Staphylococcus aureus host range and human-bovine host shift

Staphylococcus aureus is a major agent of bovine mastitis. The concomitant emergence of pig-associated methicillin-resistant S. aureus (MRSA) in human carriage and infection requires a reexamination of the host range and specificity of human- and cow-associated S. aureus strains, something which has not been systematically studied previously. The genetic relatedness of 500 S. aureus isolates from bovine mastitis cases, 57 isolates from nasal carriage of farmers, and 133 isolates from nonfarmers was determined by amplified fragment length polymorphism (AFLP) analysis and spa typing. Multilocus sequence typing (MLST) was conducted on a subset of isolates to match AFLP clusters with MLST clonal complexes (CCs). This data set allowed us to study host range and host specificity and to estimate the extent of bovine-to-human transmission. The genotype compositions of S. aureus isolates from farmers and nonfarmers were very similar, while the mastitis isolates were quite distinct. Overall, transmission was low, but specific genotypes did show increased cow-to-human transmission. Unexpectedly, more than one-third of mastitis isolates belonged to CC8, a lineage which has not been considered to be bovine mastitis associated, but it is well known from human carriage and infection (i.e., USA300). Despite the fact that we did detect some transmission of other genotypes from cows to farmers, no transmission of CC8 isolates to farmers was detected, except for one tentative case. This was despite the close genetic relatedness of mastitis CC8 strains to nonfarmer carriage strains. These results suggest that the emergence of the new bovine-adapted genotype was due to a recent host shift from humans to cows concurrent with a loss of the ability to colonize humans. More broadly, our results indicate that host specificity is a lineage-specific trait that can rapidly evolve.     

Parasite host range and the evolution of host resistance

Parasite host range plays a pivotal role in the evolution and ecology of hosts and the emergence of infectious disease. Although the factors that promote host range and the epidemiological consequences of variation in host range are relatively well characterized, the effect of parasite host range on host resistance evolution is less well understood. In this study, we tested the impact of parasite host range on host resistance evolution. To do so, we used the host bacterium Pseudomonas fluorescens SBW25 and a diverse suite of coevolved viral parasites (lytic bacteriophage Φ2) with variable host ranges (defined here as the number of host genotypes that can be infected) as our experimental model organisms. Our results show that resistance evolution to coevolved phages occurred at a much lower rate than to ancestral phage (approximately 50% vs. 100%), but the host range of coevolved phages did not influence the likelihood of resistance evolution. We also show that the host range of both single parasites and populations of parasites does not affect the breadth of the resulting resistance range in a naïve host but that hosts that evolve resistance to single parasites are more likely to resist other (genetically) more closely related parasites as a correlated response. These findings have important implications for our understanding of resistance evolution in natural populations of bacteria and viruses and other host–parasite combinations with similar underlying infection genetics, as well as the development of phage therapy.     

Growth of host dependent Bdellovibrio in host cell free system

A particulate, subcellular fraction of Escherichia coli was shown to promote the growth of host dependent (H-D) Bdellovibrio in the absence of host cells. The growth promoting activity was enhanced by both cations and trypsin, and destroyed by pronase. During the axenic growth unipolar spheres appear in the elongating Bdellovibrio forms. Thymidine monophosphate was more readily incorporated than thymidine into the Bdellovibrio DNA during growth in the host free system.     

Effect of host behaviour on host preference in Stomoxys calcitrans

Field observations suggest that, in the U.K., cattle are the preferred host of Stomoxys calcitrans (L.), followed by horses. Differences were observed in the numbers of flies feeding on individual animals both in the field and under controlled conditions. Analysis of the behaviour of four Friesian calves under attack from S. calcitrans in controlled conditions revealed that the differences in the levels of attack between individual hosts are dependent on the reactions of the host when under attack. Those hosts which respond vigorously by tail flicks, foot stamps and head-swings suffer less from attack by S. calcitrans than their more placid contemporaries.     

