In vitro activity of antibiotics potentially effective against difficult-to-treat strains of Gram-negative rods: retrospective study

Bacterial resistance surveillance is one of the main outputs of microbiological laboratories and its results are important part of antimicrobial stewardship (AMS). In this study, the susceptibility of specific bacteria to selected antimicrobial agents was tested. The susceptibility of 90 unique isolates of pathogens of critical priority obtained from clinically valid samples of ICU patients in 2017-2021 was tested. 50% of these fulfilled difficult-to-treat resistance (DTR) criteria and 50% were susceptible to all antibiotics included in the definition. 10 Enterobacterales strains met DTR criteria, and 2 (20%) were resistant to colistin (COL), 2 (20%) to cefiderocol (FCR), 7 (70%) to imipenem/cilastatin/relebactam (I/R), 3 (30%) to ceftazidime/avibactam (CAT) and 5 (50%) to fosfomycin (FOS). For Enterobacterales we also tested aztreonam/avibactam (AZA) for which there are no breakpoints yet. The highest MIC of AZA observed was 1 mg/l, MIC range in the susceptible cohort was 0.032-0.064 mg/l and in the DTR cohort (incl. class B beta-lactamase producers) it was 0.064-1 mg/l. Two (13.3%) isolates of Pseudomonas aeruginosa (15 DTR strains) were resistant to COL, 1 (6.7%) to FCR, 13 (86.7%) to I/R, 5 (33.3%) to CAT, and 5 (33.3%) to ceftolozane/tazobactam. All isolates of Acinetobacter baumannii with DTR were susceptible to COL and FCR, and at the same time resistant to I/R and ampicillin/sulbactam. New antimicrobial agents are not 100% effective against DTR. Therefore, it is necessary to perform susceptibility testing of these antibiotics, use the data for surveillance (including local surveillance) and conform to AMS standards.

Activity of aztreonam/avibactam and ceftazidime/avibactam against Enterobacterales with carbapenemase-independent carbapenem resistance

Enterobacterales with carbapenemase-independent resistance to carbapenems are sometimes selected during therapy and, on rare occasions, cause outbreaks. Most have extended-spectrum or AmpC β-lactamases, together with changes to permeability or penicillin-binding proteins (PBPs). Newer β-lactam-β-lactamase inhibitor combinations may present useful options for infections due to these organisms. Accordingly, Clinical and Laboratory Standards Institute/European Committee on Antimicrobial Susceptibility Testing broth-microdilution was used to measure the minimum inhibitory concentrations (MICs) of ceftazidime/avibactam and aztreonam/avibactam for 51 carbapenemase-negative Enterobacterales with resistance or reduced susceptibility to carbapenems: genomic sequencing of the least-susceptible organisms was also undertaken. MICs of the two avibactam combinations cross-correlated closely, but with fewer MICs (2/51 vs. 10/51) exceeding 8+4 mg/L in the case of ceftazidime/avibactam. Raised MICs for Escherichia coli were associated with PBP3 inserts together with CMY-42 β-lactamase; correlates among Enterobacter cloacae complex isolates remain elusive, with AmpC and PBP3 sequences found to be species specific. In the case of Klebsiella spp., no MICs exceeding 2 mg/L were seen for either combination. It appears that these avibactam combinations have potential against Enterobacterales with carbapenemase-independent carbapenem resistance or reduced susceptibility, with ceftazidime/avibactam being more reliably active than aztreonam/avibactam.

Activity of aztreonam/avibactam against metallo-β-Lactamase-producing Enterobacterales from the UK: impact of penicillin-binding protein-3 inserts and CMY-42 β-lactamase in Escherichia coli

Aztreonam/avibactam is being developed on the rationale that aztreonam evades metallo-β-lactamases (MBLs) whilst avibactam protects against co-produced serine β-lactamases. We measured the activity of aztreonam/avibactam against MBL-producing Enterobacterales referred to the UKHSA in 2015, 2017, 2019. MICs were determined by broth microdilution, genome sequences with Illumina technology. For Klebsiella and Enterobacter spp. with NDM, IMP or VIM enzymes, the MICs of aztreonam/avibactam were unimodally distributed, with >90% of isolates inhibited at 1+4 mg/L, and all inhibited at 8+4 mg/L. Over 85% of Escherichia coli with NDM carbapenemases were inhibited at 8+4 mg/L, but their MIC distribution was multimodal, with major peaks at 0.12 and 8 mg/L. Forty-eight of 50 NDM E. coli with 'high' aztreonam/avibactam MICs (defined as ≥8 mg/L) had YRIK inserted after amino-acid 333 of penicillin-binding protein (PBP)3 or had a YRIN insert plus an acquired AmpC β-lactamase, commonly CMY-42. Ten of 15 E. coli with 'moderately-raised' aztreonam/avibactam MICs (0.5-4 mg/L) had YRIN inserts without acquired AmpC. Twenty-two of 24 E. coli isolates with 'normal' MICs (0.03-0.25 mg/L) lacked PBP3 inserts. YRIK inserts were associated with E. coli ST405 and YRIN with ST167; however, many isolates with high or moderately-raised MICs were clonally diverse. No substantive MIC distribution shifts occurred across the 3 survey years; ST405 isolates with YRIK comprised more high-MIC organisms in 2019 versus earlier years, but the apparent excess lacked significance (P >0.05).

