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Assessment and diversity of tick bacterial microbiomes
- Author / Creator
- Sperling, Janet L.
Ticks vector diverse pathogenic bacteria that are important to identify in public health. Recent advances in high throughput sequencing have allowed a comprehensive catalog of these bacteria to be compiled for individual ticks. In this thesis I identify the bacterial assemblages associated with wild-collected Ixodes scapularis in three regions of Canada, provide the first microbiome survey for Ixodes angustus and expand our knowledge of the microbiome of Dermacentor albipictus in Alberta. I also evaluate the strengths and limitations of 16S amplicon sequencing for determining the identity and proportions of bacteria associated with ticks.
Bacterial diversity surveys can give substantially different results depending on the 16S rRNA variable region or regions that are examined, which means that a potential causative agent may be missed even when an illness is known to be associated with a tick bite. I determined that if a single 16S variable region is used, the V4 region generally detects the highest estimated bacterial diversity for ticks. However, more than one marker region or protocol is needed to provide a comprehensive assessment of a tick bacterial assemblage, since any one set of primers or methodology does not detect all bacterial taxa equally. This illustrates the potential for continued discovery of previously unknown microbiome taxa in medically important ticks.
A number of studies have shown very high numbers of bacterial taxa associated with ticks, but recent research has suggested that this high diversity is an artifact of surface contamination. These interpretations are further confounded by microbiome variation that is known to be due to biological factors such as tick species, life stage, blood meal and geographic location. To more rigorously address the extent of bacterial diversity in ticks, I used a one-host tick species, Dermacentor albipictus, and constrained sampling to multiple ticks at a single site and one host species, in order to limit the ecological factors normally associated with microbiome diversity. I still found substantial bacterial diversity, with a complex interaction between richness and evenness in comparisons among tick life stages, as demonstrated by using the Hill number series. Male ticks had a significantly greater number of bacterial genera than females or nymphs, while males had greater evenness than females and similar evenness to nymphs. I concluded that the high diversity of bacteria associated with ticks is probably biologically real and not simply due to technological artifacts. However, the high taxonomic variability of the minor components of the tick microbiome suggests that they should be examined further for functional significance.
Lyme disease is the most common tick-borne disease in Canada and is one of more than a dozen tick-borne illnesses that can occur as single infections or as co-infections in Ixodes ticks. The Lyme disease-causing bacterium, Borrelia burgdorferi, is the focus of surveillance programs across Canada, including in Alberta where it has been reported in 10-19% of Ixodes ticks during the last 7 years. However, the current focus on a single pathogen, B. burgdorferi, neglects other pathogens that may be transmitted by these ticks. I performed 16S rRNA bacterial surveys on female I. scapularis from Alberta that were previously qPCR-tested in a Lyme disease surveillance program. Both 16S and qPCR methods were concordant for the presence of Borrelia, with the 16S studies providing an additional profile of associated bacteria. Ticks that were qPCR-positive for Borrelia had significantly greater bacterial diversity than Borrelia-negative ticks and no pathogens other than B. burgdorferi were identified in these samples. This work supports and extends Lyme surveillance in Alberta, and highlights the potential to investigate the microbial community context and sources of Borrelia in Alberta.
- Graduation date
- Spring 2021
- Type of Item
- Doctor of Philosophy
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.