High rates of antimicrobial co-resistance among Enterobacteriaceae: comparative analysis between clinical isolates resistant and susceptible to third-generation cephalosporins.

We compared the antimicrobial co-resistance of 3,402 clinical isolates of Enterobacteriaceae resistant to third-generation cephalosporins (2,569 ESBL-producing and 833 AmpC overproducing) with that of 16,220 susceptible isolates, in order to determine the impact of resistance to third-generation cephalosporins on the likelihood of resistance to other antimicrobial classes. Enterobacteriaceae resistant to third-generation cephalosporins, independently of their mechanism of resistance, were significantly more resistant to other classes of antimicrobials than susceptible isolates (p <0.001). Percentages of co-resistance to ciprofloxacin, gentamicin, tobramycin and trimethoprim-sulfamethoxazole of resistant isolates were: 51%, 39%, 53% and 51%, respectively. However, among the susceptible isolates, percentages were 17%, 7%, 6% and 19%, respectively. Fosfomycin exhibited excellent in vitro activity against urinary isolates (92%), mainly against ESBL-producing organisms (90%), and is a good alternative treatment of infections caused by multidrug resistant Enterobacteriaceae. Amikacin and imipenem were the most active antimicrobials against all species tested.