OprD loss and hyperexpression of AmpC, MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM were evaluated among 120 Pseudomonas aeruginosa isolates collected during 2012 in U.S. hospitals and selected based on ceftazidime MIC values (1 to >32 g/ml). AmpC derepression (10-fold greater than that with the control) and OprD loss (decreased/no band) were the most prevalent resistance mechanisms: 47.5 and 45.8% of the isolates were considered positive, respectively. Elevated expression of the efflux pumps MexAB-OprM, MexCD-OprJ, MexEF-OprN, and MexXY-OprM was observed in 32.5, 8.3, 0.0, and 28.4% of the isolates, respectively. A total of 21 different combinations of resistance mechanisms were noted, and the most prevalent included AmpC derepression with OprD loss with and without efflux hyperexpression (38 and 10 isolates, respectively). A total of 26 isolates had no changes in the resistance mechanisms tested and had lower MIC values for all ␤-lactams or ␤-lactam/ ␤-lactamase inhibitor combinations analyzed. OprD loss had a strong correlation with elevated MIC results for imipenem and meropenem (median MIC values of 8 and 4 g/ml, respectively), with all combinations displaying OprD loss also displaying elevated median MIC values for these carbapenems (4 to >8 g/ml). AmpC expression levels were greater in isolates displaying elevated cefepime, ceftazidime, or piperacillin-tazobactam MIC values (>4, >4, and >16 g/ml, respectively). Isolates displaying derepressed AmpC had ceftolozane-tazobactam MIC values ranging from 1 to 16 g/ml. No strong correlation was noticed with MIC values for this ␤-lactam/␤-lactamase inhibitor combination and OprD loss or hyperexpression of efflux systems. Two KPC-producing isolates were detected among 16 isolates displaying ceftolozane-tazobactam MIC values of >8 g/ml.

Ceftazidime-avibactam (MIC 50/90 , 0.12/0.25 g/ml) inhibited 99.9% (20,698/20,709) of Enterobacteriaceae isolates at <8 g/ml. This compound was active against resistant subsets, including ceftazidime-nonsusceptible Enterobacter cloacae (MIC 50/90 , 0.25/ 0.5 g/ml) and extended-spectrum ␤-lactamase (ESBL) phenotype isolates. An ESBL phenotype was noted among 12.4% (1,696/ 13,692 isolates from targeted species) of the isolates, including 776 Escherichia coli (12.0% for this species; MIC 50/90 , 0.12/0.25 g/ml), 721 Klebsiella pneumoniae (16.3%; MIC 50/90 , 0.12/0.25 g/ml), 119 Klebsiella oxytoca (10.3%; MIC 50/90 , 0.06/0.25 g/ ml), and 80 Proteus mirabilis (4.9%; MIC 50/90 , 0.06/0.12 g/ml) isolates. The most common enzymes detected among ESBL phenotype isolates from 2013 (n ‫؍‬ 743) screened using a microarray-based assay were CTX-M-15-like (n ‫؍‬ 307), KPC (n ‫؍‬ 120), SHV ESBLs (n ‫؍‬ 118), and CTX-M-14-like (n ‫؍‬ 110). KPC producers were highly resistant to comparators, and ceftazidimeavibactam (MIC 50/90 , 0.5/2 g/ml) and tigecycline (MIC 50/90 , 0.5/1 g/ml; 98.3% susceptible) were the most active agents against these strains. Meropenem (MIC 50/90 , <0.06/<0.06 g/ml) and ceftazidime-avibactam (MIC 50/90 , 0.12/0.25 g/ml) were active against CTX-M-producing isolates. Other enzymes were also observed, and ceftazidime-avibactam displayed good activity against the isolates producing less common enzymes. Among 11 isolates displaying ceftazidime-avibactam MIC values of >8 g/ml, three were K. pneumoniae strains producing metallo-␤-lactamases (all ceftazidime-avibactam MICs, >32 g/ml), with two NDM-1 producers and one K. pneumoniae strain carrying the bla KPC-2 and bla VIM-4 genes. Therapeutic options for isolates producing ␤-lactamases may be limited, and ceftazidime-avibactam, which displayed good activity against strains, including those producing KPC enzymes, merits further study in infections where such organisms occur. E nterobacteriaceae species cause a variety of infection types and these organisms, including those producing extended-spectrum ␤-lactamases (ESBLs) and carbapenemases, have been implicated in severe health care-associated infections (HAIs) that are a leading cause of morbidity and mortality worldwide (1, 2). Among Gram-negative organisms associated with HAI in the United States, 15% of Klebsiella pneumoniae or Klebsiella oxytoca isolates and 2% of Escherichia coli isolates have been shown to be resistant to three or more antimicrobial classes and were categorized as multidrug resistant (MDR) (2). These isolates were the cause of central line-associated bloodstream infections, catheterassociated urinary tract infections, ventilator-associated pneumonia, and surgical site infections. Additionally, among hospitals reporting severe HAIs, 20% described the occurrence of carbapenem-resistant Klebsiella isolates that are usually MDR (2) and more recently, pandrug-resistant (PDR) isolates producing carbapenemases have been reported (3). Due to steadily increasing levels of antimicrobial resistance amo...

