RESUMEN
1. Escherichia coli isolated from lesions (Avian Pathogenic E. coli - APEC) of layer hens affected by colibacillosis and from intestinal contents of clinically-healthy birds (Avian Faecal E. coli - AFEC) were serotyped. All the isolates were investigated for the presence of virulence genes to determine which genes were more closely related to those from lesions. 2. A number of different serogroups were detected, O78 being predominant among the isolates from colibacillosis. 3. E. coli isolated from lesions were not linked to a specific pathotype (set of common virulence genes). 4. The presence of the virulence genes, with the exception of astA, was associated more generally with APEC strains. 5. Statistically, genes such as cva/cvi, tsh, iss, irp2 and iucD were more related to isolates from colibacillosis. 6. It is suggested that the detection of these genes in a rapid and inexpensive test for field practitioners could provide useful information about the potential virulence of E. coli isolated in commercial layer flocks.