After an enrichment step ESBL producers were isolated from 90 (26.9%) of the investigated 334 fecal samples, and one ESBL producer (1.5%) was found in 67 E. coli mastitis milk isolates, but none was isolated from either minced meat (pork and beef) or bulk tank milk samples. The ESBL prevalence among cattle was 13.7%, 25.3% among calves (animals under 6 months), 8.6% among sheep, and 15.3% among pigs. For chickens (herd level) a very high prevalence of 63.4% was determined (Table ). 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 (cephalosporin)/MIC (cephalosporin plus clavulanic acid) were calculated.
Occurrence of ESBL producers in food-producing animals at slaughter as well as in minced meat, bulk tank milk and isolates from bovine mastitis in Switzerland
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 ).
Identification and further characterization of the 91 ESBL producers isolated from 334 healthy food-producing animals at slaughter and from 67 Escherichia coli mastitis milk samples in Switzerland
The ESBL-encoding genes of all isolates were further characterized by PCR. A total of 78 isolates (85.7%) produced CTX-M group 1 ESBLs while six isolates (6.6%) produced CTX-M group 9 enzymes. Five isolates (5.5%) were detected as producers of the SHV-ESBLs and 2 isolates (2.2%) exclusively produced TEM-type enzymes. Twenty-seven CTX-M carriers were additionally PCR-positive for blaTEM
genes. Of the 91 ESBL producing isolates, 53 were selected for sequencing of the involved bla
genes (Figure ). Thirty-four isolates were CTX-M-1 producers, eight expressed additional TEM-1 and one isolate--from a pig-- additionally expressed a TEM-type enzyme, TEM-186 http://www.lahey.org/Studies/
, never found before (nucleotide sequence accession number JN227084). Six isolates carried CTX-M-14 with TEM-1 and five isolates specified CTX-M-15, one of which producing additional TEM-1. One isolate from a calf produced TEM-1 in combination with CTX-M-117 http://www.lahey.org/Studies/
, a novel CTX-M group 1 ESBL with an amino acid sequence never found before (nucleotide sequence accession number JN227085). Finally, two TEM-52 ESBL producers, and 5 SHV-12 carriers were found, three of the latter featuring additional TEM-1 (Table ).
Figure 1 Prevalence of different blaESBL genes in Enterobacteriaceae isolated from food producing animals in Switzerland in comparison to isolates from healthy humans. Prevalence of different blaESBL genes in Enterobacteriaceae isolated from food producing animals (more ...)
Besides the β-lactam resistances, the isolates were also tested for resistance to other classes of antibiotics. We found 76 (cattle: 13/17, pig: 6/9, sheep: 5/5, chicken: 51/62, milk: 1/1) out of 91 isolates resistant to tetracycline (83.5%), 59 isolates (cattle: 13/17, pig: 6/9, sheep: 2/5, chicken: 37/62, milk: 1/1) resistant to trimethoprim-sulfamethoxazole (64.8%), 43 isolates (cattle: 8/17, pig: 6/9, sheep: 4/5, chicken: 24/62, milk: 1/1) resistant to nalidixic acid (47.3%) and 31 (cattle: 13/17, pig: 6/9, sheep: 5/5, chicken: 6/62, milk: 1/1) resistant to at least one aminoglycoside (34.0%). Furthermore, 20 isolates (cattle: 11/17, pig: 2/9, sheep: 4/5, chicken: 2/62, milk: 1/1) showed resistance against chloramphenicol (22.0%), and 18 isolates (cattle: 7/17, pig: 3/9, sheep: 3/5, chicken: 4/62, milk: 1/1) showed resistance against ciprofloxacin (19.8%). One isolate from chicken faeces showed only resistance to ß-lactam-antibiotics, and none of the tested isolates was resistant to imipenem (Table ).