Prevalence and molecular characterisation of CTX-M-15-positive Escherichia coli and Salmonella isolates from food and livestock farms in Germany

Objective: The aim of the study was to determine the prevalence of CTX-M-15 producing E. coli and Salmonella in German livestock farms and food and to characterise the CTX-M-15-carrying isolates and their plasmids by molecular analysis. Methods: 289 E. coli and 157 Salmonella isolates showing resistance to 3rd generation cephalosporins originating from routine submissions to the Federal Institute for Risk Assessment (BfR) obtained between 2003 and June 2012 were examined. Furthermore 271 isolates (66 different farms) from the RESET-project collected in 2011 were analysed. Isolates were tested for the presence of ESBL-/AmpC-/carbapenemase-genes (PCR-sequencing) and E. coli isolates were analysed by phylogenetic grouping. CTX-M-15 positive isolates were characterised by XbaI-PFGE, MLST and transformation experiments. CTX-M-15 harbouring plasmids were analysed by Inc-rep-typing, S1-nuclease-PFGE and Southern-blot/hybridisation experiments. Results: A total of 13 E. coli and two Salmonella isolates from the routine surveys carried a blaCTX-M-15 gene. The first blaCTX-M-15 positive E. coli isolate was detected in 2010 (one isolate out of 72 - 1,4%), in 2011 seven E. coli (9% out of 75 isolates) and in 2012 (until June) four E. coli (9% of 44 isolates). Within the RESET-project, 11 different farms (17% out of 66 farms) tested positive for CTX-M-15. Molecular characterisation revealed different MLST-types, XbaI-PFGE patterns and phylogenetic groups (A, D and B1) with predominance of phylogroups A and B1. Additionally hybridisation experiments showed that most of blaCTX-M-15 genes harboured on plasmids belonging to different sizes (80-320kb) and Inc-groups (especially IncI1 and IncF-groups). Seven isolates belonging to MLST-Type ST410, phylogroup A showed a highly related PFGE-pattern and seem to carry a chromosomally located blaCTX-M-15 gene. Two CTX-M-15 positive Salmonella (Typhimurium and Bredeney) were detected (one out of 12-8% in 2005, and one out of 24-4% in 2009). The gene was located on IncI1 (90 kb) and IncHI2 (320 kb) plasmids. Conclusion: The prevalence of CTX-M-15 positive E. coli strains is increasing since 2010. The increase is caused by the spread of different E. coli isolates and plasmids. Especially a clonal lineage belonging to ST410 and plasmids belonging to IncI1, and IncF-groups seem to play, at least in livestock, an important role. The presence of this gene in Salmonella supports its horizontal gene transfer.