Yersinia pestis can infect the Pawlowsky glands of human body lice and be transmitted by louse bite.

Yersinia pestis, the causative agent of plague, is a highly lethal vector-borne pathogen responsible for killing large portions of Europe's population during the Black Death of the Middle Ages. In the wild, Y. pestis cycles between fleas and rodents; occasionally spilling over into humans bitten by infectious fleas. For this reason, fleas and the rats harboring them have been considered the main epidemiological drivers of previous plague pandemics. Human ectoparasites, such as the body louse (Pediculus humanus humanus), have largely been discounted due to their reputation as inefficient vectors of plague bacilli. Using a membrane-feeder adapted strain of body lice, we show that the digestive tract of some body lice become chronically infected with Y. pestis at bacteremia as low as 1 × 105 CFU/ml, and these lice routinely defecate Y. pestis. At higher bacteremia (≥1 × 107 CFU/ml), a subset of the lice develop an infection within the Pawlowsky glands (PGs), a pair of putative accessory salivary glands in the louse head. Lice that developed PG infection transmitted Y. pestis more consistently than those with bacteria only in the digestive tract. These glands are thought to secrete lubricant onto the mouthparts, and we hypothesize that when infected, their secretions contaminate the mouthparts prior to feeding, resulting in bite-based transmission of Y. pestis. The body louse's high level of susceptibility to infection by gram-negative bacteria and their potential to transmit plague bacilli by multiple mechanisms supports the hypothesis that they may have played a role in previous human plague pandemics and local outbreaks.

Acquisition of yersinia murine toxin enabled Yersinia pestis to expand the range of mammalian hosts that sustain flea-borne plague.

Yersinia murine toxin (Ymt) is a phospholipase D encoded on a plasmid acquired by Yersinia pestis after its recent divergence from a Yersinia pseudotuberculosis progenitor. Despite its name, Ymt is not required for virulence but acts to enhance bacterial survival in the flea digestive tract. Certain Y. pestis strains circulating in the Bronze Age lacked Ymt, suggesting that they were not transmitted by fleas. However, we show that the importance of Ymt varies with host blood source. In accordance with the original description, Ymt greatly enhanced Y. pestis survival in fleas infected with bacteremic mouse, human, or black rat blood. In contrast, Ymt was much less important when fleas were infected using brown rat blood. A Y. pestis Ymt- mutant infected fleas nearly as well as the Ymt+ parent strain after feeding on bacteremic brown rat blood, and the mutant was transmitted efficiently by flea bite during the first weeks after infection. The protective function of Ymt correlated with red blood cell digestion kinetics in the flea gut. Thus, early Y. pestis strains that lacked Ymt could have been maintained in flea-brown rat transmission cycles, and perhaps in other hosts with similar blood characteristics. Acquisition of Ymt, however, served to greatly expand the range of hosts that could support flea-borne plague.

"Fleaing" the Plague: Adaptations of Yersinia pestis to Its Insect Vector That Lead to Transmission.

Interest in arthropod-borne pathogens focuses primarily on how they cause disease in humans. How they produce a transmissible infection in their arthropod host is just as critical to their life cycle, however. Yersinia pestis adopts a unique life stage in the digestive tract of its flea vector, characterized by rapid formation of a bacterial biofilm that is enveloped in a complex extracellular polymeric substance. Localization and adherence of the biofilm to the flea foregut is essential for transmission. Here, we review the molecular and genetic mechanisms of these processes and present a comparative evaluation and updated model of two related transmission mechanisms.

Infectious blood source alters early foregut infection and regurgitative transmission of Yersinia pestis by rodent fleas.

Fleas can transmit Yersinia pestis by two mechanisms, early-phase transmission (EPT) and biofilm-dependent transmission (BDT). Transmission efficiency varies among flea species and the results from different studies have not always been consistent. One complicating variable is the species of rodent blood used for the infectious blood meal. To gain insight into the mechanism of EPT and the effect that host blood has on it, fleas were fed bacteremic mouse, rat, guinea pig, or gerbil blood; and the location and characteristics of the infection in the digestive tract and transmissibility of Y. pestis were assessed 1 to 3 days after infection. Surprisingly, 10-28% of two rodent flea species fed bacteremic rat or guinea pig blood refluxed a portion of the infected blood meal into the esophagus within 24 h of feeding. We term this phenomenon post-infection esophageal reflux (PIER). In contrast, PIER was rarely observed in rodent fleas fed bacteremic mouse or gerbil blood. PIER correlated with the accumulation of a dense mixed aggregate of Y. pestis, red blood cell stroma, and oxyhemoglobin crystals that filled the proventriculus. At their next feeding, fleas with PIER were 3-25 times more likely to appear partially blocked, with fresh blood retained within the esophagus, than were fleas without PIER. Three days after feeding on bacteremic rat blood, groups of Oropsylla montana transmitted significantly more CFU than did groups infected using mouse blood, and this enhanced transmission was biofilm-dependent. Our data support a model in which EPT results from regurgitation of Y. pestis from a partially obstructed flea foregut and that EPT and BDT can sometimes temporally overlap. The relative insolubility of the hemoglobin of rats and Sciurids and the slower digestion of their blood appears to promote regurgitative transmission, which may be one reason why these rodents are particularly prominent in plague ecology.

