Which bacterium is transmitted to humans via an arthropod vector?

Yersinia pestis is the causative agent of plague and is primarily transmitted to humans through the bite of infected fleas, which are arthropod vectors. These fleas typically infest rodents, and when they bite humans, they can transmit the bacteria, leading to various forms of plague, including bubonic and pneumonic. The association with arthropod vectors is particularly significant in understanding the epidemiology of Yersinia pestis, as outbreaks of the disease often correlate with flea infestations in rodent populations.

In contrast, the other bacteria listed are not primarily transmitted through arthropod vectors. Pseudomonas aeruginosa is mainly acquired through contact with contaminated surfaces or through opportunistic infections in immunocompromised individuals. Legionella pneumophila is primarily spread through aerosols from contaminated water sources, not through arthropods. Brucella abortus is transmitted to humans through direct contact with infected animals or consumption of unpasteurized dairy products, rather than through insect vectors. Thus, Yersinia pestis stands out as the correct answer due to its specific transmission route involving an arthropod.