Aeromonas hydrophila AH-3 strains carrying mutations in mgtE. which encodes a M2- and Co2+ transport system. showed a 50% reduction of in vitro adherence to HEp-2 cells, a reduction in swarming in semisolid swarming agar, and decrease in biofilm formation of over 60%, in comparison to the wild-type strain. The cloned A. hydrophila mgtE expressed From a plasmid complements a Salmonella typhimurium strain deleted for all Mg2+ transporters both phenotypically and by measurement of Co-57(2+) uptake. Likewise, plasmid-borne mgtE was able to complement the changes observed in A. hydrophila mgtE mutants. We suggest that MgtE and thus Mg2+ and possibly Co2+ have a rule in A. hydrophila related to their swarming ability and related consequences such as adherence and biofilm formation. (C) 2001 published by Elsevier Science B.V. on behalf of the Federation of European Microbiological Societies.