Abstract
Background:
Bartonella ancashensis is a recently described Bartonella species endemic to Peru, where it causes verruga peruana in humans. While the arthropod vector of B. ancashensis transmission is unknown, human coinfections with Bartonella bacilliformis suggest that phlebotomine sand flies are a vector.
Materials and Methods:
To address the hypothesis that sand flies are involved in the bacterium's transmission, Lutzomyia longipalpis sand flies were used as an infection model, together with green fluorescent protein-expressing B. ancashensis.
Results:
Results showed that bacterial infections were clearly established, limited to the anterior midgut of the female fly, and maintained for roughly 7 days. At 3–7 days postinfection, a prominent microcolony of aggregated bacteria was observed in the anterior midgut, immediately distal to the stomodeal valve of the esophagus. In contrast, eggs, diuretic fluid, feces, and other tissues were not infected.
Conclusion:
These results suggest that certain sand fly species within the endemic zone for B. ancashensis may play a role in the bacterium's maintenance and possibly in its transmission to humans.
Keywords: Bartonella ancashensis, sand fly, infection model, arthropod vector, Lutzomyia longipalpis
Introduction
Bartonella ancashensis is a gram-negative alphaproteobacterium, first isolated from two children in 2003 during a drug trial screening of 127 verruga peruana patients in Caraz, Ancash, Peru (Blazes et al., 2013; Mullins et al., 2015; Mullins et al., 2013). Genomic sequencing, multilocus sequence typing, and intergenic spacer typing indicated that the species was novel, with its closest relative being the gram-negative alphaproteobacterium, Bartonella bacilliformis (Mullins et al., 2015). Before this discovery, verruga peruana was presumed to be a chronic manifestation of Carrión's disease; a sand fly-vectored human illness caused by a B. bacilliformis infection, that is endemic to South America.
Carrión's disease manifests as Oroya fever; a potentially life-threatening hematic syndrome involving acute hemolytic anemia, and/or verruga peruana; a chronic infection of vascular endothelial cells characterized by nonlife-threatening hemangiomas of the skin and bacteremia (Minnick et al., 2014). While the geographic distribution of B. ancashensis overlaps that of B. bacilliformis, the bacterium has not been isolated from patients with Oroya fever, to date. As such, B. ancashensis is thought to be less virulent (Blazes et al., 2013; Mullins et al., 2017; Mullins et al., 2015; Mullins et al., 2013). The apparent overlap in geographic ranges between these two pathogens, similarities in their chronic pathological manifestations during infections, and possible coinfections involving the two bacteria (Mullins et al., 2017) obfuscates our understanding of the epidemiology of Carrión's disease.
This situation also complicates the proper diagnosis and treatment of Carrión's disease, which is currently based on the patient's symptoms, blood smears, and blood cultures (Ellis et al., 1999). We hypothesize that, such as B. bacilliformis, B. ancashensis transmission involves a sand fly vector. To investigate this, we used a Lutzomyia longipalpis sand fly infection model to follow tissue colonization by green fluorescent protein (GFP)-expressing B. ancashensis in vitro over time.
Materials and Methods
Ethics statement
The Institutional Biosafety Committee and Institutional Review Board at the University of Montana granted approval for the experimental use of B. ancashensis, L. longipalpis, and human blood (IBC 2022-002; IRB 120-20). Formal consent was obtained from the blood donor (coauthor MFM).
Bacterial culture
B. ancashensis type strain 20.00 (Mullins et al., 2015) was cultivated on HIBB medium (Bacto heart infusion agar [Becton Dickinson; Franklin Lakes, NJ] supplemented with 4% defibrinated sheep blood and 2% sheep serum [Quad-Five, Ryegate, MT] by volume). Cultures were routinely grown for 4–5 days at 30°C with 5% CO2. HIBB plates were supplemented with 25 μg/mL kanamycin (HIBB-kan25) to cultivate B. ancashensis transformed with pJMB-GFP. When needed, B. ancashensis was freshly harvested from HIBB plates using a sterile razor blade, as previously described (Battisti and Minnick, 2008), and suspended in 300 μL ice-cold, sterile physiological saline (0.9% NaCl; w/v) per plate.
