Aerobactin in human and animal disease Mark Roberts The aerobactin iron uptake system is specified by a cluster of five genes, four for the synthesis of the siderophore aerobactin and the fifth for its cognate outer membrane protein receptor. The aerobactin genes have been cloned and subjected to molecular analysis. The role of the product of one of these genes (the 50,000 dalton protein) was in dispute and had been reported to be involved in both the transport and the biosynthesis of aerobactin. Complementation studies showed that the protein is intimately involved in aerobactin biosynthesis. Also, strains carrying plasmids with transposon insertions that abolished production of the 50 K protein were still able to transport aerobactin, showing that the outer membrane receptor is the only protein specified by the aerobactin system necessary for the uptake of ferri-aerobactin. E. coli strains isolated from domestic animals were screened for aerobactin and colicin V production by bioassay and by colony hybridization using DNA probes derived from the aerobactin biosynthesis and colicin V activity regions of ColV-K30. There was a high incidence of aerobactin production among septicaemic strains compared with a very low incidence among faecal strains. Colicin V production correlated with aerobactin production but the association was not absolute. Southern hybridization experiments indicated that the aerobactin genes of animal strains almost always reside on large (usually ColV) plasmids. Diarrheagenic strains of E. coli were also screened for aerobactin production. No enterotoxigenic strains produced aerobactin but strains belonging to the enteropathogenic, enteroinvasive and facultatively enteropathogenic E. coli groups contained members that produced aerobactin. The aerobactin biosynthesis probe hybridized to small plasmids of enteropathogenic E. coli strains in Southern blots. Antisera were raised in rabbits to both native and denatured aerobactin receptor protein (IutA). The antisera raised to denatured protein reacted with denatured protein in Western blots (WB) and immunoprecipitation (IP) but did not inhibit aerobactin or cloacin binding to IutA. The serum raised against the native protein inhibited cloacin binding but did not react in WB or IP. The cloned aerobactin genes restored the virulence in a murine peritonitis model of a wild type animal strain of E. coli cured of its CoIV plasmid. Prior growth in iron-restricted medium enhanced the virulence of the infecting strain. None of the anti-IutA sera passively protected mice from infection with a wild type iutA+ strain.