Differential infectivity of the Lyme disease spirochete Borrelia burgdorferi derived from Ixodes scapularis salivary glands and midgut

Laodelphax striatellus saliva mucin enables the formation of stylet sheathes to facilitate its feeding and rice stripe virus transmission

BACKGROUND

Laodelphax striatellus transmits rice stripe virus (RSV) during sap feeding on the rice plant. The insect saliva proteins have direct and indirect roles in mediating RSV transmission; however, the function of most saliva proteins remains unclear.

RESULTS

In this study, we sequenced L. striatellus saliva proteins using shotgun liquid chromatography-electrospray ionization-tandem mass spectrometry. We identified 41 secreted saliva proteins, among which a saliva mucin-like protein, designated LssaMP, was the most abundant. In silico analysis revealed the sequence conservation among planthoppers. We revealed that the LssaMP gene is specifically expressed in the salivary glands and the protein is secreted as a component of gel saliva. Using LssaMP-specific double-stranded RNA (dsRNA) to silence gene expression, we revealed that LssaMP is required for formation of the salivary sheath, an important structure for sap feeding. Disrupting LssaMP expression resulted in inefficient formation of the feeding structure, thereby stopping insects from secreting watery saliva and acquiring sufficient nutrients from the phloem sap. We confirmed that RSV is mainly released via the watery saliva, which passes through the salivary sheathes into the plant phloem. An insufficient feeding structure results in decreased release of watery saliva, as well as the arboviruses.

CONCLUSION

This study clarified the function of an insect saliva protein in mediating insect feeding, as well as arbovirus transmission. © 2022 Society of Chemical Industry.

Competition Between Strains of Borrelia afzelii in Immature Ixodes ricinus Ticks Is Not Affected by Season

Vector-borne pathogens often consist of genetically distinct strains that can establish co-infections in the vertebrate host and the arthropod vector. Co-infections (or mixed infections) can result in competitive interactions between strains with important consequences for strain abundance and transmission. Here we used the spirochete bacterium, Borrelia afzelii, as a model system to investigate the interactions between strains inside its tick vector, Ixodes ricinus. Larvae were fed on mice infected with either one or two strains of B. afzelii. Engorged larvae were allowed to molt into nymphs that were subsequently exposed to three seasonal treatments (artificial summer, artificial winter, and natural winter), which differed in temperature and light conditions. We used strain-specific qPCRs to quantify the presence and abundance of each strain in the immature ticks. Co-infection in the mice reduced host-to-tick transmission to larval ticks and this effect was maintained in the resultant nymphs at 1 and 4 months after the larva-to-nymph molt. Competition between strains in co-infected ticks reduced the abundance of both strains. This inter-strain competition occurred in the three life stages that we investigated: engorged larvae, recently molted nymphs, and overwintered nymphs. The abundance of B. afzelii in the nymphs declined by 40.5% over a period of 3 months, but this phenomenon was not influenced by the seasonal treatment. Future studies should investigate whether inter-strain competition in the tick influences the subsequent strain-specific transmission success from the tick to the vertebrate host.

Sleeper cells: the stringent response and persistence in the Borreliella (Borrelia) burgdorferi enzootic cycle

Infections with tick-transmitted Borreliella (Borrelia) burgdorferi, the cause of Lyme disease, represent an increasingly large public health problem in North America and Europe. The ability of these spirochetes to maintain themselves for extended periods of time in their tick vectors and vertebrate reservoirs is crucial for continuance of the enzootic cycle as well as for the increasing exposure of humans to them. The stringent response mediated by the alarmone (p)ppGpp has been determined to be a master regulator in B. burgdorferi. It modulates the expression of identified and unidentified open reading frames needed to deal with and overcome the many nutritional stresses and other challenges faced by the spirochete in ticks and animal reservoirs. The metabolic and morphologic changes resulting from activation of the stringent response in B. burgdorferi may also be involved in the recently described non-genetic phenotypic phenomenon of tolerance to otherwise lethal doses of antimicrobials and to other antimicrobial activities. It may thus constitute a linchpin in multiple aspects of infections with Lyme disease borrelia, providing a link between the micro-ecological challenges of its enzootic life-cycle and long-term residence in the tissues of its animal reservoirs, with the evolutionary side effect of potential persistence in incidental human hosts.

The abundance of the Lyme disease pathogen Borrelia afzelii declines over time in the tick vector Ixodes ricinus

Background

The population dynamics of vector-borne pathogens inside the arthropod vector can have important consequences for vector-to-host transmission. Tick-borne spirochete bacteria of the Borrelia burgdorferi (sensu lato) species complex cause Lyme borreliosis in humans and spend long periods of time (>12 months) in their Ixodes tick vectors. To date, few studies have investigated the dynamics of Borrelia spirochete populations in unfed Ixodes nymphal ticks.