Reevaluation of the Role of Blocked Oropsylla hirsuta Prairie Dog Fleas (Siphonaptera: Ceratophyllidae) in Yersinia pestis (Enterobacterales: Enterobacteriaceae) Transmission.

Prairie dogs in the western United States experience periodic epizootics of plague, caused by the flea-borne bacterial pathogen Yersinia pestis. An early study indicated that Oropsylla hirsuta (Baker), often the most abundant prairie dog flea vector of plague, seldom transmits Y. pestis by the classic blocked flea mechanism. More recently, an alternative early-phase mode of transmission has been proposed as the driving force behind prairie dog epizootics. In this study, using the same flea infection protocol used previously to evaluate early-phase transmission, we assessed the vector competence of O. hirsuta for both modes of transmission. Proventricular blockage was evident during the first two weeks after infection and transmission during this time was at least as efficient as early-phase transmission 2 d after infection. Thus, both modes of transmission likely contribute to plague epizootics in prairie dogs.

Poor vector competence of the human flea, Pulex irritans, to transmit Yersinia pestis.

BACKGROUND: The human flea, Pulex irritans, is widespread globally and has a long association with humans, one of its principal hosts. Its role in plague transmission is still under discussion, although its high prevalence in plague-endemic regions and the presence of infected fleas of this species during plague outbreaks has led to proposals that it has been a significant vector in human-to-human transmission in some historical and present-day epidemiologic situations. However, based on a limited number of studies, P. irritans is considered to be a poor vector and receives very little attention from public health policymakers. In this study we examined the vector competence of P. irritans collected from foxes and owls in the western United States, using a standard protocol and artificial infection system. METHODS: Wild-caught fleas were maintained in the laboratory and infected by allowing them to feed on human or rat blood containing 2 × 108 to 1 × 109 Y. pestis/ml. The fleas were then monitored periodically for infection rate and bacterial load, mortality, feeding rate, bacterial biofilm formation in the foregut (proventricular blockage), and ability to transmit Y. pestis after their single infectious blood meal. RESULTS: P. irritans were susceptible to infection, with more than 30% maintaining high bacterial loads for up to 20 days. Transmission during this time was infrequent and inefficient, however. Consistent with previous studies, a low level of early-phase transmission (3 days after the infectious blood meal) was detected in some trials. Transmission at later time points was also sporadic, and the incidence of proventricular blockage, required for this mode of transmission, was low in fleas infected using rat blood and never occurred in fleas infected using human blood. The highest level of blockage and transmission was seen in fleas infected using rat blood and allowed to feed intermittently rather than daily, indicating that host blood and feeding frequency influence vector competence. CONCLUSIONS: Our results affirm the reputation of P. irritans as a feeble vector compared to rodent flea species examined similarly, and its vector competence may be lower when infected by feeding on bacteremic human blood.

Molecular and Genetic Mechanisms That Mediate Transmission of Yersinia pestis by Fleas.

The ability to cause plague in mammals represents only half of the life history of Yersinia pestis. It is also able to colonize and produce a transmissible infection in the digestive tract of the flea, its insect host. Parallel to studies of the molecular mechanisms by which Y. pestis is able to overcome the immune response of its mammalian hosts, disseminate, and produce septicemia, studies of Y. pestis-flea interactions have led to the identification and characterization of important factors that lead to transmission by flea bite. Y. pestis adapts to the unique conditions in the flea gut by altering its metabolic physiology in ways that promote biofilm development, a common strategy by which bacteria cope with a nutrient-limited environment. Biofilm localization to the flea foregut disrupts normal fluid dynamics of blood feeding, resulting in regurgitative transmission. Many of the important genes, regulatory pathways, and molecules required for this process have been identified and are reviewed here.

Correction: Comparative Ability of Oropsylla montana and Xenopsylla cheopis Fleas to Transmit Yersinia pestis by Two Different Mechanisms.

[This corrects the article DOI: 10.1371/journal.pntd.0005276.].

Transcriptomic profiling of the digestive tract of the rat flea, Xenopsylla cheopis, following blood feeding and infection with Yersinia pestis.

