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. 2016 Apr 22;60(5):3163–3169. doi: 10.1128/AAC.03042-15

In Vitro Susceptibility to Ceftazidime-Avibactam of Carbapenem-Nonsusceptible Enterobacteriaceae Isolates Collected during the INFORM Global Surveillance Study (2012 to 2014)

Boudewijn L M de Jonge a, James A Karlowsky b, Krystyna M Kazmierczak b,✉, Douglas J Biedenbach b, Daniel F Sahm b, Wright W Nichols a
PMCID: PMC4862516  PMID: 26926648

Abstract

The activity of ceftazidime-avibactam was assessed against 961 isolates of meropenem-nonsusceptible Enterobacteriaceae. Most meropenem-nonsusceptible metallo-β-lactamase (MBL)-negative isolates (97.7%) were susceptible to ceftazidime-avibactam. Isolates that carried KPC or OXA-48-like β-lactamases, both alone and in combination with extended-spectrum β-lactamases (ESBLs) and/or AmpC β-lactamases, were 98.7% and 98.5% susceptible to ceftazidime-avibactam, respectively. Meropenem-nonsusceptible, carbapenemase-negative isolates demonstrated 94.7% susceptibility to ceftazidime-avibactam. Ceftazidime-avibactam activity was compromised only in isolates for which carbapenem resistance was mediated through metallo-β-lactamases.

TEXT

Carbapenems are bactericidal β-lactam antibiotics that are recommended for therapy against infections caused by extended-spectrum β-lactamase (ESBL)- and/or AmpC-cephalosporinase-producing Enterobacteriaceae (1–4). Two clinically important mechanisms of resistance to carbapenems among Enterobacteriaceae have been identified. One is the production of carbapenemases, such as serine carbapenemases (KPC and OXA) and metallo-β-lactamases (VIM, IMP, and NDM) (5), and the other is the production of ESBLs or Ambler class C β-lactamases coupled with reduced expression or loss of function of one or more outer membrane pore-forming proteins (6–8).

Avibactam is a non-β-lactam–β-lactamase inhibitor that inhibits the activities of Ambler class A β-lactamases, including ESBLs and Klebsiella pneumoniae carbapenemase (KPC), class C β-lactamases, and some class D β-lactamases (9). Ceftazidime-avibactam displays antibacterial activity in vitro against KPC-producing clinical isolates of Enterobacteriaceae (10), including isolates carrying ompK36 mutations (11), and against AmpC-producing Enterobacter spp. and ESBL-producing K. pneumoniae strains with impaired permeability (12). Ceftazidime-avibactam also displayed low MICs (≤8 μg/ml) against noncarbapenemase-producing, carbapenem-nonsusceptible (NS) Enterobacteriaceae isolated from patients in France (13). Moreover, while recognizing that ceftazidime is not hydrolyzed significantly by OXA-48, but that blaOXA-48-containing isolates of Enterobacteriaceae commonly also carry genes encoding ESBLs that do hydrolyze ceftazidime, ceftazidime-avibactam was found to be active against carbapenem-resistant, blaOXA-48-positive Enterobacteriaceae (12, 14). In contrast, metallo-β-lactamase (MBL)-producing Enterobacteriaceae are generally not susceptible to carbapenems, nor are they susceptible to ceftazidime-avibactam (9, 12), because avibactam does not inhibit MBLs (15).

The aim of the present study was to characterize the in vitro activity of ceftazidime-avibactam against contemporary carbapenem-nonsusceptible, bla (β-lactamase gene)-characterized clinical isolates of Enterobacteriaceae collected from hospitalized patients over a 3-year time period in a global surveillance program (International Network for Optimal Resistance Monitoring [INFORM]). In the years 2012 to 2014, inclusive, the INFORM program received 34,062 isolates of Enterobacteriaceae collected by medical center laboratories in Europe (19 countries from 93 laboratories), Asia/Pacific (9 countries from 41 laboratories), Latin America (6 countries from 26 laboratories), and the Middle East/Africa (5 countries from 16 laboratories). Of the 34,062 isolates of Enterobacteriaceae, 961 (2.8%) isolates were meropenem nonsusceptible (used as a marker of nonsusceptibility to carbapenems), and these were the focus of this study.