Generation of GFP-expressing B. ancashensis
B. ancashensis (type strain 20.00) was transformed with pJMB-GFP by electroporation, as previously described for B. bacilliformis (Battisti and Minnick, 1999). Positive clones were identified by kanamycin resistance on HIBB-kan25, pJMB-GFP plasmid content, and an intense GFP signal when samples of bacterial colonies were observed by ultraviolet (UV) fluorescence microscopy.
Sand fly handling and maintenance
L. longipalpis (LLJB, Brazil; L3/pupae, NR-44001) was obtained from BEI Resources (Manassas, VA). Adults were released daily from commercial larval “pots” into 30.5 cm3 plexiglass holding cages (21st Century Plastics, Missoula, MT) until the pots were depleted of adults. Holding cages were maintained in a growth chamber (Percival Model 136NL) at 25°C, 100% relative humidity, in total darkness. All work was done in a dedicated and secure insectary with air filters and negative ventilation. Flies were fed ad libitum on sterile, 30% sucrose-water in moist cotton balls placed on the tops of the cages and were replaced every 48 h. Sand flies were allowed to mature for ≥10 days before artificial bloodmeal feedings.
Artificial bloodmeal feedings
Fresh human blood (6 mL) was collected by venipuncture into anticoagulant acid-citrate dextrose solution B tubes (BD Vacutainer 364816; Becton Dickinson). After gentle mixing, blood cells were centrifuged (1000 × g for 5 min at 4°C), and the plasma supernatant aseptically drawn off and discarded. Blood cells were subsequently washed three times by gently mixing the pellet in 4 mL ice-cold, sterile physiological saline, followed by centrifugation (1000 × g for 5 min at 4°C). The final cell pellet was brought to 6 mL with physiological saline and used immediately or was refrigerated overnight at 10°C before use.
Artificial bloodmeals were prepared by combining freshly-harvested B. ancashensis cells and washed erythrocytes (∼1 × 109 cells of each type) to yield a multiplicity of infection (MOI) of 1.0. Cell counts were performed using a hemocytometer for erythrocytes, and by counting viable (GFP+) bacteria in 10, 0.22-mm fields on three slides using UV fluorescence microscopy and a fluorescein isothiocyanate (FITC) filter (1000 × magnification). The number of bacteria per mL was calculated using the average number of bacteria per field times the dilution factor times a conversion factor of 1.273 × 106. The bloodmeal mix was incubated for 4 h at 37°C, before sand fly feeding, to allow for equilibration and infection of erythrocytes by bacteria.
Artificial bloodmeal feedings were performed using a 14 mm glass mosquito feeder (Chemglass Life Sciences, Vineland, NJ) overlaid with the defeathered skin of a 1-day-old frozen chick (Layne Laboratories, Arroyo Grande, CA), as previously described (Battisti et al., 2015). The feeder was connected to a constantly circulating water bath to maintain the bloodmeal at 39°C. The center vestibule of the feeder was filled with the bloodmeal mixture, while the outer vestibule contained circulating warm water. The glass feeder (chick skin facing downwards) was placed on top of a fly feeding cup that was covered with tulle cloth, containing 50–100 sand flies transferred from the holding cage. A damp sponge and aluminum foil were placed around the entire feeder and cup, and feeding occurred for 60 min in the dark at 25°C.
Afterward, two cotton balls soaked with 10% sucrose water plus kanamycin (40 μg/mL) were provided for feeding the flies ad libitum. Infected insects were maintained in feeding cups in the growth chamber for up to 9 days with cotton balls replaced at 48 h intervals.
