Location: Animal Disease ResearchTitle: Identification of multi-locus genetic heterogeneity in anaplasma marginale ss. centrale and its restriction following tick-borne transmission.) Author
Submitted to: Infection and Immunity
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/7/2013
Publication Date: 3/18/2013
Publication URL: iai.asm.org/content/81/5/1852.full
Citation: Herndon, D.R., Ueti, M.W., Reif, K.E., Noh, S.M., Brayton, K.A., Agnes, J.T., Palmer, G.H. 2013. Identification of multi-locus genetic heterogeneity in anaplasma marginale ss. centrale and its restriction following tick-borne transmission. Infection and Immunity. 81(5):1852-8. Interpretive Summary: Anaplasma marginale is an important disease causing tick-borne parasite of cattle. One control strategy for bovine anaplasmosis is vaccination with the naturally less virulent and poorly tick transmitted subspecies Anaplasma centrale. The genome sequence of A. centrale is composed of multiple unique individual genotypes, likely caused by many years of animal to animal passage. Our goal was to determine if these multiple genotypes are maintained following tick transmission and to determine if a change in tick transmission efficiency corresponded with reduction in genotypic diversity. Several genotypes were tracked by DNA sequencing following natural and intravenous transmission of the pathogen. The results indicate that genotypic diversity can be reduced when A. centrale is passage through the tick however this reduction of diversity does not alter tick transmission efficiency. The outcome of this research will assist in our efforts at understanding the mechanism of protection for the vaccine strain and will enhance our understanding of the genomes of tick-born bacterial pathogens.
Technical Abstract: Anaplasma marginale ss. centrale was the first vaccine used to protect against a rickettsial disease and continues in widespread use a century after initial implementation. As its use preceded development of either cryopreservation or cell culture, the vaccine strain was maintained for decades by sequential passage among donor animals, excluding the natural tick-borne transmission cycle that provides a selective pressure or population “bottleneck”. We demonstrated that the vaccine strain is genetically heterogeneous at 46 chromosomal loci and that heterogeneity was maintained upon inoculation into recipient animals. The number of variants per site ranged from 2-11 with a mean of 2.8/locus and a mode and median of 2/locus; variants included SNPs, indels, polynucleotide tracts, and different numbers of perfect repeats. The genetic heterogeneity is highly unlikely to be a result of strain contamination based on analysis using a panel of eight gene markers with a high power for strain discrimination. In contrast, heterogeneity appears to be a result of genetic drift in the absence of the restriction of tick passage. Heterogeneity could be reduced following tick passage and the reduced heterogeneity maintained in sequential intravenous and tick-borne passages. The reduction in vaccine strain heterogeneity following tick passage did not confer an enhanced transmission phenotype, indicating that a stochastically determined population bottleneck was likely responsible as opposed to a positive selective pressure. These findings demonstrate the plasticity of an otherwise highly constrained, “closed core” genome and highlight the role of natural transmission cycles in shaping and maintaining the bacterial genome.