Among 69 of 139 (49.6%) carbapenem-nonsusceptible Enterobacteriaceae carrying bla KPC , 1 Klebsiella pneumoniae was also positive for bla VIM . The isolate belonged to sequence type 258 (ST258) and carried bla KPC-2 on a copy of Tn4401a and bla VIM-4 on a class 1 integron. Genes were located on distinct plasmids belonging to Inc types A/C and FII. Elevated expression of the efflux pump AcrAB-TolC (acrA, 15.3 times) and reduced expression of outer membrane protein genes ompK35 and ompK37 (0.16 and 0.081 times, respectively) associated with various amino acid alterations on OmpK37 were observed. The presence of two carbapenemases in ST258 K. pneumoniae is of great concern due to the ability of this organism to widely disseminate.T he first reported Klebsiella pneumoniae carbapenemase (KPC)-producing isolate was a Klebsiella pneumoniae collected in 1996 from a patient hospitalized in North Carolina (1). Over the past decade, KPC variants became the most clinically significant ␤-lactamases among class A derivatives, and KPC-producing Enterobacteriaceae have been reported worldwide. Within the United States, there are differences in the prevalence of KPC-producing isolates, which have been reported in all states except Alaska, Maine, and Idaho; however, these strains seem to be endemic in the New York City area (2).(This study was presented at the 114th General Meeting of the American Society for Microbiology, Boston, MA, 2014.) Overall, the dissemination of bla KPC genes has been associated with clonally related strains, and the majority of bla KPC -carrying K. pneumoniae belong to multilocus sequence type (MLST) 258 and double-locus variants, which may have contributed to the worldwide dissemination of KPC-producing strains (3). Moreover, there is mounting evidence that bla KPC genes are consistently associated with a specific genetic element (i.e., transposon Tn4401) (3). Tn4401 is a Tn3-like transposon that has been identified in distinct plasmids carried by KPC-producing Enterobacteriaceae and Pseudomonas aeruginosa isolates from various geographic areas (4).During 2013, a total of 139 Enterobacteriaceae isolates with elevated imipenem or meropenem MIC values (Ն2 g/ml) were collected from U.S. medical centers as part of the SENTRY Antimicrobial Surveillance Program. Susceptibilities were performed on the isolates using the reference broth microdilution method described by the Clinical and Laboratory Standards Institute (CLSI) (5). Categorical interpretations from CLSI M100-S25 were used for all antimicrobials (6), and quality control (QC) was performed using Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853. All QC results were within ranges specified by CLSI (6).Carbapenem-nonsusceptible isolates were screened for the presence of bla KPC , bla NDM , bla VIM , bla IMP , bla SME , bla IMI , bla NMC-A , and bla OXA-48 in a multiplex PCR. Amplicons generated were sequenced on both strands; nucleotide and deduced amino acid sequences were analyzed using the Lasergene software package (DNAStar, Madi...

The in vitro activities of four new beta-lactam antibiotics and dibekacin against aerobic bacteria were compared. The new cephalosporins were more broadly active against gram-negative bacteria than were presently available cephalosporins, but were less active against staphylococci.