The resistance of Acinetobacter baumannii strains to carbapenems is a worrying problem in hospital settings. The main mechanism of carbapenem resistance is the expression of ␤-lactamases (metalloenzymes or class D enzymes). The mechanisms of the dissemination of these genes among A. baumannii strains are not fully understood. In this study we used two carbapenem-resistant clinical strains of A. baumannii (AbH12O-A2 and AbH12O-CU3) expressing the plasmid-borne bla OXA-24 gene (plasmids pMMA2 and pMMCU3, respectively) to demonstrate that A. baumannii releases outer membrane vesicles (OMVs) during in vitro growth. The use of hybridization studies enabled us to show that these OMVs harbored the bla OXA-24 gene. The incubation of these OMVs with the carbapenem-susceptible A. baumannii ATCC 17978 host strain yielded full resistance to carbapenems. The presence of the original plasmids harboring the bla OXA-24 gene was detected in strain ATCC 17978 after the transformation of OMVs. New OMVs harboring bla OXA-24 were released by A. baumannii ATCC 17978 after it was transformed with the original OMV-mediated plasmids, indicating the universality of the process. We present the first experimental evidence that clinical isolates of A. baumannii may release OMVs as a mechanism of horizontal gene transfer whereby carbapenem resistance genes are delivered to surrounding A. baumannii bacterial isolates.

A clinical strain of Acinetobacter calcoaceticus resistant to carbapenems was isolated from a blood culture sample from an inpatient in a hospital in Madrid (Spain) during a large outbreak of infection (affecting more than 300 inpatients), caused by a multidrug-resistant Acinetobacter baumannii clone. The carbapenem resistance in both the A. calcoaceticus and A. baumannii clones was due to a bla OXA-24 gene harbored in different plasmids. The plasmids were fully sequenced, revealing the presence of site-specific recombination binding sites putatively involved in mobilization of the bla OXA-24 gene. Comparison of plasmids contained in the two strains revealed possible horizontal transmission of resistance genes between the Acinetobacter species.

hospitals (about 40,000 hospital beds) prospectively collected nonduplicate Enterobacteriaceae using the screening cutoff recommended by EUCAST. Carbapenemase characterization was performed by phenotypic methods and confirmed by PCR and sequencing. Multilocus sequencing types (MLST) were determined for Klebsiella pneumoniae and Escherichia coli. A total of 702 Enterobacteriaceae isolates met the inclusion criteria; 379 (54%) were CPE. OXA-48 (71.5%) and VIM-1 (25.3%) were the most frequent carbapenemases, and K. pneumoniae (74.4%), Enterobacter cloacae (10.3%), and E. coli (8.4%) were the species most affected. Susceptibility to colistin, amikacin, and meropenem was 95.5%, 81.3%, and 74.7%, respectively. The most prevalent sequence types (STs) were ST11 and ST405 for K. pneumoniae and ST131 for E. coli. Forty-five (54.1%) of the hospitals had at least one CPE case. For K. pneumoniae, ST11/OXA-48, ST15/OXA-48, ST405/ OXA-48, and ST11/VIM-1 were detected in two or more Spanish provinces. ST11 isolates carried four carbapenemases (VIM-1, OXA-48, KPC-2, and OXA-245), but ST405 isolates carried OXA-48 only. A wide interregional spread of CPE in Spain was observed, mainly due to a few successful clones of OXA-48-producing K. pneumoniae (e.g., ST11 and ST405). The dissemination of OXA-48-producing E. coli is a new finding of public health concern. According to the susceptibilities determined in vitro, most of the CPE (94.5%) had three or more options for antibiotic treatment. C arbapenemase-producing Enterobacteriaceae (CPE), mainly Klebsiella pneumoniae, are an emerging threat to public and individual health worldwide. These microorganisms are often resistant to almost all available antibiotics (1, 2), so there are few alternative treatment options. The most common carbapenemases are KPC (class A); VIM, IMP, and NDM (class B); and the OXA-48 types (class D). However, the extent to which health care systems have been affected and the carbapenemase types that are predominant differ substantially from country to country (3).A multicenter study performed in Spain in 2009 revealed 43 (0.04%) cases of CPE, which were mostly VIM-1 and IMP-22 (4). After that, we reported a rapid increase in the number of cases of CPE, mainly OXA-48-producing K. pneumoniae, in this country from 2010 to 2012 (5-7).Because previous studies (5, 6) were based on voluntary reports without taking into account key important issues, in this paper, we present data on the impact of CPE as obtained from a prospective, multicenter, and population-based study. We show that carbapenemase production in this country is widely and irregularly distributed; however, the rates of susceptibility to meropenem and colistin were still high.(The preliminary results of this study were presented in part at the 24th European Congress of Clinical Microbiology and Infectious Diseases Annual Meeting, 10 to 13 May 2014 in Barcelona, Spain,).