Yersinia pestis, the causative agent of plague, is a highly lethal pathogen transmitted by the bite of infected fleas. Once ingested by a flea, Y. pestis establish a replicative niche in the gut and produce a biofilm that promotes foregut colonization and transmission. The rat flea Xenopsylla cheopis is an important vector to several zoonotic bacterial pathogens including Y. pestis. Some fleas naturally clear themselves of infection; however, the physiological and immunological mechanisms by which this occurs are largely uncharacterized. To address this, RNA was extracted, sequenced, and distinct transcript profiles were assembled de novo from X. cheopis digestive tracts isolated from fleas that were either: 1) not fed for 5 days; 2) fed sterile blood; or 3) fed blood containing ~5x108 CFU/ml Y. pestis KIM6+. Analysis and comparison of the transcript profiles resulted in identification of 23 annotated (and 11 unknown or uncharacterized) digestive tract transcripts that comprise the early transcriptional response of the rat flea gut to infection with Y. pestis. The data indicate that production of antimicrobial peptides regulated by the immune-deficiency pathway (IMD) is the primary flea immune response to infection with Y. pestis. The remaining infection-responsive transcripts, not obviously associated with the immune response, were involved in at least one of 3 physiological themes: 1) alterations to chemosensation and gut peristalsis; 2) modification of digestion and metabolism; and 3) production of chitin-binding proteins (peritrophins). Despite producing several peritrophin transcripts shortly after feeding, including a subset that were infection-responsive, no thick peritrophic membrane was detectable by histochemistry or electron microscopy of rat flea guts for the first 24 hours following blood-feeding. Here we discuss the physiological implications of rat flea infection-responsive transcripts, the function of X. cheopis peritrophins, and the mechanisms by which Y. pestis may be cleared from the flea gut.

Comparative Ability of Oropsylla montana and Xenopsylla cheopis Fleas to Transmit Yersinia pestis by Two Different Mechanisms.

BACKGROUND: Transmission of Yersinia pestis by flea bite can occur by two mechanisms. After taking a blood meal from a bacteremic mammal, fleas have the potential to transmit the very next time they feed. This early-phase transmission resembles mechanical transmission in some respects, but the mechanism is unknown. Thereafter, transmission occurs after Yersinia pestis forms a biofilm in the proventricular valve in the flea foregut. The biofilm can impede and sometimes completely block the ingestion of blood, resulting in regurgitative transmission of bacteria into the bite site. In this study, we compared the relative efficiency of the two modes of transmission for Xenopsylla cheopis, a flea known to become completely blocked at a high rate, and Oropsylla montana, a flea that has been considered to rarely develop proventricular blockage. METHODOLOGY/PRINCIPAL FINDINGS: Fleas that took an infectious blood meal containing Y. pestis were maintained and monitored for four weeks for infection and proventricular blockage. The number of Y. pestis transmitted by groups of fleas by the two modes of transmission was also determined. O. montana readily developed complete proventricular blockage, and large numbers of Y. pestis were transmitted by that mechanism both by it and by X. cheopis, a flea known to block at a high rate. In contrast, few bacteria were transmitted in the early phase by either species. CONCLUSIONS: A model system incorporating standardized experimental conditions and viability controls was developed to more reliably compare the infection, proventricular blockage and transmission dynamics of different flea vectors, and was used to resolve a long-standing uncertainty concerning the vector competence of O. montana. Both X. cheopis and O. montana are fully capable of transmitting Y. pestis by the proventricular biofilm-dependent mechanism.

Feeding Behavior Modulates Biofilm-Mediated Transmission of Yersinia pestis by the Cat Flea, Ctenocephalides felis.

BACKGROUND: The cat flea, Ctenocephalides felis, is prevalent worldwide, will parasitize animal reservoirs of plague, and is associated with human habitations in known plague foci. Despite its pervasiveness, limited information is available about the cat flea's competence as a vector for Yersinia pestis. It is generally considered to be a poor vector, based on studies examining early-phase transmission during the first week after infection, but transmission potential by the biofilm-dependent proventricular-blocking mechanism has never been systematically evaluated. In this study, we assessed the vector competence of cat fleas by both mechanisms. Because the feeding behavior of cat fleas differs markedly from important rat flea vectors, we also examined the influence of feeding behavior on transmission dynamics. METHODOLOGY/PRINCIPAL FINDINGS: Groups of cat fleas were infected with Y. pestis and subsequently provided access to sterile blood meals twice-weekly, 5 times per week, or daily for 4 weeks and monitored for infection, the development of proventricular biofilm and blockage, mortality, and the ability to transmit. In cat fleas allowed prolonged, daily access to blood meals, mimicking their natural feeding behavior, Y. pestis did not efficiently colonize the digestive tract and could only be transmitted during the first week after infection. In contrast, cat fleas that were fed intermittently, mimicking the feeding behavior of the efficient vector Xenopsylla cheopis, could become blocked and regularly transmitted Y. pestis for 3-4 weeks by the biofilm-mediated mechanism, but early-phase transmission was not detected. CONCLUSIONS: The normal feeding behavior of C. felis, more than an intrinsic resistance to infection or blockage by Y. pestis, limits its vector competence. Rapid turnover of midgut contents results in bacterial clearance and disruption of biofilm accumulation in the proventriculus. Anatomical features of the cat flea foregut may also restrict transmission by both early-phase and proventricular biofilm-dependent mechanisms.

