Molecular epidemiological studies on multidrug-resistant Escherichia coli isolates from newborn Holstein calves, cow of each, and calf-breeding areas

Of all Escherichia coli isolates that were obtained from 2 newborn Holstein calves (designated calf A and B), two cows (designated cow A and B, respectively), and calf-breeding areas (trough, floor, fence, litter bed, sawdust, water trough, Holstein cattle breeding area around <calves C and D, and cows E and F>), a total of 120 multidrug resistant E. coli isolates with resistance to at least 3 out of 7 tested antibiotics [ampicillin, chloramphenicol, chlortetracycline, kanamycin, streptomycin, nalidixic acid, and sulfadimethoxine] were obtained in 2002. Each isolate was tested for clonal relationships by plasmid profile analysis and/or pulsed-field gel electrophoresis (PFGE) analysis. Plasmids were found in 65% (78/120) of all E. coli isolates tested. A total of 9 different plasmid profiles (i.e., type I - IX) were identified based on the number and size of the plasmids. Type VII was the most widely distributed type in strains from various sources, including calf A, cow A, and calf-breeding areas. The number and type of plasmid profile observed in each Holstein was: calf A, 3 types (II, III, and VII); cow A, one type (VII); calf B, 4 types (I, III, IV, and V); cow B, 2 types (IV and VI). These results show that the calves had more varied profiles than the cows, and that both calves also carried E. coli strains with plasmid profiles that were different from those of the calf-breeding areas. PFGE following restriction digestion with XbaI was performed on 120 of the isolates, because approximately one-third of the isolates did not carry plasmids and could not be typed by plasmid profile analysis. The 120 isolates were divided into 49 PFGE patterns, which showed that some genetically indistinguishable isolates were present in calf A and calf-breeding areas (floor, trough, sawdust, calf C, or calf D), and some genetically closely related isolates were also present in calf A and calf-breeding areas (floor, trough, sawdust, or water trough). Furthermore, none of the indistinguishable or closely related isolates were shared between calf A and cow A, or calf B and cow B, respectively. These results indicate that there may be other environmental sources, besides the calf-breeding areas (floor, trough, sawdust, water trough, calf breeding locations) identified in this study, that are potential sources of multidrug-resistant E. coli infection in newborn Holstein calves.