The afa-related gene cluster in necrotoxigenic and other Escherichia coli from animals belongs to the afa-8 variant

Abstract Six hundred and nine necrotoxigenic Escherichia coli type 1 and 2 (NTEC1 and NTEC2) and non-NTEC isolated in Western and Southern Europe, North Africa and Canada from diseased calves, pigs, humans, poultry, and 55 isolated from asymptomatic calves were studied for the identification of afa -related sequences to the recently described afa-7 and afa-8 gene cluster variants from two bovine Escherichia coli ( Lalioui et al., 1999 ). Colony hybridization and PCR assays for the afaD - 7 , afaE - 7 , afaD - 8 and afaE - 8 identified the afa -related sequences to the afa - 8 gene cluster in most (67/79; 85%) of the E. coli positive with the Afa-f family probe and in 14 additional strains negative with the Afa-f probe. No E. coli was positive for the afa - 7 gene cluster. The existence of afa - 8 positive strains was thus confirmed among bovine E. coli and for the first time among porcine, poultry and human E. coli . Sequencing of the afaE - 8 amplicon of nine strains from the different host species showed a high degree of conservation (>95% at the DNA level; >92% at the amino-acid level). The afa - 8 gene cluster was more frequent in E. coli from diseased calves (18%) than from piglets (12%), humans (6%) and poultry (5%). Bovine NTEC2 (26%) were more frequently positive than NTEC 1 (20%) and non-NTEC (11%). E. coli isolated from asymptomatic calves were rarely positive: one NTEC2 (3%) and no non-NTEC. The afa - 8 gene cluster was located on the Vir plasmid in 11/23 NTEC2, but no plasmid localization was detected in NTEC1 or non-NTEC.