Methods

Larval ticks from our laboratory colony of I. ricinus were experimentally infected with B. afzelii, and killed at 1 month and 4 months after the larva-to-nymph moult. The spirochete load was also compared between engorged larval ticks and unfed nymphs (from the same cohort) and between unfed nymphs and unfed adult ticks (from the same cohort). The spirochete load of B. afzelii in each tick was estimated using qPCR.

Results

The mean spirochete load in the 1-month-old nymphs (~14,000 spirochetes) was seven times higher than the 4-month-old nymphs (~2000 spirochetes). Thus, the nymphal spirochete load declined by 80% over a period of 3 months. An engorged larval tick acquired ~100 spirochetes, and this population was 20 times larger in a young, unfed nymph. The spirochete load also appeared to decline in adult ticks. Comparison between wild and laboratory populations found that lab ticks were more susceptible to acquiring B. afzelii.

Conclusion

The spirochete load of B. afzelii declines dramatically over time in domesticated I. ricinus nymphs under laboratory conditions. Future studies should investigate whether temporal declines in spirochete load occur in wild Ixodes ticks under natural conditions and whether these declines influence the tick-to-host transmission of Borrelia.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2187-4) contains supplementary material, which is available to authorized users.

Lyme borreliosis: a review of data on transmission time after tick attachment

Lyme borreliosis is increasing rapidly in many parts of the world and is the most commonly occurring vector-borne disease in Europe and the USA. The disease is transmitted by ticks of the genus Ixodes. They require a blood meal at each stage of their life cycle and feed on a wide variety of wild and domestic animals as well as birds and reptiles. Transmission to humans is incidental and can occur during visits to a vector habitat, when host mammals and their associated ticks migrate into the urban environment, or when companion animals bring ticks into areas of human habitation. It is frequently stated that the risk of infection is very low if the tick is removed within 24–48 hours, with some claims that there is no risk if an attached tick is removed within 24 hours or 48 hours. A literature review has determined that in animal models, transmission can occur in <16 hours, and the minimum attachment time for transmission of infection has never been established. Mechanisms for early transmission of spirochetes have been proposed based on their presence in different organs of the tick. Studies have found systemic infection and the presence of spirochetes in the tick salivary glands prior to feeding, which could result in cases of rapid transmission. Also, there is evidence that spirochete transmission times and virulence depend upon the tick and Borrelia species. These factors support anecdotal evidence that Borrelia infection can occur in humans within a short time after tick attachment.

Protective value of prophylactic antibiotic treatment of tick bite for Lyme disease prevention: an animal model

Clinical studies have demonstrated that prophylactic antibiotic treatment of tick bites by Ixodes scapularis in Lyme disease hyperendemic regions in the northeastern United States can be effective in preventing infection with Borrelia burgdorferi sensu stricto, the Lyme disease spirochete. A large clinical trial in Westchester County, NY (USA), demonstrated that treatment of tick bite with 200mg of oral doxycycline was 87% effective in preventing Lyme disease in tick-bite victims (Nadelman, R.B., Nowakowski, J., Fish, D., Falco, R.C., Freeman, K., McKenna, D., Welch, P., Marcus, R., Agúero-Rosenfeld, M.E., Dennis, D.T., Wormser, G.P., 2001. Prophylaxis with single-dose doxycycline for the prevention of Lyme disease after an Ixodes scapularis tick bite. N. Engl. J. Med. 345, 79-84.). Although this excellent clinical trial provided much needed information, the authors enrolled subjects if the tick bite occurred within 3 days of their clinical visit, but did not analyze the data based on the exact time between tick removal and delivery of prophylaxis. An animal model allows for controlled experiments designed to determine the point in time after tick bite when delivery of oral antibiotics would be too late to prevent infection with B. burgdorferi. Accordingly, we developed a tick-bite prophylaxis model in mice that gave a level of prophylactic protection similar to what had been observed in clinical trials and then varied the time post tick bite of antibiotic delivery. We found that two treatments of doxycycline delivered by oral gavage to mice on the day of removal of a single potentially infectious nymphal I. scapularis protected 74% of test mice compared to controls. When treatment was delayed until 24 h after tick removal, only 47% of mice were protected; prophylactic treatment was totally ineffective when delivered ≥2 days after tick removal. Although the dynamics of antibiotic treatment in mice may differ from humans, and translation of animal studies to patient management must be approached with caution, we believe our results emphasize the point that antibiotic prophylactic treatment of tick bite to prevent Lyme disease is more likely to be efficacious if delivered promptly after potentially infectious ticks are removed from patients. There is only a very narrow window for prophylactic treatment to be effective post tick removal.

