on this dynamic. Eighty E. coli strains isolated from fecal samples of pigs receiving a diet containing chlortetracycline and penicillin,with or without 2% clinoptilolite, were examined for antimicrobial resistance to 15 antimicrobial agents. Overall, an increased resistance to 10 antimicrobials was observed with time. Supplementation with clinoptilolite was associated
with an early increase but later decrease in blaCMY−2, in isolates, as shown by DNA probe. Concurrently, a later
increase in the frequency of blaCMY−2 and the virulence genes iucD and tsh was observed in the control pig isolates,
being significantly greater than in the supplemented pigs at day 28. Our results suggest that, in the long
term, supplementation with clinoptilolite could decrease the prevalence of E. coli carrying certain antimicrobial
resistance and virulence genes.