Disclosed herein is a method for inhibiting or preventing Vibrio cholera toxin production in a subject, the method comprising enterally administering to the subject a pharmaceutically effective amount of a fatty acid dissolved or suspended in a pharmaceutically acceptable carrier, wherein the fatty acid contains 10 to 30 carbon atoms, such as an unsaturated fatty acid such as a cis-2-unsaturated fatty acid, such as a fatty acid having the formula:

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wherein n is an integer of 6-26, and the fatty acid optionally includes a second carbon-carbon double bond resulting from removal of two hydrogen atoms on adjacent carbon atoms. Also disclosed herein is a method for treating or preventing a Vibrio infection comprising administering to a subject in need of treatment an effective amount of a genetically engineered bacterium, wherein the genetically engineered bacterium comprises an exogenous nucleic acid encoding an enzyme that produces a diffusible signal factor (DSF) by introducing a cis-2 double bond to a fatty acid.