In vitro activity of aztreonam/avibactam against a global collection of Klebsiella pneumoniae collected from defined culture sources in 2016 and 2017

OBJECTIVES

This study reports on the activity of aztreonam/avibactam (ATM-AVI) against a collection of Klebsiella pneumoniae collected in 2016 and 2017.

METHODS

Non-duplicate K. pneumoniae isolates were collected from four regions (Africa/Middle East, n = 785; Asia-Pacific, n = 1433; Europe, n = 4236; Latin America, n = 1499) and five culture sources (blood, n = 902; intra-abdominal, n = 992; urinary tract, n = 2148; skin and skin structure, n = 1409; lower respiratory tract, n = 2502). MICs were determined at a central laboratory using Clinical and Laboratory Standards Institute (CLSI) broth microdilution methodology. Susceptibility was determined using European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints.

RESULTS

For all culture sources, against all K. pneumoniae, the highest rates of susceptibility were seen for amikacin (>84%), ceftazidime/avibactam (>94%), colistin (>92%) and meropenem (>83%), and >99.9% of isolates were inhibited at an ATM-AVI MIC of ≤4 mg/L. Among meropenem-resistant (MEM-R, n = 583) and meropenem-resistant metallo-β-lactamase-negative (MEM-R-MBLN; n = 469) isolates, susceptibility was highest to ceftazidime/avibactam (79.9% and 99.4%, respectively) and colistin (67.2% and 62.7%, respectively). All MEM-R-MBLN isolates from blood, intra-abdominal, urinary tract and skin and skin structure sources, and all but one isolate from respiratory sources, were inhibited at an ATM-AVI MIC of ≤2 mg/L. Against the meropenem-resistant metallo-β-lactamase positive (MEM-R-MBLP; n = 114) isolates, susceptibility to colistin was between 75.0% (urinary tract isolates) and 93.3% (lower respiratory tract isolates). All MEM-R-MBLP isolates were inhibited at an ATM-AVI MIC of ≤0.5 mg/L.

CONCLUSIONS

ATM-AVI is active against K. pneumoniae isolates from a range of culture sources across Africa/Middle East, Asia-Pacific, Europe and Latin America. ATM-AVI also has activity against MEM-R-MBLN and MEM-R-MBLP isolates.

Genetic diversity and in vitro activity of ceftazidime/avibactam and aztreonam/avibactam against imipenem-resistant Enterobacteriaceae isolates in Southwest China: A single-centre study

OBJECTIVES

The aim of this study was to investigate the molecular mechanisms of imipenem resistance in Enterobacteriaceae and to assess the antimicrobial activities of ceftazidime/avibactam (CAZ/AVI) and aztreonam/avibactam (ATM/AVI) against imipenem-resistant clinical isolates in a tertiary hospital in China.

METHODS

A total of 91 imipenem-resistant Enterobacteriaceae were collected and genes encoding carbapenemases, ESBLs, AmpC β-lactamases and porins were detected using PCR. MICs and susceptibility were determined using in-house-prepared broth microdilution panels and were interpreted according to CLSI breakpoints.

RESULTS

Imipenem-resistant isolates comprising 54 Klebsiella pneumoniae, 18 Escherichia coli, 8 Enterobacter cloacae, 6 Serratia marcescens, 3 Klebsiella oxytoca and 2 Klebsiella aerogenes were collected independently. Five different carbapenemase genes were identified, namely bla_KPC-2 (n = 60), bla_NDM-5 (n = 14), bla_NDM-1 (n = 11), bla_KPC-3 (n = 4) and bla_IMP-4 (n = 1). Among the 91 carbapenem-resistant Enterobacteriaceae (CRE), 85 isolates harboured at least one ESBL and/or AmpC gene, including 5 strains without carbapenemase-encoding genes. Moreover, 31 K. pneumoniae carried ompK35 and/or ompK36 mutations. MLST results showed that the K. pneumoniae belonged to 12 different STs, with ST11 being predominant (29/54; 53.7%). Overall, 17.6%, 25.3%, 41.8%, 65.9% and 100% of the CRE strains were susceptible to amikacin, trimethoprim/sulfamethoxazole, tetracycline, CAZ/AVI and ATM/AVI, respectively.

CONCLUSION

This study revealed that CRE isolates differ significantly in their species, STs, porins and carbapenemase genes in a single Chinese hospital. ATM/AVI exhibited potent activity against CRE isolates, even for the most notorious double-carbapenemase-producers with porin defects, whereas CAZ/AVI was active against all the non-metallo-β-lactamase-producing isolates.