Resistance to Innate Immunity Contributes to Colonization of the Insect Gut by Yersinia pestis.

Yersinia pestis, the causative agent of bubonic and pneumonic plague, is typically a zoonotic vector-borne disease of wild rodents. Bacterial biofilm formation in the proventriculus of the flea contributes to chronic infection of fleas and facilitates efficient disease transmission. However prior to biofilm formation, ingested bacteria must survive within the flea midgut, and yet little is known about vector-pathogen interactions that are required for flea gut colonization. Here we establish a Drosophila melanogaster model system to gain insight into Y. pestis colonization of the insect vector. We show that Y. pestis establishes a stable infection in the anterior midgut of fly larvae, and we used this model system to study the roles of genes involved in biofilm production and/or resistance to gut immunity stressors. We find that PhoP and GmhA both contribute to colonization and resistance to antimicrobial peptides in flies, and furthermore, the data suggest biofilm formation may afford protection against antimicrobial peptides. Production of reactive oxygen species in the fly gut, as in fleas, also serves to limit bacterial infection, and OxyR mediates Y. pestis survival in both insect models. Overall, our data establish the fruit fly as an informative model to elucidate the relationship between Y. pestis and its flea vector.

Imaging early pathogenesis of bubonic plague: are neutrophils commandeered for lymphatic transport of bacteria?

Vector-borne infections begin in the dermis when a pathogen is introduced by an arthropod during a blood meal. Several barriers separate an invading pathogen from its replicative niche, including phagocytic cells in the dermis that activate immunity by engulfing would-be pathogens and migrating to the lymph node. In addition, neutrophils circulating in the blood are rapidly recruited when the dermal barriers are penetrated. For flea-borne disease, no insect-encoded immune-suppressive molecules have yet been described that might influence the establishment of infection, leaving the bacteria on their own to defend against the mammalian immune system. Shortly after a flea transmits Yersinia pestis to a mammalian host, the bacteria are transported to the lymph node, where they grow logarithmically and later spread systemically. Even a single cell of Y. pestis can initiate a lethal case of plague. In their article, J. G. Shannon et al. [mBio 4(5):e00170-13, 2013, doi:10.1128/mBio.00170-13] used intravital microscopy to visualize trafficking of Y. pestis in transgenic mice in vivo, which allowed them to examine interactions between bacteria and specific immune cells. Bacteria appeared to preferentially interact with neutrophils but had no detectable interactions with dendritic cells. These findings suggest that Y. pestis infection of neutrophils not only prevents their activation but may even result in their return to circulation and migration to distal sites.

Novel genetic tools for diaminopimelic acid selection in virulence studies of Yersinia pestis.

Molecular studies of bacterial virulence are enhanced by expression of recombinant DNA during infection to allow complementation of mutants and expression of reporter proteins in vivo. For highly pathogenic bacteria, such as Yersinia pestis, these studies are currently limited because deliberate introduction of antibiotic resistance is restricted to those few which are not human treatment options. In this work, we report the development of alternatives to antibiotics as tools for host-pathogen research during Yersinia pestis infections focusing on the diaminopimelic acid (DAP) pathway, a requirement for cell wall synthesis in eubacteria. We generated a mutation in the dapA-nlpB(dapX) operon of Yersinia pestis KIM D27 and CO92 which eliminated the expression of both genes. The resulting strains were auxotrophic for diaminopimelic acid and this phenotype was complemented in trans by expressing dapA in single and multi-copy. In vivo, we found that plasmids derived from the p15a replicon were cured without selection, while selection for DAP enhanced stability without detectable loss of any of the three resident virulence plasmids. The dapAX mutation rendered Y. pestis avirulent in mouse models of bubonic and septicemic plague which could be complemented when dapAX was inserted in single or multi-copy, restoring development of disease that was indistinguishable from the wild type parent strain. We further identified a high level, constitutive promoter in Y. pestis that could be used to drive expression of fluorescent reporters in dapAX strains that had minimal impact to virulence in mouse models while enabling sensitive detection of bacteria during infection. Thus, diaminopimelic acid selection for single or multi-copy genetic systems in Yersinia pestis offers an improved alternative to antibiotics for in vivo studies that causes minimal disruption to virulence.