Graduation date: 2006Biological agents and their products commonly cause foodborne illnesses. In the United States it is estimated that there are over 76 million cases of foodborne illnesses each year, resulting in an economic loss of approximately $40 billion. These high figures demonstrate the need for bioassays that display a rapid and accurate response to biologically active agents such as the products produced by pathogenic bacteria. Current detection techniques consist of selective enrichment plating, which requires culturing time, or molecular probing techniques, that detect the presence of an agent, but not its actual biological toxicity. A rapid assay that detects biologically active agents would provide a superior method of pathogen detection and could act as a warning of environmental contamination. One pathogen of concern, Bacillus cereus, is ubiquitous in the environment and most notably known for causing food poisoning. This research describes a chromatophore-based biosensor from Betta splendens (Siamese fighting fish), which can detect B. cereus at levels at or below the infectious dose needed to cause food poisoning. Chromatophores are pigmented cells found in cold-blooded animals that react physiologically to biologically active agents by altering their optical density. The red-pigmented chromatophores, or erythrophores, were used to determine the response limit to B. cereus, and the response patterns to B. cereus grown in different media. This research found the lower limit of detection of B. splendens erythrophores for B. cereus cultured in Luria Bertani broth to be 104 viable B. cereus cells per milliliter. This sensitivity is equivalent to that expected of the sensitivity pertinent to food safety standards. Erythrophore responses to B. cereus cultured in Brain Heart Infusion broth supplemented with glucose support the hypothesis that culturing in this fashion results in maximally expressed enterotoxins. These results establish Brain Heart Infusion broth supplemented with glucose as the preferred medium for use in B. cereus gene expression analyses that assess enterotoxin regulation and function.