Student Research: Christopher Wilkerson

, , 2003
Faculty Advisor:

Antibiotic Resistance of Escherichia coli O157 Isolated from Humans and Cattle


This study provides a characterization of the antibiotic resistance abilities of the human pathogen Escherichia coli O157. Cattle are a common reservoir of the pathogen and most illness is a result of contamination of meat of other foodstuffs with manure. A total of 901 Escherichia coli O157 isolates collected from health departments and cattle feedlots were tested using the Kirby-Bauer disk diffusion method for the occurrence of resistance to 12 antimicrobial agents. Antibiotics tested included; ampicillin, ampicillin / sulbactam, ceftriaxone, nalidixic acid, ciproflaxin, kanamycin, gentamycin, amikacin, streptomycin, tetracycline, and trimethoprim / sulfamethoxazole.

Of the 663 isolates of bovine origin 619 (93.4%) were susceptible to all antibiotics tested. The 238 human pathogens isolated 207 (87.0%) were susceptible to all antibiotics tested. Tetracycline resistance was most common among both cows and humans with 43 (6.5%) of bovine isolates and 16 (6.7%) of human isolates possessing resistance. Streptomycin resistance was also relatively high with 29 (4.4%) of cows and 13 (5.5%) of human isolates resistant. Resistance within cows to all other antibiotics was very rare; less than 1% for ampicillin, ampicillin / sulbactam, gentamycin, kanamycin, nalidixic acid and trimethoprim / sulfamethoxazole. Ampicillin resistance was significantly higher within human derived isolates with 8 (3.4%) isolates resistant. Ampicillin / sulbactam, gentamycin, kanamycin and trimethoprim / sulfamethoxazole and nalidixic acid resistance were all found to be higher in human isolates as well. All of the pathogens regardless of source were susceptible to amikacin, ciproflaxin, and ceftriaxone.

Thirty (4.5%) of the bovine isolates were multi-drug resistant. Sixteen (6.7%) of the human isolates were multi-drug resistant. The majority of MDR isolates shared resistance to tetracycline and streptomycin.

The resistance levels in general were lower than previously tested collections. This study detracts from previous studies and common perceptions that have suggested that agricultural isolates possessed greater resistance levels and were the source of resistance within humans. Due to the likelihood that cows were the reservoir and initial source of the human infections more research is needed to determine if a subset of the O157 e. coli population is both more infective and antibiotic resistant or whether e. coli O157 isolates acquiring resistance due to horizontal transfer form native enteric bacteria within human hosts.