Influence of Environmental Temperature on the Bacterial Microbiomes of Ixodes scapularis Ticks

The blacklegged tick, Ixodes scapularis, is the primary vector of Lyme disease in North America. Lyme disease, endemic to approximately 65 countries worldwide, is number one vector-borne illness in the United States. In the eastern United States, Lyme diseases is transmitted by the bite of I. scapularis ticks infected with the spirochete bacterium Borrelia burgdorferi. If left untreated, Lyme infection can result in a variety of complications related to the joints, heart and nervous systems. Though the disease is highly endemic to the northeastern and upper midwestern regions of the United States, these areas represent only a portion of the vector's and host's total range. The influence of environmental factors (i.e. temperature, humidity) on the seasonal and regional incidence of Lyme disease are well-known, but their impacts on the microbial communities within the tick remain unknown. A better understanding of the influence of environmental variables on the tick microbial community maybe crucial given the recent reports that vector competence and transmission of certain tick-borne diseases can be influenced by the endogenous tick microbial communities. Using high throughput next generation sequencing, the present study investigated the effects of temperature (a major environmental variable) on the tick microbial community by comparing the sequenced amplicons of the bacterial 16S V4 rRNA gene of colony-reared I. scapularis ticks exposed to environments with different incubation temperatures (4ºC, 20ºC, 30ºC and 37ºC) at a constant humidity of [greater than]80% in a controlled lab setting for 10 days to that of the baseline, unexposed controls. Results showed that the bacterial community composition of colony-reared I. scapularis ticks is distinct between male and female adults. The microbiome of male ticks is more diverse than the females, which are entirely dominated by Rickettsia. The bacterial microbiome of I. scapularis changed upon incubation at 30ºC for a week and 37ºC for more than five days. Additionally, the male ticks incubated at 37°C revealed a significantly different bacterial diversity compared to the baseline microbiomes, and the change was dependent on the length of exposure. In-silico removal of Rickettsia from the female data revealed a shift in the underlying bacterial population, with a significantly different bacterial diversity at 37°C compared to that of 4°C and 20°C treatments. These findings led us to further investigate if the bacterial communities in natural populations of I. scapularis from diverse geographic and climatic regions also differed. Our results demonstrated that the bacterial community structure in field-collected male ticks from Texas was different compared to the males from Massachusetts. Despite the dominance of Rickettsia in female ticks from both states, the females from Massachusetts contained Borrelia, and Anaplasma, but not from Texas. Interestingly, the bacterial composition in female I. scapularis collected from dogs in Texas was more complex in comparison to the field-collected female ticks from both locations. Taken together, these results provide experimental evidence that environmental temperature can impact the bacterial microbiome composition of I. scapularis ticks in a controlled laboratory setting and may have implications to the regional differences observed in the bacterial community structures among the natural populations of the tick. Future studies on the mechanisms of how environmental temperature and other abiotic factors influence the tick microbiome will improve our understanding of the impacts of climate change on the ticks' ability to carry and transmit pathogens, with possible ramifications on strategies to control tick-borne and other zoonotic pathogens.