Abstract
BackgroundThe impact of food animals as a possible reservoir for extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae, and the dissemination of such strains into the food production chain need to be assessed. In this study 334 fecal samples from pigs, cattle, chicken and sheep were investigated at slaughter. Additionally, 100 raw milk samples, representing bulk tank milk of 100 different dairy farms, 104 minced meat (pork and beef) samples and 67 E. coli isolates from cattle E. coli mastitis were analyzed.ResultsAs many as 15.3% of the porcine, 13.7% of the bovine, 8.6% of the sheep and 63.4% of the chicken fecal samples yielded ESBL producers after an enrichment step. In contrast, none of the minced meat, none of the bulk tank milk samples and only one of the mastitis milk samples contained ESBL producing strains. Of the total of 91 isolates, 89 were E. coli, one was Citrobacter youngae and one was Enterobacter cloacae. PCR analysis revealed that 78 isolates (85.7%) produced CTX-M group 1 ESBLs while six isolates (6.6%) produced CTX-M group 9 enzymes. Five detected ESBLs (5.5%) belonged to the SHV group and 2 isolates (2.2%) contained a TEM-type enzyme. A total of 27 CTX-M producers were additionally PCR-positive for TEM-beta-lactamase. The ESBL-encoding genes of 53 isolates were sequenced of which 34 produced CTX-M-1, 6 produced CTX-M-14, 5 produced CTX-M-15 and also 5 produced SHV-12. Two isolates produced TEM-52 and one isolate expressed a novel CTX-M group 1 ESBL, CTX-M-117. One isolate--aside from a CTX-M ESBL-- contained an additional novel TEM-type broad-spectrum beta-lactamase, TEM-186.ConclusionsThe relatively high rates of ESBL producers in food animals and the high genetic diversity among these isolates are worrisome and indicate an established reservoir in farm animals.
Highlights
The impact of food animals as a possible reservoir for extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae, and the dissemination of such strains into the food production chain need to be assessed
All suspected isolates were phenotypically confirmed, in that they showed a synergy effect with at least 1 of 3 strips when tested with Etest-ESBL strips containing cefepime, cefotaxime or ceftazidime, and they yielded factors > 8 when ratios of MIC/MIC were calculated
Almost all isolated ESBL producers were E. coli (89 out of 91), the exceptions being one Enterobacter cloacae isolated from a sheep, and one Citrobacter youngae isolated from a calf (Table 2)
Summary
The impact of food animals as a possible reservoir for extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae, and the dissemination of such strains into the food production chain need to be assessed. 100 raw milk samples, representing bulk tank milk of 100 different dairy farms, 104 minced meat (pork and beef) samples and 67 E. coli isolates from cattle E. coli mastitis were analyzed. One of the currently most important resistance mechanisms in Enterobacteriaceae, which reduces the efficacy even of modern expanded-spectrum cephalosporins (except cephamycins and carbapenems) and monobactams is based on plasmid-mediated production of enzymes that inactivate these compounds by hydrolyzing their b-lactam ring. Such resistance is encoded by an increasing more than 600 ESBL variants are known http:// www.lahey.org/Studies/ (last accessed January 2012). As a matter of growing concern, resistance caused by ESBLs is often associated with resistance to other classes of antibiotics like fluoroquinolones, aminoglycosides and trimethoprim-sulfmethoxazole [1,4]
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