Carbapenem and multidrug resistance in Gram-negative bacteria in a single centre in Italy: considerations on in vitro assay of active drugs

tIn intensive care units (ICUs), the most important causes of nosocomial bacterial infections are mainlymultidrug-resistant (MDR) and extensively drug-resistant (XDR) Acinetobacter baumannii and Klebsiellapneumoniae strains. Mortality related to these infections is very high due to lack of effective therapy andthe severity of patient conditions. This study aimed to assess the prevalence of carbapenem resistancegenes in 77 carbapenem-resistant Gram-negative bacteria isolated from severe infections (bloodstream,pulmonary and urinary tract) during the period 1 January to 31 July 2013 in a general ICU in Catania,Italy, and to examine their susceptibility to tigecycline and colistin using two different methods. In total,52 A. baumannii belonging to the same sequence type (ST) 2 clone and carrying the blaOXA-23gene aswell as 25 K. pneumoniae carrying blaKPC-3were isolated. Four distinct pulsotypes were identified in K.pneumoniae, which correlated with four distinct STs: ST258 and ST512, spread worldwide, and ST147and ST395 detected for the first time in Italy. A. baumannii isolates showed an XDR profile and werefully susceptible only to colistin; all KPC-producing K. pneumoniae isolates were MDR, whilst colistin wasactive against 19 of 25 strains. These results show that broth microdilution (BMD) is a reliable in vitrosusceptibility test for colistin, above all K. pneumoniae, whilst both the gradient test and BMD are suitablefor tigecycline susceptibility testing of A. baumannii.