Interaction of the calcium ionophore A.23187 (Calcimycin) with Bacillus cereus and Escherichia coli

Ionophores isolated from bacterial strains, and especially A.23187, are efficient antibiotics against Gram‐positive bacteria and devoid of activity on Gram‐negative species. This difference in activity was attributed to the outer membrane of Gramnegative bacteria which is presumably impermeable to these very hydrophobic compounds. In this context, the partition of the calcium ionophore A.23187 between bacteria and the medium was studied on Escherichia coli (Gram‐negative) and Bacillus cereus (Gram‐positive) using, on the one hand, fluorimetric measurements and, on the other hand, radioautographic analysis of bacteria incubated with the [3H]‐labelled ionophore. Although the first method did not give a definitive answer, the second one clearly showed that the tritiated metabolite was only incorporated into B. cereus.