Production of toxins (enterotoxins, verotoxins, and necrotoxins) and colicins by Escherichia coli strains isolated from septicemic and healthy chickens: relationship with in vivo pathogenicity.

Since the mechanism of virulence of Escherichia coli strains pathogenic to birds is not fully understood, the prevalence of toxic factors produced by E. coli strains pathogenic to other animals was investigated. A total of 625 E. coli strains isolated from visceral organs of chickens with colisepticemia and from feces of healthy chickens in Spain were tested for production of enterotoxins (heat labile [LT] and heat stable [STa]), verotoxins (VT1, VT2, and VT2v), cytotoxic necrotizing factors (CNF1 and CNF2), alpha-hemolysin (Hly), enterohemolysin (EntHly), colicin V (Col V) and other types of colicins, and necrotic and lethal activities. Only 45 (7%) of avian E. coli strains were toxigenic: 20 strains produced a cytotoxic response in HeLa but not in Vero cells, indicating the production of a cytotoxin not related to the VTs; 16 were EntHly+; 5 produced a new cytotonic product that causes the appearance of whitish vacuola in Vero and HeLa cells; 3 synthesized soluble factors that cause lethal activity in mice; and 1 elaborated LT. None of 625 avian E. coli strains was positive for production of VTs or CNFs. In contrast, colicinogenicity occurred in 335 (73%) of the 458 septicemic strains and 97 (58%) of 167 fecal isolates (P < 0.01), and this property was correlated with in vivo pathogenicity of strains. Thus, 80% (P < 0.001) and 66% (P < 0.001) of strains producing Col V and other types of colicins were characterized as being of high pathogenicity, whereas only 15% of the noncolicinogenic strains were classified as highly pathogenic. Our results clearly support the special pathogenicity theory, because 60% of the E. coli strains belonging to 18 serogroups (O1, O2, O5, O8, O12, O14, O15, O18, O20, O53, O78, O81, O83, O102, O103, O115, O116, and O132) most frequently identified among clinical septicemic strains were classified as highly pathogenic in in vivo assays, whereas only 24% of the strains with O serogroups less prevalent among diseased chickens were considered highly pathogenic (P < 0.01).