. Many insects live in obligate relationship with different ecto- and endosymbiotic bacteria, which are needed to maintain their hosts’ fitness in their natural environment, to the point of even relying on them for survival. The case of cockroaches (Blattodea) is paradigmatic, as both symbiotic systems coexist in the same organism in two separated compartments: an intracellular endosymbiont (Blattabacterium) inside bacteriocytes located in the fat body, and a rich and complex microbiota in the hindgut. The German cockroach Blattella germanica is a good model for the study of symbiotic interactions, as it can be maintained in the laboratory in controlled populations, allowing the perturbations of the two symbiotic systems in order to study the communication and integration of the tripartite organization of the host–endosymbiont–microbiota, and to evaluate the role of symbiotic antimicrobial peptides (AMPs) in host control over their symbionts. The importance of cockroaches as reservoirs and transmission vectors of antibiotic resistance sequences, and their putative interest to search for AMPs to deal with the problem, is also discussed.