Quantification of B. ancashensis over the course of infection
Alimentary tracts of infected sand flies were aseptically isolated daily for 7 days postfeeding using a dissecting scope and sterile microtools (i.e., insect needles embedded in wooden applicator sticks). Individual abdominal midguts were suspended in 50 μL sterile physiological saline and macerated in 1.5 mL microcentrifuge tubes using a pestle and vortexer. Resulting cell suspensions were 10-fold serially diluted with heart infusion broth, vortexed, then spread-plated onto HIBB-kan25 plates. Colony counts were performed visually after incubating for 10 days at 30°C with 5% CO2. A GFP+ phenotype, by fluorescence microscopy, was used to verify that the colonies were B. ancashensis.
Feces and diuretic fluid sampling from infected sand flies
Three randomly selected dried fecal piles and diuretic fluids were collected from feeder cups at 6 days postinfection using 10 μL sterile physiological saline and a micropipettor, then cultured for 7 days on HIBB-kan25 plates, as previously described (Battisti et al., 2015). Three additional samples of each type were collected and observed by UV fluorescence microscopy.
Imaging
Microscopic imaging was done using a BX31 UV fluorescence microscope (Olympus; Waltham, MA) and a FITC filter. Images in results are representative of at least three different insects per time point postinfection, from five separate bloodmeal feedings, and were captured using a DP74 microscope digital camera (Olympus) and cellSens imaging software version 3.1 (Olympus).
Limitations of the study
A colony of a bona fide sand fly vector for B. bacilliformis (e.g., Lutzomyia verrucarum) was not available at the time of the study, thus we used a model organism (Lutzomyia longipalpis). Uninfected sand flies were not examined as a control in this study.
Results
Microscopic examination of L. longipalpis colonization by B. ancashensis
To examine the possibility that sand flies could maintain B. ancashensis, we artificially infected L. longipalpis using fresh, washed human blood spiked with a GFP+ strain of the bacterium at an MOI of 1.0. Bacterial colonization of sand flies was followed over time by observing various tissues by UV fluorescence microscopy until no apparent infection was observed in any of the sampled insects. Overall, the results of these experiments showed that L. longipalpis remained infected with viable (GFP+) bacteria for up to 7 days, and the infections were restricted to the abdominal midgut.
At 1 day postinfection, female sand flies that had taken a bloodmeal were easily identified, as they appeared larger, darker, and had a prominent reddish-colored abdomen (Supplementary Fig. S1). Dissections and UV microscopy were not conducted at this time point to allow the peritrophic membrane (PM) to fully develop around the bloodmeal.
At 2 days postinfection, the PM was well formed and provided for easy dissection and removal of the entire alimentary tract from the sand flies. When observed microscopically, the entire anterior midgut was filled with intact human erythrocytes and GFP+ B. ancashensis (Fig. 1). However, no other tissues were found to contain bacteria (not shown).
By 3 days postinfection, a prominent bacterial aggregation (microcolony) appeared at the anterior end of the abdominal midgut, immediately distal to the stomodeal valve (SV) of the esophagus, in all insects observed (Fig. 2). In a few instances, a small number of GFP+ bacteria had apparently escaped the PM and adhered to its exterior (Fig. 2B). The bloodmeal content of the anterior midgut from these insects was indistinguishable from flies observed at 2 days and consisted of a high concentration of erythrocytes and GFP+ B. ancashensis (Fig. 2C, D). As on day 2, GFP+ bacteria were confined to the anterior midgut of the insect and were not detectable in the head, thorax, or thoracic midgut (Supplementary Fig. S2).
At 4–6 days postinfection, the bloodmeal in infected sand flies was partially digested and rusty-brown in color. Despite autofluorescence of various insect body parts under UV fluorescence microscopy, GFP+ bacteria were readily discernible throughout the anterior midgut. Over the 4–6 days time period, the microcolony at the anterior of the anterior midgut became more prominent, in contrast to a waning bacterial population located elsewhere in the anterior midgut (Figs. 3–5). In a few instances, GFP+ bacteria that had apparently escaped the PM and adhered to its exterior were also observed (Fig. 5C).