All study isolates were shipped to a central reference laboratory at International Health Management Associates, Inc. (IHMA; Schaumburg, IL, USA), where their identities were confirmed using a Bruker Biotyper matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry instrument (Bruker Daltonics, Billerica, MA, USA). All antimicrobial susceptibility testing was performed by using in-house-prepared 96-well broth microdilution panels, according to Clinical and Laboratory Standards Institute (CLSI) standards (16, 17). Avibactam was tested at a fixed concentration of 4 μg/ml in combination with doubling dilutions of ceftazidime (16). MICs were interpreted using CLSI breakpoints (16), where available. Ceftazidime-avibactam MICs were interpreted using U.S. FDA MIC breakpoints for Enterobacteriaceae, with susceptibility at an MIC of ≤8 μg/ml and resistance at an MIC of ≥16 μg/ml (18). U.S. FDA MIC interpretative breakpoints were also used for tigecycline (19). For colistin, EUCAST MIC interpretative breakpoints against Enterobacteriaceae were used (20).

All 961 meropenem-nonsusceptible isolates were screened, using a combination of the microarray-based assay Check-MDR CT101 kit (Check-Points, Wageningen, the Netherlands) and published multiplex PCR assays, to detect and identify genes encoding carbapenemases (KPC, OXA-48, GES, IMP, VIM, NDM, and SPM), ESBLs (TEM, SHV, CTX-M, VEB, PER, and GES), original-spectrum β-lactamases (OSBLs) (TEM and SHV that did not contain substitutions at amino acid positions 104, 164, or 238 [TEM] or 146, 238, or 240 [SHV] associated with ESBL activity), and plasmid-mediated AmpC β-lactamases (ACC, ACT, CMY, DHA, FOX, MIR, and MOX), as previously described (21). Enzyme variants were identified by amplification of full-length β-lactamase genes, followed by DNA sequencing, and these were compared against the National Center for Biotechnology Information database (www.ncbi.nlm.nih.gov) and the Lahey Clinic website (www.lahey.org/studies). The distributions of KPC and MBLs in these isolates were described recently in great detail (reference 22 and K. M. Kazmierczak, D. J. Biedenbach, M. Hackel, S. Rabine, B. L. M. de Jonge, S. K. Bouchillon, D. F. Sahm, and P. A. Bradford, unpublished data).

Of the 34,062 isolates of Enterobacteriaceae that were tested, 99.5% (33,877 isolates) were susceptible to ceftazidime-avibactam (MIC, ≤8 μg/ml), and 2.8% were meropenem nonsusceptible (Table 1). The considerably lower percentage of susceptibility to imipenem (85.1%) compared to that to doripenem (97.3%) and meropenem (97.2%) was attributable to the presence of 4,572 isolates of Proteeae species (13.4% of all isolates tested) in this set, as these species demonstrated intrinsic elevated MICs for imipenem (data not shown) (16). Meropenem-nonsusceptible isolates were more susceptible to ceftazidime-avibactam (83.5% susceptible) than to all other β-lactams tested, which showed rates of susceptibility of ≤12% (Table 1).

TABLE 1.

In vitro activities of ceftazidime-avibactam and comparative antimicrobial agents tested against Enterobacteriaceae collected by the INFORM global surveillance program from 2012 to 2014

