PATHOGENIC ESCHERICHIA COLI:GENETIC CHARACTERIZATION OF STRAINS ISOLATED FROM DOMESTIC ANIMALS AND POTENTIAL HUMAN HEALTH HAZARDS

Pathogenic Escherichia coli (E. coli) are one of the most important groups of bacteria causing diarrhea and extraintestinal infections in humans and animals. In particular, in farm animals diseases due to pathogenic E. coli strains are a recognized cause of strong economic losses and reduction of animal welfare. Moreover, farm animals, especially cattle, are an identified source of zoonotic E. coli strains, such as Shiga-toxin producing E. coli (STEC), which are responsible of serious human illnesses. In this thesis both the roles of E. coli (E. coli as an animal pathogen and E. coli as a zoonotic agent) were investigated. Virulence factors and antimicrobial resistance of pathogenic E. coli strains isolated from diseased and healthy calves were characterized. Identification of virulence factors and antimicrobial susceptibly testing were also carried out for E. coli isolated from sows with urinary tract infections (UTI). Finally, a survey was conducted to investigate the presence of STEC in bovine dairy herds of Lodi province. E. coli is frequently responsible for diseases in calves. Virulence genes and pathotypes of 54 E. coli selected isolates from septicemic, diarrheic and healthy calves from Lodi, Italy were characterized. Three (5.6%) enterotoxigenic E. coli (ETEC), 24 (44.4%) extraintestinal pathogenic E. coli (ExPEC), 25 (46.3%) E. coli strains harbouring a combination of virulence factors not related to a particular pathotype and 2 (3.7%) potentially non-pathogenic strains were identified by PCR and microarray analysis. Different E. coli pathotypes were isolated from calves with similar diseases and analogous pathotypes were observed in animals with different symptoms. Virulence factors such as iss and microcins were found in ETEC isolates from septicemic animals and the iroN gene was significantly associated with ExPEC isolates from symptomatic calves. Our results suggest that particular E. coli pathotypes may not always be associated with a specific syndrome and the presence of other virulence, host and/or environmental factors contribute to the disease. Antimicrobial resistance of the 54 E. coli strains was also investigated. Antimicrobial susceptibly testing and detection of antimicrobial resistance-related genes by microarray analysis highlighted the presence of a great number of multi-resistance E. coli strains. Alarming resistance to third and fourth generation cephalosporins and to fluoroquinolones was also identified. E. coli strains are also recognized as a major cause of UTI in sows, 77 E. coli strains isolated from urine of sows with acute UTI were investigated to identify virulence genes and phylogroups. The most prevalent gene identified was fimH alone or in combination with other virulence genes, especially iutA and iucD genes. The fimH gene was also the most common gene found in the 33 E. coli strains isolated from sows’ faecal samples. Differences with human uropathogenic isolates were observed related to phylogenetic groups and adhesion profile. Strains isolated from sows belonged to E. coli groups A and B1 while pathogenic strains in humans belong to groups B2 and D. Moreover, P and S fimbriae were detected less frequently in porcine strain. Overall, the majority of strains isolated from UTI in sows harboured few virulence factors, suggesting that other factors related to animals and environment could contribute to the pathogenesis of infection. In addition, further investigation are required to clearly understand the real relationship between faecal and UTI strains, in particular for those strains showing fimH gene only. Antimicrobial resistance profile of the 77 isolates from UTI showed that the majority of these strains was susceptible to the antimicrobial agents tested, with the exception of the association sulfamethoxazole-trimethoprim. Finally a survey, to investigate the presence of the zoonotic STEC pathotype was carried out in 315 bovine dairy herds of Lodi province, Italy. Six-hundred bulk tank milk samples were analysed by PCR to detect stx genes. Stx-Positive samples were characterized for the presence of eae gene and serogroup specific-associated genes (O157, O26, O103, O111, O121, O145 and O1O4:H4). Twenty-five (4.2%) raw milk samples were found positive for stx gene, in particular Stx2 was the most common type of toxin. This toxin was detected both alone and in association with eae gene. Serogroup was identified in 10 (40%) of 25 STEC positive bulk tank milk samples. The determination of the serogroup was not possible in 15 (60%) samples with the PCR assay used. The results reported highlight the presence of human potentially pathogenic strains in dairy herds of Lodi province.