NmcA Carbapenem-hydrolyzing Enzyme in Enterobacter cloacae in North America1

An imipenem-resistant Enterobacter cloacae isolate was recovered from the blood of a patient with a hematologic malignancy. Analytical isoelectric focusing, inhibitor studies, hydrolysis, induction assays, and molecular sequencing methods confirmed the presence of a NmcA carbapenem-hydrolyzing enzyme. This first report of NmcA detected in North America warrants further investigation into its distribution and clinical impact

AmpC Disk Test for Detection of Plasmid-Mediated AmpC β-Lactamases in Enterobacteriaceae Lacking Chromosomal AmpC β-Lactamases

Although plasmid-mediated AmpC β-lactamases were first reported in the late 1980s, many infectious disease personnel remain unaware of their clinical importance. These enzymes are typically produced by isolates of Escherichia coli, Klebsiella spp., Proteus mirabilis, and Salmonella spp. and are associated with multiple antibiotic resistance that leaves few therapeutic options. Plasmid-mediated AmpC β-lactamases have been associated with false in vitro susceptibility to cephalosporins. Many laboratories do not test for this resistance mechanism because current tests are inconvenient, subjective, lack sensitivity and/or specificity, or require reagents that are not readily available. In this study a new test, the AmpC disk test, based on filter paper disks impregnated with EDTA, was found to be a highly sensitive, specific, and convenient means of detection of plasmid-mediated AmpC β-lactamases in organisms lacking a chromosomally mediated AmpC β-lactamase. Using cefoxitin insusceptibility as a screen, the test accurately distinguished AmpC and extended-spectrum β-lactamase production and differentiated AmpCs from non-β-lactamase mechanisms of cefoxitin insusceptibility, such as reduced outer membrane permeability. The test is a potentially useful diagnostic tool. It can provide important infection control information and help to ensure that infected patients receive appropriate antibiotic therapy

Kinetics Study of KPC-3, a Plasmid-Encoded Class A Carbapenem-Hydrolyzing β-Lactamase

The kinetic activity of KPC-3, a plasmid-encoded class A carbapenemase, was studied. It hydrolyzed penicillins, cephalosporins, carbapenems, and even sulbactam. The best substrate was cephalothin (k(cat/K)m = 3.48 μM(−1) s(−1)). The efficiency of the enzyme was similar for imipenem and meropenem (k(cat)/K(m), 1.4 and 1.94 μM(−1) s(−1), respectively)

In Vitro Activities of DX-619 and Comparison Quinolones against Gram-Positive Cocci

The in vitro activity of the novel quinolone DX-619 was compared to those of currently available quinolones against U.S. clinical isolates of Staphylococcus aureus, coagulase-negative staphylococci, Enterococcus spp., Streptococcus pyogenes, and Streptococcus pneumoniae. DX-619 was the most potent quinolone overall, indicating possible utility as an anti-gram-positive quinolone

In Vitro Activity of DX-619, a Novel Des-Fluoro(6) Quinolone, against a Panel of Streptococcus pneumoniae Mutants with Characterized Resistance Mechanisms

The in vitro activities of DX-619 and four other quinolones were compared against Streptococcus pneumoniae mutants that contained a variety of alterations within the quinolone resistance-determining regions. DX-619 was the most potent quinolone and was least affected by the mutations

βlasEN: Microdilution Panel for Identifying β-Lactamases Present in Isolates of Enterobacteriaceae

A dried investigational use-only microdilution panel named βlasEN (a short named derived from the panel’s purpose, to identify β-lactamases in Enterobacteriaceae) containing 10 β-lactam drugs with and without β-lactamase inhibitors was developed to identify β-lactamases among clinical isolates of Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, Citrobacter koseri, Citrobacter freundii group, Enterobacter spp., and Serratia marcescens. The MICs obtained with a collection of 383 organisms containing well-characterized β-lactamases were used to develop numeric codes and logic pathways for computerized analysis of results. The resultant logic pathways and βlasEN panel were then used to test and identify β-lactamases among 885 isolates of Enterobacteriaceae recovered in cultures obtained at six different hospital laboratories across the United States. β-Lactamases present in 801 (90.5%) of the 885 isolates were identified by βlasEN by using the existing logic pathways and codes or after minor modifications were made to the existing codes. The 84 strains that gave codes that βlasEN could not identify were collected, reidentified, and retested by using βlasEN. Three strains had been misidentified, 54 strains gave different codes upon repeat testing that could be identified by βlasEN, and 27 strains repeated new codes. The β-lactamases in these strains were identified, and the new codes were added to the βlasEN logic pathways. These results indicate that βlasEN can identify clinically important β-lactamases among most isolates of Enterobacteriaceae. The results also show that good quality control and attention to proper performance of the tests are essential to the correct performance of βlasEN

Ability of the VITEK 2 Advanced Expert System To Identify β-Lactam Phenotypes in Isolates of Enterobacteriaceae and Pseudomonas aeruginosa

The Advanced Expert System (AES) was used in conjunction with the VITEK 2 automated antimicrobial susceptibility test system to ascertain the β-lactam phenotypes of 196 isolates of the family Enterobacteriaceae and the species Pseudomonas aeruginosa. These isolates represented a panel of strains that had been collected from laboratories worldwide and whose β-lactam phenotypes had been characterized by biochemical and molecular techniques. The antimicrobial susceptibility of each isolate was determined with the VITEK 2 instrument, and the results were analyzed with the AES to ascertain the β-lactam phenotype. The results were then compared to the β-lactam resistance mechanism determined by biochemical and molecular techniques. Overall, the AES was able to ascertain a β-lactam phenotype for 183 of the 196 (93.4%) isolates tested. For 111 of these 183 (60.7%) isolates, the correct β-lactam phenotype was identified definitively in a single choice by the AES, while for an additional 46 isolates (25.1%), the AES identified the correct β-lactam phenotype provisionally within two or more choices. For the remaining 26 isolates (14.2%), the β-lactam phenotype identified by the AES was incorrect. However, for a number of these isolates, the error was due to remediable problems. These results suggest that the AES is capable of accurate identification of the β-lactam phenotypes of gram-negative isolates and that certain modifications can improve its performance even further