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Salmonella typhimurium responds chemotactically to gradients of divalent cations in the presence of citrate ions. The actual chemoeffector is the citrate-metal ion complex, which acts as an attractant. Citrate (which is also a chemoeffector for Salmonella) and the citrate-metal ion complex are recognized by different receptors. The response of Salmonells, which can transport citrate through its membrane, is quite different than that of Escherichia coli, which cannot.
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Ingolia et al. (1979) studied this question.
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