The evolution of parasite host range in heterogeneous host populations

Theory on the evolution of niche width argues that resource heterogeneity selects for niche breadth. For parasites, this theory predicts that parasite populations will evolve, or maintain, broader host ranges when selected in genetically diverse host populations relative to homogeneous host populations. To test this prediction, we selected the bacterial parasite Serratia marcescens to kill Caenorhabditis elegans in populations that were genetically heterogeneous (50% mix of two experimental genotypes) or homogeneous (100% of either genotype). After 20 rounds of selection, we compared the host range of selected parasites by measuring parasite fitness (i.e. virulence, the selected fitness trait) on the two focal host genotypes and on a novel host genotype. As predicted, heterogeneous host populations selected for parasites with a broader host range: these parasite populations gained or maintained virulence on all host genotypes. This result contrasted with selection in homogeneous populations of one host genotype. Here, host range contracted, with parasite populations gaining virulence on the focal host genotype and losing virulence on the novel host genotype. This pattern was not, however, repeated with selection in homogeneous populations of the second host genotype: these parasite populations did not gain virulence on the focal host genotype, nor did they lose virulence on the novel host genotype. Our results indicate that host heterogeneity can maintain broader host ranges in parasite populations. Individual host genotypes, however, vary in the degree to which they select for specialization in parasite populations.     

Characterising insect plant host relationships facilitates understanding multiple host use

Weed biocontrol relies on host specificity testing, usually carried out under quarantine conditions to predict the future host range of candidate control agents. The predictive power of host testing can be scrutinised directly with Aconophora compressa, previously released against the weed Lantana camara L. (lantana) because its ecology in its new range (Australia) is known and includes the unanticipated use of several host species. Glasshouse based predictions of field host use from experiments designed a posteriori can therefore be compared against known field host use. Adult survival, reproductive output and egg maturation were quantified. Adult survival did not differ statistically across the four verbenaceous hosts used in Australia. Oviposition was significantly highest on fiddlewood (Citharexylum spinosum L.), followed by lantana, on which oviposition was significantly higher than on two varieties of Duranta erecta (“geisha girl” and “Sheena’s gold”; all Verbenaceae). Oviposition rates across Duranta varieties were not significantly different from each other but were significantly higher than on the two non-verbenaceous hosts (Jacaranda mimosifolia D. Don: Bignoneaceae (jacaranda) and Myoporum acuminatum R. Br.: Myoporaceae (Myoporum)). Production of adult A. compressa was modelled across the hosts tested. The only major discrepancy between model output and their relative abundance across hosts in the field was that densities on lantana in the field were much lower than predicted by the model. The adults may, therefore, not locate lantana under field conditions and/or adults may find lantana but leave after laying relatively few eggs. Fiddlewood is the only primary host plant of A. compressa in Australia, whereas lantana and the others are used secondarily or incidentally. The distinction between primary, secondary and incidental hosts of a herbivore species helps to predict the intensity and regularity of host use by that herbivore. Populations of the primary host plants of a released biological control agent are most likely to be consistently impacted by the herbivore, whereas secondary and incidental host plant species are unlikely to be impacted consistently. As a consequence, potential biocontrol agents should be released only against hosts to which they have been shown to be primarily adapted.     

不同离体寄主及其成熟度对亚洲柑桔木虱存活的影响

【目的】亚洲柑桔木虱Diaphorina citri Kuwayama的虫口密度及活动与柑桔黄龙病的田间传播、流行有十分密切的关系,寻找合适的室内饲养条件能够便于观察和研究其生物学特性和传病机制。【方法】本文以九里香Murraya exotica(L.)Mant.、酸桔Citrus sunki Hort.ex Tanaka和马水桔Citrus reticulata Blanco.cv.Mashuiju 3种寄主植物不同成熟度离体梢为研究材料,(1)比较亚洲柑桔木虱卵分别在3种寄主植株和离体嫩梢上的孵化率;(2)比较3种寄主不同成熟度离体梢对各龄若虫存活率和蜕皮的影响;(3)比较3种寄主不同成熟度离体梢对成虫存活率的影响。【结果】(1)亚洲柑桔木虱在植株嫩梢上卵的孵化率高于离体嫩梢,九里香表现最明显;(2)低龄若虫在叶片未完全展开的离体嫩梢上存活率最高,而高龄若虫在完全展开的嫩梢上最高;(3)用离体成熟梢饲养柑桔木虱成虫存活率高于离体嫩梢,酸桔和马水桔离体梢饲养的存活率较九里香高。【结论】用寄主植物离体梢饲养的亚洲柑桔木虱卵孵化率和若虫、成虫存活率都较高而且较稳定,该方法可用于这种木虱的室内繁殖中。     

