Multidrug resistance associated to cephalosporins-resistant Escherichia coli from bovine mastitis

The aim was to compare the rate of multidrug resistance in Escherichia coli selected randomly and based on resistance to ceftiofur. The study targeted strains isolated from bovine mastitis. E. coli (n = 107) isolated from bovine subclinical and clinical mastitis in Northern Italy dairy farms were included. E. coli isolation and identification were carried out following the National Mastitis Council guidelines (1). The isolates were selected following two criteria: A) 53 E. coli were randomly chosen to represent each farm; B) 54 E. coli were selected on MacConkey agar with 8 mg/L of ceftiofur (2). Ceftiofur was used as a representative of cephalosporins class (3). Antimicrobial susceptibility was tested in vitro by minimal inhibitory concentrations (MICs) of 16 antibiotics using Sensititre susceptibility plates according to supplier instructions (Trek Diagnostics System, East Grinstead, UK). CLSI recommendations and resistance breakpoints were used (4). A χ2 test was used to compare the two group A and B. Results were considered significant if P < 0.05. All the isolates, regardless to the group, were susceptible to Amikacin, Imipenem and Cefoxitin (P=0.061). The resistance frequencies to most antimicrobials were significantly different between group A and B. The E. coli of group B were more frequently resistant to other antibiotic classes than group A. P was 0.001 for ampicillin, ticarcillin, cefpodoxime, Ticarcillin/clavulanic acid, Trimethoprim/sulfamethoxazole, Cefazolin, Doxicycline, Ceftiofur, Marbofloxacin and Chloramphenicol, 0.030 for Amoxicillin/clavulanic acid and 0.070 for Gentamicin. The results confirmed the usefulness of ceftiofur to detect resistance to cephalosporins (3) and the link with multidrug resistance. A growing number of cephalosporin-resistant E. coli are extended-spectrum beta-lactamases (ESβL) producers and multidrug-resistant (5). A monitoring based on a selective medium could reveal a higher than expected prevalence of resistant strains.