Sand flies observed at the 7-day time point exhibited considerable variation regarding their infection status. While roughly half of the flies showed reduced bloodmeal sizes (Fig. 6), the other half had no detectable bloodmeal remaining when observed by dissection or light microscopy. These qualitative differences possibly resulted from the variable bloodmeal volumes initially taken by the sand flies. In flies that still possessed a visible bloodmeal, the number of GFP+ B. ancashensis cells was markedly reduced compared to insects observed at earlier time points.
However, bacteria remained at low density throughout the abdominal midgut, and remained concentrated as a microcolony in the anterior end of the anterior midgut (Fig. 6). When anterior midguts from insects at the 7 days postinfection time were aseptically isolated and cultured on HIBB-kan25, GFP+ bacteria were rescued, suggesting that the B. ancashensis present remained viable for at least a week in the sand fly anterior midgut (not shown).
At 8–9 days postinfection, GFP+ B. ancashensis was not detected in female sand flies by dissection, light, or fluorescence microscopy, and the bloodmeals were completely digested. However, the presence of eggs in these insects indicated that they had previously fed on blood, since oviposition requires a bloodmeal by L. longipalpis (Milleron et al., 2008). However, none of the eggs observed in flies after 8 days postinfection contained GFP+ B. ancashensis by UV microscopy (Supplementary Fig. S3).
Diuretic fluid and feces from infected sand flies did not possess GFP+ B. ancashensis cells when observed by fluorescence microscopy at 1000 × magnification. Moreover, B. ancashensis was not recovered from these samples when cultured on HIBB-kan25 plates.
Viability of B. ancashensis in L. longipalpis
Quantification of bacteria by plate counts using isolated abdominal midguts from different insects gave inconsistent results (not shown). Although disappointing, this was not unexpected, especially considering the wide range of bloodmeal volumes and bacterial loads taken up during feeding by individual sand flies. As a corollary, we also observed considerable variation in the sizes of bloodmeals on any given day during the dissections. To demonstrate bacterial load variability, plate counts on one sand fly at 3 days postinfection yielded 4.47 × 107 B. ancashensis CFU's/mL, while in another sand fly at 1 day postinfection, the midgut yielded 5 × 105 B. ancashensis CFU's/mL. Despite variability in the bacterial load quantification results, we were able to culture viable B. ancashensis from isolated abdominal midguts/bloodmeals each day for up to 7 days postinfection.
Discussion
In this study, we addressed the hypothesis that B. ancashensis colonizes and persists in a sand fly vector, by using a L. longipalpis sand fly model of infection and GFP fluorescently tagged bacteria. Results of the study showed that L. longipalpis can be infected and maintain viable, GFP+ B. ancashensis in the anterior midgut for 7 days. These results are similar to those we reported previously for GFP+ B. bacilliformis in the L. longipalpis infection model (Battisti et al., 2015). Interestingly, both B. bacilliformis and B. ancashensis infections of L. longipalpis were restricted to the bloodmeal of the abdominal midgut. In contrast, B. bacilliformis infection of L. verrucarum (a bona fide sand fly vector for B. bacilliformis [Noguchi et al., 1929]) showed a much longer infection (>14 days) and involved the anterior midgut lumen outside the blood meal (i.e., in the intraperitrophic space).
Once the bona fide sand fly vector(s) for B. ancashensis is identified, it would be prudent to determine if it also provides for longer term maintenance of the bacterium versus L. longipalpis. One noticeable difference between B. bacilliformis and B. ancashesis, in the L. longipalpis model, was the appearance of a small number of B. ancashensis cells that had apparently escaped from the blood meal and adhered to the exterior of the PM in a subset of sand flies (Figs. 2B and 5C). The mechanism of bacterial escape to this location is unknown. Taken as a whole, the results suggest that one or more sand fly species inhabiting the endemic zone of B. ancashensis could conceivably maintain the pathogen for at least a short term and possibly serve as a vector of transmission during this time.
