Undergraduate Honors Theses

Thesis Defended

Spring 2016

Document Type


Type of Thesis

Departmental Honors


Integrative Physiology

First Advisor

Christopher A. Lowry

Second Advisor

Steven K. Schmidt

Third Advisor

David E. Sherwood


The relationship between the brain and digestive health has been understood for many centuries, but understanding of how the human microbiota communicates with the nervous system and modifies behavior is only recently emerging. Evidence suggests that the intestinal microbiota communicates with the central nervous system through immune, neural and endocrine signals along a pathway called the microbiota-gut-brain axis. Few studies have examined the potential role of nonpathogenic environmental bacteria, such as Mycobacterium vaccae, on the functioning of the central nervous system and animal behavior. In this research, we show that rats immunized subcutaneously with heat-killed M. vaccae following fear conditioning demonstrated a reduction in fear-potentiated startle (FPS) responses and enhanced extinction learning. The group treated with M. vaccae and the control group were both tested using the fear-potentiated startle paradigm, in which the amplitude of the startle reflex is increased in the presence of a conditioned stimulus (light) previously paired with an unconditioned stimulus (footshock). M. vaccae-immunized rats had an overall reduction in percent FPS as compared to the control group, with significant reductions in percent FPS seen on days 2 and 3 of extinction trials. Collectively, the results suggest a beneficial effect of subcutaneously immunizing rats with heat-killed M. vaccae on FPS responses and the rate of extinction learning, supporting a positive role of nonpathogenic microbes in the modulation of animal behavior.