AEM Accepts, published online ahead of print on 10 November 2006

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Appl. Environ. Microbiol. doi:10.1128/AEM.01551-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Descriptive and Molecular Epidemiology of Tetracycline-Resistant Gram-Negative Enteric Bacteria from Dairy Cattle

Ashish A. Sawant, Narasimha V. Hegde, Beth A. Straley, Sarah C. Donaldson, Brenda C. Love, Stephen J. Knabel, and Bhushan. M. Jayarao^*

Department of Veterinary and Biomedical Sciences, and Department of Food Science; The Pennsylvania State University, University Park, Pennsylvania

^* To whom correspondence should be addressed. Email: bmj3{at}psu.edu.

A study was conducted to understand the descriptive and molecular epidemiology of antimicrobial-resistant Gram-negative enteric bacteria in the feces of healthy lactating dairy cattle. Gram-negative enteric bacteria resistant to ampicillin, florfenicol, spectinomycin, and tetracycline were isolated from feces of 35, 8, 5, and 42% of 213 lactating cattle on 74, 39, 9, 26, and 82% of 23 farms surveyed, respectively. Antimicrobial resistant Gram-negative bacteria accounted for 5 (florfenicol) to 14% (tetracycline) of total Gram-negative enteric microflora. Nine bacterial species were isolated, of which Escherichia coli (87%) was the most predominant species. Minimum inhibitory concentrations showing reduced susceptibility to ampicillin, ceftiofur, chloramphenicol, florfenicol, spectinomycin, streptomycin, and tetracycline were observed in E. coli isolates. Isolates exhibited resistance to ampicillin (48%), ceftiofur (11%), chloramphenicol (20%), florfenicol (78%), spectinomycin (18%), and tetracycline (93%). Multidrug resistance ( 3 to 6 antimicrobials) was seen in 40% of E. coli isolates from healthy lactating cattle. Of 113 tetracycline resistant E. coli, tet(B) was the predominant resistance determinant and was detected in 93%, while remaining 7% isolates carried the tet(A) determinant. DNA-DNA hybridization assays revealed that tet determinants were located on the chromosome. Pulsed field gel electrophoresis revealed that tetracycline-resistant E. coli (n=99 isolates) belonged to 60 subtypes suggestive of a highly diverse population of tetracycline-resistant organisms. On most occasions, E. coli subtypes although shared between cows within the herd, were mostly confined to a dairy herd. The findings of this study suggest that commensal enteric E. coli from healthy lactating cattle can be an important reservoir for tetracycline- and perhaps other antimicrobial-resistance determinants.

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