Although bona fide sand fly vectors of B. ancashensis have not yet been identified, those phlebotomine sand fly species that have been shown to harbor B. bacilliformis or its DNA are logical starting points for screening. These include Lutzomyia maranonensis (Ulloa et al., 2018), Lutzomyia noguchii (Noguchi et al., 1929), Lutzomyia peruensis (Villaseca et al., 1999), Lutzomyia robusta (Carrazco-Montalvo, 2017), and L. verrucarum (Noguchi et al., 1929; Romero, 2004). A closer examination of the various sand fly species inhabiting the endemic zone for B. ancashensis and B. bacilliformis is needed to better understand the epidemiology of this understudied pathogen. It may also be prudent to screen other insects living in the endemic zone, especially considering the increasing number of vectors involved in the transmission of other Bartonella species.
An additional novel discovery was the consistent formation of a bacterial aggregate or “microcolony” distal to the SV of the esophagus at the anterior portion of the abdominal midgut. These microcolonies were observed as early as 3 days postinfection (Fig. 2) and maintained for up to 7 days in infected sand flies (Fig. 6). It is possible that the microcolonies resulted from biofilm formation, as previously reported for Bartonella henselae (Okaro et al., 2021). However, the actual composition of the B. ancashensis microcolony remains to be determined. Assuming that the microcolony also forms in the anterior portion of the anterior midgut of a bona fide sand fly vector(s), it could conceivably enhance transmission by allowing bacteria to be regurgitated into the thoracic midgut, mouth parts, and finally into a human host's circulatory system during hematophagy.
A similar transmission scenario is seen in Yersinia pestis-infected fleas, where biofilm formation in the proventricular valve of the foregut eventually blocks its closure and allows the bacterium to be regurgitated during hematophagy (Hinnebusch et al., 2017).
Conclusions and Future Directions
In conclusion, this study has shown that B. ancashensis can establish an infection of the anterior midgut of female L. longipalpis sand flies, and the infection is maintained for ∼7 days. At 3–7 days postinfection, a prominent microcolony of aggregated bacteria forms in the anterior midgut of the fly, immediately distal to the SV of the esophagus. Eggs, diuretic fluid, feces, and other tissues are not infected. While our results suggest that certain sand fly species within the endemic zone for B. ancashensis may play a role in the bacterium's maintenance and possibly in its transmission to humans, numerous questions remain unanswered.
For example, how many of the reported cases of verruga peruana in historically nonendemic areas of South America were actually caused by B. ancashensis but attributed to B. bacilliformis (Alexander, 1995; Amano et al., 1997; Cooper et al., 1997; Ellis et al., 1999; Kosek et al., 2000; Maco et al., 2004; Maguiña and Gotuzzo, 2000; Maguiña-Vargas, 1998; Pachas-Cavez, 2001)? Is B. ancashensis actually vectored by sand flies? Do coinfections involving B. ancashensis and B. bacilliformis in sand flies result in cotransmission through a single vector (Mullins et al., 2017)? We believe that the study has paved the way to begin to address these questions.
Supplementary Material
Acknowledgments
We sincerely thank Dr. Al Richards for the generous gift of B. ancashensis strain 20.00 and Dr. James M. Battisti for technical advice on sand fly handling. We are grateful to the Walter Reed Army Institute of Research for Lutzomyia longipalpis (LLJB, Brazil; L3/pupae, NR-44001) obtained through BEI Resources, National Institute of Allergy and Infectious Diseases (NIAID), and National Institute of Health (NIH). Material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its presentation and/or publication. The opinions or assertions contained herein are the private views of the author, and are not to be construed as official, or as reflecting true views of the Department of the Army or the Department of Defense.
Author Disclosure Statement
The authors declare that they have no known competing financial interests or personal relationships that could constitute a conflict of interest or to have influenced the work reported in this paper.
Funding Information
This work was supported by NIH grant R03AI146927 (to M.F.M.).
Supplementary Material
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