Nosocomial ceftazidime-resistant Klebsiella pneumoniae may be resistant to the bactericidal activity of all cephalosporins and cephamycins. Such isolates appear susceptible to cephalosporins other than ceftazidime by routine disc diffusion testing. Ineffective therapy, delayed detection of resistance, and epidemic spread are potential consequences. Imipenem provides consistent bactericidal activity. Ceftazidime restriction and barrier precautions for colonized and infected patients are effective control measures.

Nineteen isolates of carbapenem-resistant Klebsiella species were recovered from 7 hospitals in New York City. Most K. pneumoniae belonged to a single ribotype. Nucleotide sequencing identified KPC-2, a carbapenem-hydrolyzing beta -lactamase. In 3 strains, TEM-30, an inhibitor-resistant beta -lactamase, was detected. Carbapenem-resistant Klebsiella species possessing KPC-2 are endemic in New York City. This study documents the identification of an inhibitor-resistant TEM beta -lactamase in the United States.

Most current fluoroquinolone-resistant E. coli clinical isolates, and the largest share of multidrug-resistant isolates, represent a highly clonal subgroup that likely originated from a single rapidly expanded and disseminated ST131 strain. Focused attention to this strain will be required to control the fluoroquinolone and multidrug-resistant E. coli epidemic.

No abstract

Six Escherichia coli and 12 Klebsiella pneumoniae isolates from a single hospital expressed a common beta-lactamase with a pI of approximately 9.0 and were resistant to cefoxitin and cefotetan (MIC ranges, 64 to > 128 and 16 to > 128 micrograms/ml, respectively). Seventeen of the 18 strains produced multiple beta-lactamases. Most significantly, three K. pneumoniae strains were also resistant to imipenem (MICs, 8 to 32 micrograms/ml). Spectrophotometric beta-lactamase assays with purified enzyme indicated hydrolysis of cephamycins, in addition to cephaloridine and benzylpenicillin. The 4ene encoding the pI 9.0 beta-lactamase (designated ACT-1 for AmpC type) was cloned and sequenced, which revealed an ampC-type beta-lactamase gene that originated from Enterobacter cloacae and that had 86% sequence homology to the P99 beta-lactamase and 94% homology to the partial sequence of MIR-1. Southern blotting revealed that the gene encoding ACT-1 was on a large plasmid in some of the K. pneumoniae strains as well as on the chromosomes of all of the strains, suggesting that the gene is located on an easily mobilized element. Outer membrane protein profiles of the K. pneumoniae strains revealed that the three imipenem-resistant strains were lacking a major outer membrane protein of approximately 42 kDa which was present in the imipenem-susceptible strains. ACT-1 is the first plasmid-mediated AmpC-type beta-lactamase derived from Enterobacter which has been completely sequenced. This work demonstrates that in addition to resistance to cephamycins, imipenem resistance can occur in K. pneumoniae when a high level of the ACT-1 beta-lactamase is produced in combination with the loss of a major outer membrane protein.

We sought to control infection due to multidrug-resistant Acinetobacter baumannii (MDR-Ab) by identifying isolates as clonally related, leading to enhanced infection-control measures, including cohorting, surveillance, contact precaution, initial therapy with ampicillin/sulbactam and local polymyxin B, and, more recently, therapy with synergistic antibiotic combinations. Class restriction of cephalosporins has been associated with a reduction in cephalosporins-cephamycin-carbapenem resistance among nosocomial Klebsiella isolates. This has been supplemented by restriction of carbapenem use after an initial 24-h period in an effort to reduce the selection of porin-deficient, carbapenem-resistant A. baumannii and Pseudomonas aeruginosa. Evidence is reviewed suggesting that eradication of MDR-Ab nosocomial colonization may prevent subsequent infection. Relatively few standard antibacterial drugs remain active against MDR-Ab. Published clinical results of therapy with these agents are reviewed, and in vitro evidence of synergy between them is presented that suggests that combination therapy should be studied for enhanced clinical activity.

