The Enterobacter cloacae complex (ECC) is a group of diverse environmental and clinically relevant bacterial species associated with a variety of infections in humans. ECC have emerged as one of the leading causes of nosocomial infections worldwide. The purpose of this paper is to evaluate the activity of NOSO-502 and colistin (CST) against a panel of ECC clinical isolates, including different Hoffmann's clusters strains, and to investigate the associated resistance mechanisms. NOSO-502 is the first preclinical candidate of a novel antibiotic class, the odilorhabdins (ODLs). MIC50 and MIC90 of NOSO-502 against ECC are 1 μg/mL and 2 μg/mL, respectively, with a MIC range from 0.5 μg/mL to 32 μg/mL. Only strains belonging to clusters XI and XII showed decreased susceptibility to both NOSO-502 and CST while isolates from clusters I, II, IV, and IX were only resistant to CST. To understand this phenomenon, E. cloacae ATCC 13047 from cluster XI was chosen for further study. Results revealed that the two-component system ECL_01761-ECL_01762 (ortholog of CrrAB from Klebsiella pneumoniae) induces NOSO-502 hetero-resistance by expression regulation of the ECL_01758 efflux pump component (ortholog of KexD from K. pneumoniae) which could compete with AcrB to work with the multidrug efflux pump proteins AcrA and TolC. In E. cloacae ATCC 13047, CST-hetero-resistance is conferred via modification of the lipid A by addition of 4-amino-4-deoxy-l-arabinose controlled by PhoPQ. We identified that the response regulator ECL_01761 is also involved in this resistance pathway by regulating the expression of the ECL_01760 membrane transporter.

The rise of antibiotic-resistant Mycobacterium tuberculosis and non-tuberculous mycobacterial infections has placed ever-increasing importance on discovering new antibiotics to treat these diseases. Recently, a new penem, T405, was discovered to have strong antimicrobial activity against M. tuberculosis and Mycobacteroides abscessus. Here, a penem library of C2 side-chain variants was synthesized, and their antimicrobial activities were evaluated against M. tuberculosis H37Rv and M. abscessus ATCC 19977. Several new penems with antimicrobial activity stronger than the standard-of-care carbapenem antibiotics were identified with some candidates improving on the activity of the lead compound, T405. Moreover, many candidates showed little or no increase in the minimum inhibitory concentration in the presence of serum compared to the highly protein-bound T405. The penems with the strongest activity identified in this study were then biochemically characterized by reaction with the representative l,d-transpeptidase LdtMt2 and the representative penicillin-binding protein d,d-carboxypeptidase DacB2.

The increasing incidence of carbapenem resistance in Acinetobacter baumannii is a critical concern worldwide owing to the limitations of therapeutic alternatives. The most important carbapenem resistance mechanism for A. baumannii is the enzymatic hydrolysis mediated by carbapenemases, mostly OXA-type carbapenemases (class D) and, to a lesser extent, metallo-β-lactamases (class B). Therefore, early and accurate detection of carbapenemase-producing A. baumannii is required to achieve the therapeutic efficacy of such infections. Many methods for carbapenemase detection have been proposed as effective tests for A. baumannii; however, none of them are officially recommended. In this study, three carbapenemase detection methods, namely, CarbaAcineto NP test, modified carbapenem inactivation method (mCIM), and simplified carbapenem inactivation method (sCIM) were evaluated for phenotypic detection of clinically isolated A. baumannii. The MICs of imipenem, meropenem, and doripenem were determined for 123 clinically isolated A. baumannii strains before performing three phenotypic detections. The overall sensitivity and specificity values were 89.09%/100% for the carbAcineto NP test, 71.82%/100% for sCIM, and 32.73%/33.13% for mCIM. CarbAcineto NP test and sCIM performed excellently (100% sensitivity) when both Class B and Class D carbapenemases were present in the same isolate. Based on the results, the combined detection method of sCIM and CarbAcineto NP test was proposed to detect carbapenemase-producing A. baumannii rather than a single assay, significantly increasing the sensitivity of detection to 98.18%. The proposed algorithm was more reliable and cost-effective than the CarbAcineto NP test alone. It can be easily applied in routine microbiology laboratories for developing countries with limited resources.

CTX-M-type extended-spectrum ß-lactamases (ESBL) are widespread among Enterobacterales worldwide. The most common variant is CTX-M-15 hydrolyzing ceftazidime at high rate, but sparing carbapenems. We identified here CTX-M-33, a point mutant derivative of CTX-M-15 (Asp to Ser substitution at Ambler position 109), exhibiting a low carbapenemase activity. ß-Lactamase CTX-M-33 was identified in a Klebsiella pneumoniae isolate belonging to ST405, lacking the outer membrane protein OmpK36, that was resistant to broad-spectrum cephalosporins and ß-lactam/ß-lactamase inhibitor combinations, and displayed a decreased susceptibility to carbapenems. Comparative hydrolytic activity assays showed that CTX-M-33 hydrolyzed ceftazidime at a lower level than CTX-M-15, but significantly hydrolyzed meropenem. In addition, CTX-M-33 showed higher Mutant Prevention Concentration values and wider mutant selection window in presence of meropenem, in accordance with its observed hydrolytic properties. We identified here the very first CTX-M enzyme possessing a weak carbapenemase activity, that may correspond to an emerging phenomenon when considering its possibility to evolve from the widespread ESBL CTX-M-15.
Copyright © 2019 American Society for Microbiology.