Discrimination of Gram-Positive versus Gram-Negative Bacteria by the Social Amoeba Dictyostelium discoideum

Professional phagocytes detect, pursue, engulf, and kill bacteria. While all professional phagocytes use chemotaxis to locate bacteria, little is known about whether they can discriminate among different types of bacteria, responding preferentially to some over others. We examined the chemotaxis of the soil amoeba and professional phagocyte Dictyostelium discoideum in assays where amoebae were presented with a paired choice between different bacteria. We observed variation in the extent to which they pursue different bacteria, with preferential migration towards Gram-negative over Gram-positive bacteria. Response profiles were similar for amoebae isolated from wide-ranging locations, suggesting that chemotaxis preference was not strongly influenced by geographic variation in the bacterial community. Because cyclic adenosine monophosphate (cAMP) is a known chemoattractant for D. discoideum , we tested whether it mediated the preference for Gram-negative bacteria. Chemotaxis was diminished in response to a cAMP-deficient strain of Escherichia coli and enhanced in response to an E. coli strain that overproduces cAMP. We conclude that D. discoideum discriminates at a distance among bacteria and that discrimination is mediated in part by sensing of cAMP. Preferential sensing and response to some bacteria over others may explain why some amoeba-bacterial associations are more prevalent in nature than others.