Using host traits to predict reservoir host species of rabies virus

Wildlife are important reservoirs for many pathogens, yet the role that different species play in pathogen maintenance frequently remains unknown. This is the case for rabies, a viral disease of mammals. While Carnivora (carnivores) and Chiroptera (bats) are the canonical mammalian orders known to be responsible for the maintenance and onward transmission of rabies Lyssavirus (RABV), the role of most species within these orders remains unknown and is continually changing as a result of contemporary host shifting. We combined a trait-based analytical approach with gradient boosting machine learning models to identify physiological and ecological host features associated with being a reservoir for RABV. We then used a cooperative game theory approach to determine species-specific traits associated with known RABV reservoirs. Being a carnivore reservoir for RABV was associated with phylogenetic similarity to known RABV reservoirs, along with other traits such as having larger litters and earlier sexual maturity. For bats, location in the Americas and geographic range were the most important predictors of RABV reservoir status, along with having a large litter. Our models identified 44 carnivore and 34 bat species that are currently not recognized as RABV reservoirs, but that have trait profiles suggesting their capacity to be or become reservoirs. Further, our findings suggest that potential reservoir species among bats and carnivores occur both within and outside of areas with current RABV circulation. These results show the ability of a trait-based approach to detect potential reservoirs of infection and could inform rabies control programs and surveillance efforts by identifying the types of species and traits that facilitate RABV maintenance and transmission.     

Rhagoletis sibling species and host races differ in host odor recognition

Electronantennograms (EAG) were recorded from the apple and hawthorn host race of the apple maggot fly, Rhagoletis pomonella (Walsh), and from the blueberry maggot fly, R. mendax (Curran) (Diptera: Tephritidae), in response to host fruit extracts and nine volatile host fruit odor compounds at six concentrations. Mean relative EAG response to apple odor is the same in both species, but in respect to blueberry odor, it is significantly stronger in R. mendax than in both host races of R. pomonella (P<0.05), indicating that antennal sensitivity is selectively adapted to species specific host fruit odors. Differences in antennal response to several host fruit odor compounds were found between both species as well as between the host races. This indicates differences in antennal receptor cell types and/or numbers between species and host races. The flies had no prior host fruit experience which indicates that the measured differences are genetically based. Because Rhagoletis fruit flies are highly host specific parasites which meet and mate on their respective host plants, the results suggest that antennal sensitivity plays an important role in host shifts and speciation in this genus.

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Zusammenfassung Elektroantennogramme (EAG) der Apfel- und der Weissdorn-Wirtsrasse der Apfelfruchtfliege, Rhagoletis pomonella, und der Blaubeerenfruchtfliege, Rhagoletis mendax (Diptera: Tephritidae), wurden aufgezeichnet als Reizantworten auf Wirtsfruchtextrakte und auf neun flüchtige Duftkomponenten ihrer Wirtsfrüchte in sechs Konzentrationen. Die Sensitivität der Antennen bezüglich Apfelduft ist in beiden Arten gleich, aber die Reizantwort von R. mendax auf den Duft ihrer spezifischen Wirtsfrucht, Blaubeeren, ist signifikant stärker als diejenige beider Wirtsrassen von R. pomonella (P<0.05), was darauf hinweist, dass die antennale Sensitivität möglicherwiese an den artspezifischen Wirtsfruchtduft adaptiert ist. Unterschiede in der antennalen Reizantwort auf mehrere Duftkomponenten der Wirtsfrüchte konnten sowohl zwischen den Arten als auch Wirtsrassen gefunden werden. Dies deutet auf Unterschiede in antennalen Rezeptorzelltypen und/oder Rezeptorzellzahl zwischen Arten und Wirtsrassen. Die Unterschiede wurden von Individuen aufgezeichnet, die keine vorherige Erfahrung mit den Wirtsfrüchten hatten und sind daher genetischen Ursprungs. Fruchtfliegen der Gattung Rhagoletis sind stark wirtsspezifische Pflanzenparasiten, die ihre Wirtspflanze als Treffpunkt und Paarungsort benützen. Diese Resultate deuten daher darauf hin, dass antennale Sensitivität eine wichtige Rolle in Wirtswechseln und Speziation in diesem Genus spielt.

