Rapid and widespread dissemination of multidrug-resistant blaCMY-2Salmonella Typhimurium in Mexico

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Abstract
We describe the emergence and dissemination of multidrug-resistant (MDR) Salmonella Typhimurium in humans, retail meat and food animals from Yucatan, Mexico. Salmonella Typhimurium isolates were collected through an active surveillance system and tested for susceptibility to 12 antimicrobial agents. Isolates that were non-susceptible to ceftriaxone were tested with 10 additional antimicrobials and assayed by PCR for the presence of CMY, CTX-M, SHV, TEM and OXA β-lactamase genes. Plasmid-borne phenotypes were identified by transfer to susceptible Escherichia coli. Isolates from humans, retail meat and food animals were compared by PFGE to determine genetic relatedness. MDR Salmonella Typhimurium containing a plasmid-mediated blaCMY-2 AmpC β-lactamase rose from 0% (0/27) during 2000 and 2001 to 75% (63/84) in 2004 and 2005 (P < 0.0001). MDR blaCMY-2Salmonella Typhimurium (n = 115) was most common in ill children (44.3%) and pork or swine intestine (36.5%). In several cities, MDR blaCMY-2Salmonella Typhimurium from retail meat or swine intestine exhibited PFGE patterns and antibiograms indistinguishable from those in strains recovered from hospitalized children. The CMY gene was transferred to E. coli by electroporation, along with resistance to three to six other antimicrobials. Children with MDR blaCMY-2Salmonella Typhimurium infection (n = 39) had a higher frequency of systemic infection (13% versus 0%), mortality (8% versus 0%) and hospital re-admission due to protracted diarrhoea (28% versus 17%) than children with non-MDR-Salmonella Typhimurium (n = 24), although the difference was not statistically significant. The rapid and widespread dissemination of MDR blaCMY-2Salmonella Typhimurium in Mexico calls for urgent interventions to contain this potentially fatal pathogen.