Prevalence of extended spectrum beta-lactamases among Enterobacteriaceae: an Italian survey

The prevalence of extended-spectrum beta-lactamase (ESBL) production by consecutive non-repeated isolates of Enterobacteriaceae was determined over a 6-month period. A total of 8015 strains were isolated from ten Italian laboratories and 509 (6.3%) of these were designated ESBL producers from the results of a double-disk synergy test. Escherichia coli was the most isolated microrganism, followed by Klebsiella pneumoniae and Proteus mirabilis. Providencia stuartii (28.1%) was the most frequently isolated ESBL producer, followed by K. pneumoniae and Enterobacter aerogenes (20.5%). However, amongst all ESBL producers, K. pneumoniae (38.2%) was the most represented followed by P. mirabilis (25.7%). All the strains positive to DD tests were confirmed for the carriage of TEM and SHV genes using colony-blot hybridisation (CH). A total of 447 strains (88.0%) were CH-positive, of which 42.3% hybridised with the TEM-type probe, 30.1% with the SHV-type probe and 15.6% with both probes. In conclusion, our findings indicate that 6.3% of all Enterobacteriaceae tested produced ESBLs, 42.3% of which were TEM-derived enzymes. More than 20% of P. stuartii, K. pneumoniae and E. aerogenes harbour these enzymes. The double-disk test seems to be a useful test to identify ESBL producing strains.