Background: Infections sustained by multidrug-resistant (MDR) and pan-resistant Acinetobacter baumannii have become a challenging problem in Intensive Care Units. Tigecycline provided new hope for the treatment of MDR A. baumannii infections, but isolates showing reduced susceptibility have emerged in many countries, further limiting the therapeutic options. Empirical combination therapy has become a common practice to treat patients infected with MDR A. baumannii, in spite of the limited microbiological and clinical evidence supporting its efficacy. Here, the in vitro interaction of tigecycline with seven commonly used anti-Acinetobacter drugs has been assessed.

emergence of the OXA-23 epidemic lineage could result from selective advantage conferred by the bla(OXA-23)-like determinant, which provides increased resistance to carbapenems.

Infections caused by metallo-β-lactamase (MBL)-producing Enterobacterales and Pseudomonas are increasingly reported worldwide and are usually associated with high mortality rates (>30%). Neither standard therapy nor consensus for the management of these infections exist. Aztreonam, an old β-lactam antibiotic, is not hydrolyzed by MBLs. However, since many MBL-producing strains co-produce enzymes that could hydrolyze aztreonam (e.g., AmpC, ESBL), a robust β-lactamase inhibitor such as avibactam could be given as a partner drug. We performed a systematic review including 35 in vitro and 18 in vivo studies on the combination aztreonam + avibactam for infections sustained by MBL-producing Gram-negatives. In vitro data on 2209 Gram-negatives were available, showing the high antimicrobial activity of aztreonam (MIC ≤ 4 mg/L when combined with avibactam) in 80% of MBL-producing Enterobacterales, 85% of Stenotrophomonas and 6% of MBL-producing Pseudomonas. Clinical data were available for 94 patients: 83% of them had bloodstream infections. Clinical resolution within 30 days was reported in 80% of infected patients. Analyzing only patients with bloodstream infections (64 patients), death occurred in 19% of patients treated with aztreonam + ceftazidime/avibactam. The combination aztreonam + avibactam appears to be a promising option against MBL-producing bacteria (especially Enterobacterales, much less for Pseudomonas) while waiting for new antimicrobials.

P olymyxins are old antibiotics that have recently regained popularity for treatment of severe infections caused by extensively drug-resistant (XDR) Gram-negative bacterial strains (1). As a likely consequence, emergence of polymyxin resistance is being increasingly reported in the clinical setting, especially among carbapenem-resistant Klebsiella pneumoniae isolates (2, 3). Acquired resistance to polymyxins is generally associated with chromosomal mutations (4, 5), but a new plasmid-mediated transferable resistance determinant, the mcr-1 gene, encoding a phosphoethanolamine transferase, has been described recently (6). The mcr-1 gene was originally detected in Enterobacteriaceae (mostly Escherichia coli) of animal and human origin in China (6) and subsequently also elsewhere (7-15), suggesting a broader distribution. In this communication, we report on the first detection of mcr-1 in colistin-resistant (COL-R) E. coli isolates from Italy. A retrospective analysis of the laboratory databases from the clinical microbiology laboratories of two Italian hospitals (Florence, central Italy; Lecco, northern Italy) revealed overall stable and low rates of colistin resistance among E. coli clinical isolates in the period 2012 to 2015 (Table 1). In both laboratories, routine susceptibility testing had been performed with the Vitek 2 system (bioMérieux, Marcy l'Etoile, France), and interpretation had been performed according to the EUCAST breakpoints (www.eucast .org). Eleven isolates that had been reported as colistin resistant (COL-R) were available for investigation, and nine of them were confirmed to be COL-R by reference broth microdilution (16). The COL MIC was 8 g/ml in all cases. The presence of mcr-1 was screened for by PCR and sequencing as previously described (6), using also an additional pair of primers (CLR5-F1, 5=-ATGATGCAGCATACTTCTGTGTGG; CLR5-R1, 5=-TCAGCGGATGAATGCGGTGC) targeting the extremities of the mcr-1 gene. Multilocus sequence typing (MLST) was carried out as described previously (17). Eight of the nine COL-R E. coli isolates were positive for the mcr-1 gene. In the sequenced region (positions 25 to 1576), with reference to the mcr-1 coding sequence (accession no. KP347127), the nucleotide sequences from all isolates were identical to that previously reported (6). Isolates positive for mcr-1 were detected from both centers, from inpatients in different wards, and also from two outpatients. Two mcr-1-positive isolates (LC-902/14

We investigated the colistin resistance mechanism in an Escherichia coli strain (LC711/14) isolated in Italy in 2014, from an urinary tract infection, which was previously shown to express a colistin resistance mechanism different from mcr-1. LC711/14 was found to carry a novel mutation in the pmrB gene, resulting in a leucine to proline amino acid substitution at position 10 of the PmrB sensor kinase component of the PmrAB signal transduction system. The role of this substitution in colistin resistance was documented by expression of the wild-type and mutated alleles in a pmrB deletion derivative of the E. coli reference strain MG1655, in which expression of the mutated allele conferred colistin resistance and upregulation of the endogenous pmrHFIJKLM lipid A modification system. Complementation of LC711/14 with the wild-type pmrB allele restored colistin susceptibility and decreased expression of pmrHFIJKLM, confirming the role of this PmrB mutation. Substitution of leucine at position 10 of PmrB with other amino acids (glycine and glutamine) resulted in loss of function, underscoring a key role of this residue which is located in the cytoplasmic secretion domain of the protein. This work demonstrated that mutation in this domain of the PmrB sensor kinase can be responsible for acquired colistin resistance in E. coli strains of clinical origin.

e Carbapenem-resistant Acinetobacter baumannii (CRAb) is emerging worldwide as a public health problem in various settings. The aim of this study was to investigate the prevalence of CRAb isolates in Italy and to characterize their resistance mechanisms and genetic relatedness. A countrywide cross-sectional survey was carried out at 25 centers in mid-2011. CRAb isolates were reported from all participating centers, with overall proportions of 45.7% and 22.2% among consecutive nonreplicate clinical isolates of A. baumannii from inpatients (n ‫؍‬ 508) and outpatients (n ‫؍‬ 63), respectively. Most of them were resistant to multiple antibiotics, whereas all remained susceptible to colistin, with MIC 50 and MIC 90 values of <0.5 mg/liter. The genes coding for carbapenemase production were identified by PCR and sequencing. OXA-23 enzymes (found in all centers) were by far the most common carbapenemases (81.7%), followed by OXA-58 oxacillinases (4.5%), which were found in 7 of the 25 centers. In 6 cases, CRAb isolates carried both bla OXA-23-like and bla OXA-58-like genes. A repetitive extragenic palindromic (REP)-PCR technique, multiplex PCRs for group identification, and multilocus sequence typing (MLST) were used to determine the genetic relationships among representative isolates (n ‫؍‬ 55). Two different clonal lineages were identified, including a dominant clone of sequence type 2 (ST2) related to the international clone II (sequence group 1 [SG1], SG4, and SG5) and a clone of ST78 (SG6) previously described in Italy. Overall, our results demonstrate that OXA-23 enzymes have become the most prevalent carbapenemases and are now endemic in Italy. In addition, molecular typing profiles showed the presence of international and national clonal lineages in Italy.

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