     

Mosquito host selection varies seasonally with host availability and mosquito density

Host selection by vector mosquitoes is a critical component of virus proliferation, particularly for viruses such as West Nile (WNV) that are transmitted enzootically to a variety of avian hosts, and tangentially to dead-end hosts such as humans. Culex tarsalis is a principal vector of WNV in rural areas of western North America. Based on previous work, Cx. tarsalis utilizes a variety of avian and mammalian hosts and tends to feed more frequently on mammals in the late summer than during the rest of the year. To further explore this and other temporal changes in host selection, bloodfed females were collected at a rural farmstead and heron nesting site in Northern California from May 2008 through May 2009, and bloodmeal hosts identified using either a microsphere-based array or by sequencing of the mitochondrial cytochrome c oxidase I (COI) gene. Host composition during summer was dominated by four species of nesting Ardeidae. In addition, the site was populated with various passerine species as well as domestic farm animals and humans. When present, Cx. tarsalis fed predominantly (>80%) upon the ardeids, with Black-crowned Night-Herons, a highly competent WNV host, the most prevalent summer host. As the ardeids fledged and left the area and mosquito abundance increased in late summer, Cx. tarsalis feeding shifted to include more mammals, primarily cattle, and a high diversity of avian species. In the winter, Yellow-billed Magpies and House Sparrows were the predominant hosts, and Yellow-billed Magpies and American Robins were fed upon more frequently than expected given their relative abundance. These data demonstrated that host selection was likely based both on host availability and differences in utilization, that the shift of bloodfeeding to include more mammalian hosts was likely the result of both host availability and increased mosquito abundance, and that WNV-competent hosts were fed upon by Cx. tarsalis throughout the year.     

A parasite's modification of host behavior reduces predation on its host

Parasite modification of host behavior is common, and the literature is dominated by demonstrations of enhanced predation on parasitized prey resulting in transmission of parasites to their next host. We present a case in which predation on parasitized prey is reduced. Despite theoretical modeling suggesting that this phenomenon should be common, it has been reported in only a few host–parasite–predator systems. Using a system of gregarine endosymbionts in host mosquitoes, we designed experiments to compare the vulnerability of parasitized and unparasitized mosquito larvae to predation by obligate predatory mosquito larvae and then compared behavioral features known to change in the presence of predatory cues. We exposed Aedes triseriatus larvae to the parasite Ascogregarina barretti and the predator Toxohrynchites rutilus and assessed larval mortality rate under each treatment condition. Further, we assessed behavioral differences in larvae due to infection and predation stimuli by recording larvae and scoring behaviors and positions within microcosms. Infection with gregarines reduced cohort mortality in the presence of the predator, but the parasite did not affect mortality alone. Further, infection by parasites altered behavior such that infected hosts thrashed less frequently than uninfected hosts and were found more frequently on or in a refuge within the microcosm. By reducing predation on their host, gregarines may be acting as mutualists in the presence of predation on their hosts. These results illustrate a higher‐order interaction, in which a relationship between a species pair (host–endosymbiont or predator–prey) is altered by the presence of a third species.     

寄主利它素对稻螟赤眼蜂寄生行为的影响

对稻螟赤眼蜂Trichogrammajaponicum(Ashmead)在寄主定位过程中所利用的寄主利它素的存在部位、作用方式及其应用效果作了初步研究。生测结果表明 :寄主利它素存在于三化螟Tryporyzaincertu las(Walker)卵块盖毛和成虫鳞片中 ,其中以盖毛中利它素含量最高 ,雌蛾鳞片次之 ,雄蛾鳞片含量最少。结果还表明盖毛在卵块上的完整性对稻螟赤眼蜂的寄主搜索行为有明显影响。室内应用效果表明利它素可以使稻螟赤眼蜂对寄主卵寄生率提高 1 0 %以上。