Saliva, salivary gland, and hemolymph collection from Ixodes scapularis ticks

Ticks are found worldwide and afflict humans with many tick-borne illnesses. Ticks are vectors for pathogens that cause Lyme disease and tick-borne relapsing fever (Borrelia spp.), Rocky Mountain Spotted fever (Rickettsia rickettsii), ehrlichiosis (Ehrlichia chaffeensis and E. equi), anaplasmosis (Anaplasma phagocytophilum), encephalitis (tick-borne encephalitis virus), babesiosis (Babesia spp.), Colorado tick fever (Coltivirus), and tularemia (Francisella tularensis) (1-8). To be properly transmitted into the host these infectious agents differentially regulate gene expression, interact with tick proteins, and migrate through the tick (3,9-13). For example, the Lyme disease agent, Borrelia burgdorferi, adapts through differential gene expression to the feast and famine stages of the tick's enzootic cycle (14,15). Furthermore, as an Ixodes tick consumes a bloodmeal Borrelia replicate and migrate from the midgut into the hemocoel, where they travel to the salivary glands and are transmitted into the host with the expelled saliva (9,16-19). As a tick feeds the host typically responds with a strong hemostatic and innate immune response (11,13,20-22). Despite these host responses, I. scapularis can feed for several days because tick saliva contains proteins that are immunomodulatory, lytic agents, anticoagulants, and fibrinolysins to aid the tick feeding (3,11,20,21,23). The immunomodulatory activities possessed by tick saliva or salivary gland extract (SGE) facilitate transmission, proliferation, and dissemination of numerous tick-borne pathogens (3,20,24-27). To further understand how tick-borne infectious agents cause disease it is essential to dissect actively feeding ticks and collect tick saliva. This video protocol demonstrates dissection techniques for the collection of hemolymph and the removal of salivary glands from actively feeding I. scapularis nymphs after 48 and 72 hours post mouse placement. We also demonstrate saliva collection from an adult female I. scapularis tick.

Live imaging reveals a biphasic mode of dissemination of Borrelia burgdorferi within ticks

Lyme disease is caused by transmission of the spirochete Borrelia burgdorferi from ticks to humans. Although much is known about B. burgdorferi replication, the routes and mechanisms by which it disseminates within the tick remain unclear. To better understand this process, we imaged live, infectious B. burgdorferi expressing a stably integrated, constitutively expressed GFP reporter. Using isolated tick midguts and salivary glands, we observed B. burgdorferi progress through the feeding tick via what we believe to be a novel, biphasic mode of dissemination. In the first phase, replicating spirochetes, positioned at varying depths throughout the midgut at the onset of feeding, formed networks of nonmotile organisms that advanced toward the basolateral surface of the epithelium while adhering to differentiating, hypertrophying, and detaching epithelial cells. In the second phase of dissemination, the nonmotile spirochetes transitioned into motile organisms that penetrated the basement membrane and entered the hemocoel, then migrated to and entered the salivary glands. We designated the first phase of dissemination "adherence-mediated migration" and provided evidence that it involves the inhibition of spirochete motility by one or more diffusible factors elaborated by the feeding tick midgut. Our studies, which we believe are the first to relate the transmission dynamics of spirochetes to the complex morphological and developmental changes that the midgut and salivary glands undergo during engorgement, challenge the conventional viewpoint that dissemination of Lyme disease-causing spirochetes within ticks is exclusively motility driven.

Borrelia burgdorferi bba74 is expressed exclusively during tick feeding and is regulated by both arthropod- and mammalian host-specific signals

Although BBA74 initially was described as a 28-kDa virulence-associated outer-membrane-spanning protein with porin-like function, subsequent studies revealed that it is periplasmic and downregulated in mammalian host-adapted spirochetes. To further elucidate the role of this protein in the Borrelia burgdorferi tick-mammal cycle, we conducted a thorough examination of its expression profile in comparison with the profiles of three well-characterized, differentially expressed borrelial genes (ospA, ospC, and ospE) and their proteins. In vitro, transcripts for bba74 were expressed at 23 degrees C and further enhanced by a temperature shift (37 degrees C), whereas BBA74 protein diminished at elevated temperatures; in contrast, neither transcript nor protein was expressed by spirochetes grown in dialysis membrane chambers (DMCs). Primer extension of wild-type B. burgdorferi grown in vitro, in conjunction with expression analysis of DMC-cultivated wild-type and rpoS mutant spirochetes, revealed that, like ospA, bba74 is transcribed by sigma(70) and is subject to RpoS-mediated repression within the mammalian host. A series of experiments utilizing wild-type and rpoS mutant spirochetes was conducted to determine the transcriptional and translational profiles of bba74 during the tick-mouse cycle. Results from these studies revealed (i) that bba74 is transcribed by sigma(70) exclusively during the larval and nymphal blood meals and (ii) that transcription of bba74 is bracketed by RpoS-independent and -dependent forms of repression that are induced by arthropod- and mammalian host-specific signals, respectively. Although loss of BBA74 does not impair the ability of B. burgdorferi to complete its infectious life cycle, the temporal compartmentalization of this gene's transcription suggests that BBA74 facilitates fitness of the spirochete within a narrow window of its tick phase. A reexamination of the paradigm for reciprocal regulation of ospA and ospC, performed herein, revealed that the heterogeneous expression of OspA and OspC displayed by spirochete populations during the nymphal blood meal results from the intricate sequence of transcriptional and translational changes that ensue as B. burgdorferi transitions between its arthropod vector and mammalian host.