The mec gene of a number of clinical methicillin-resistant Staphylococcus aureus isolates exhibiting a variety of heterogeneous expression modes was selectively inactivated by allelic replacement mutagenesis. While the resistance level of each of the transformants was reduced, the methicillin MIC for these transformants was well above the MIC for susceptible laboratory strains of S. aureus and was similar to the methicillin MIC for many contemporary clinical isolates which did not react with the mec-specific DNA probe but which showed a low or borderline level of resistance to methicillin. A number of those strains had no detectable I8-lactamase, and for about half of the isolates that did carry plasmid-borne j8-lactamase, elimination of the plasmid caused only partial reduction of the methicillin MIC or no reduction at all. The findings suggest that many contemporary strains of staphylococci harbor a combination of at least three distinct I8-lactam resistance mechanisms: (i) the mechanism related to the acquisition of the foreign mec gene and (ii) a ,B-lactamase-dependent and (iii) a I3-lactamase-independent mechanism, each one of which can provide a certain degree of resistance against penicillinase-resistant Il-lactam antibiotics.Methicillin-resistant clinical isolates of Staphylococcus aureus (MRSA) carry a complex, as yet only poorly understood resistance mechanism. All MRSA isolates examined so far have contained the mec gene, a 2,130-bp segment of foreign DNA coding for a low-affinity penicillin-binding protein (PBP 2A) (3,19,20). Despite the ubiquitous presence of this gene, MRSA isolates show tremendous variation in the MICs for the majority of the cells, and cultures of MRSA isolates are also heterogeneous: they contain a variable number of subpopulations for which the MICs range from very low to very high. Recently, it was shown that these complex modes of phenotypic expression are strain specific and appear to be under genetic control (37). In extreme cases, the methicillin MIC for the majority of bacteria (>99.99% of cells) may be as low as 3 p.g/ml (a MIC for susceptibility is 0.5 to 1.0 p.g/ml), despite the presence of the mec gene and its gene product (PBP 2A) in every cell. Similar, moderately increased MICs (4 to 8 p.g/ml) were also detected recently in some isolates which did not carry the mec gene, whether or not they produced f-lactamase (6, 36), the overproduction of which has been proposed as one mechanism that causes a moderate (borderline) elevation of the MIC of typically penicillinase-resistant antibiotics (6, 22, 23).One purpose of this study was to determine the relative contribution(s) of these mechanisms to the MIC for some selected MRSA isolates. Of particular interest was testing of the possibility that these distinct mechanisms may coexist in single isolates. An

Imipenem with relebactam was active against Escherichia coli, Klebsiella pneumoniae, and Enterobacter spp., including K. pneumoniae carbapenemase (KPC)-producing isolates. Loss of OmpK36 in KPC-producing K. pneumoniae isolates affected the susceptibility of this combination. Enhanced activity was evident against Pseudomonas aeruginosa, including isolates with depressed oprD and increased ampC expression. However, the addition of relebactam to imipenem did not provide added benefit against Acinetobacter baumannii. The combination of imipenem with relebactam demonstrated activity against KPC-producing Enterobacteriaceae and multidrug-resistant P. aeruginosa. The spread of carbapenemases in Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter baumannii has created therapeutic dilemmas for clinicians. In particular, the acquisition of metallo-␤-lactamases and the class A enzyme Klebsiella pneumoniae carbapenemase (KPC) affords protection against virtually all ␤-lactam therapeutic agents (1). The worldwide dissemination of bacteria possessing bla KPC has been especially striking (2). First reported in the northeastern United States in the 1990s, pathogens harboring this ␤-lactamase are now endemic in countries in Asia, South America, and Europe (2).Novel ␤-lactamase inhibitors are being developed to restore the utility of ␤-lactam antibiotics against carbapenemase-producing pathogens (3, 4). Avibactam and relebactam are diazabicyclooctane inhibitors with activity against a wide spectrum of ␤-lactamases, including class A (extended-spectrum ␤-lactamases [ESBLs] and KPC) and class C (AmpC) enzymes (3,4). In this study, we determined the activity of imipenem with relebactam against Gram-negative pathogens from medical centers in New York City.Between November 2013 and January 2014, single patient isolates of Escherichia coli, K. pneumoniae, Enterobacter spp., P. aeruginosa, and A. baumannii were collected from 11 hospitals in Brooklyn and Queens, New York. Susceptibility tests were performed in a central research laboratory using the agar dilution method, and results were interpreted according to CLSI guidelines (5). Isolates of E. coli and K. pneumoniae were presumed to harbor ESBLs if they were not susceptible to ceftazidime and/or ceftriaxone and did not have bla KPC . Imipenem was tested both with and without the presence of relebactam (fixed concentration of 4 g/ ml). For the purposes of this study, imipenem breakpoints were used to interpret susceptibility to imipenem plus relebactam.Cephalosporin-resistant isolates were tested by PCR for the presence of bla KPC , bla OXA-23-type , bla OXA-24-type , bla VIM , bla IMP , and bla NDM using previously described primers (6-9). Isolates of K. pneumoniae with bla KPC and isolates of A. baumannii with bla OXA23-type underwent genetic fingerprinting by the repetitive element palindromic PCR (rep-PCR) method with the ERIC-2 primer, as described previously (6).Susceptibility testing of imipenem with and without relebactam was also performed with a collection of prev...