Organism group (n) Antimicrobial agent MIC values (μg/ml)
 MIC interpretation (%)a
MIC50 MIC90 MIC range Susceptible Intermediate Resistant
All Enterobacteriaceae (34,062)b Ceftazidime-avibactam 0.12 0.5 ≤0.015 to >128 99.5 0.5
Ceftazidime 0.25 64 ≤0.015 to >128 75.6 1.9 22.5
Cefepimec ≤0.12 >16 ≤0.12 to >16 77.3 4.9 17.8
Aztreonam 0.12 64 ≤0.015 to >128 74.3 1.9 23.8
Piperacillin-tazobactam 2 128 ≤0.25 to >128 84.0 5.9 10.1
Doripenem 0.06 0.25 ≤0.008 to >4 97.3 0.5 2.2
Ertapenem (n = 20,885)d 0.015 0.25 ≤0.002 to >1 94.5 1.6 3.9
Imipenem 0.25 2 ≤0.03 to >8 85.1 7.6 7.3
Meropenem 0.03 0.12 ≤0.004 to >8 97.2 0.4 2.4
Amikacin 2 8 ≤0.25 to >32 96.3 1.5 2.2
Colistine ≤0.12 >4 ≤0.015 to >4 83.1 16.9
Tigecycline 0.5 2 ≤0.015 to >8 92.9 5.7 1.4
Levofloxacin 0.06 >4 ≤0.03 to >4 75.0 2.5 22.5
Meropenem nonsusceptible (961) Ceftazidime-avibactam 1 >128 ≤0.015 to >128 83.5 16.5
Ceftazidime >128 >128 0.06 to >128 5.3 2.3 92.4
Cefepime >16 >16 ≤0.12 to >16 5.4 9.0 85.6
Aztreonam >128 >128 ≤0.015 to >128 8.9 0.8 90.3
Piperacillin-tazobactam >128 >128 0.5 to >128 3.2 3.7 93.1
Doripenem >4 >4 0.03 to >4 9.6 13.2 77.2
Ertapenem (n = 534) >1 >1 0.015 to >1 2.8 0.8 96.4
Imipenem >8 >8 0.06 to >8 12.2 5.2 82.6
Meropenem >8 >8 2 to >8 0.0 16.2 83.8
Amikacin 16 >32 ≤0.25 to >32 58.1 22.0 19.9
Colistin ≤0.12 >4 ≤0.015 to >4 81.4 18.6
Tigecycline 1 2 0.06 to >8 91.0 7.5 1.5
Levofloxacin >4 >4 ≤0.03 to >4 18.6 4.3 77.1
Meropenem nonsusceptible, MBL negative (816) Ceftazidime-avibactam 1 4 ≤0.015 to >128 97.7 2.3f
Ceftazidime 128 >128 0.06 to >128 5.9 2.7 91.4
Cefepime >16 >16 ≤0.12 to >16 5.6 9.4 85.0
Aztreonam >128 >128 ≤0.015 to >128 4.7 0.6 94.7
Piperacillin-tazobactam >128 >128 0.5 to >128 2.6 3.8 93.6
Doripenem >4 >4 0.03 to >4 11.2 15.0 73.8
Ertapenem (n = 452) >1 >1 0.015 to >1 2.4 0.4 97.2
Imipenem >8 >8 0.06 to >8 14.1 5.5 80.4
Meropenem >8 >8 2 to >8 0.0 17.4 82.6
Amikacin 16 >32 ≤0.25 to >32 57.9 25.2 16.9
Colistin ≤0.12 >4 ≤0.015 to >4 81.5 18.5
Tigecycline 1 2 0.06 to 8 91.4 7.4 1.2
Levofloxacin >4 >4 ≤0.03 to >4 16.5 3.7 79.8
Meropenem nonsusceptible, carbapenemase positive, MBL negative (609) Ceftazidime-avibactam 1 4 ≤0.015 to >128 98.7 1.3
Ceftazidime 128 >128 0.06 to >128 5.6 2.9 91.5
Cefepime >16 >16 ≤0.12 to >16 5.1 9.4 85.5
Aztreonam >128 >128 ≤0.015 to >128 3.6 0.3 96.1
Piperacillin-tazobactam >128 >128 2 to >128 0.5 2.0 97.5
Doripenem >4 >4 0.5 to >4 4.8 10.2 85.0
Ertapenem (n = 329) >1 >1 0.25 to >1 0.9 0.3 98.8
Imipenem >8 >8 0.5 to >8 0.8 3.5 95.7
Meropenem >8 >8 2 to >8 0.0 8.4 91.6
Amikacin 16 >32 ≤0.25 to >32 52.1 32.0 15.9
Colistin ≤0.12 >4 ≤0.015 to >4 81.0 19.0
Tigecycline 1 2 0.06 to 8 91.5 7.7 0.8
Levofloxacin >4 >4 ≤0.03 to >4 14.8 3.9 81.3
KPC positive, MBL negative (476)g Ceftazidime-avibactam 1 4 ≤0.015 to 128 98.7 1.3
Ceftazidime >128 >128 1 to >128 2.1 3.4 94.5
Cefepime >16 >16 ≤0.12 to >16 2.9 10.7 86.4
Aztreonam >128 >128 2 to >128 0.4 0.2 99.4
Piperacillin-tazobactam >128 >128 2 to >128 0.6 2.1 97.3
Doripenem >4 >4 0.5 to >4 2.1 8.4 89.5
Ertapenem (n = 269) >1 >1 0.25 to >1 1.1 0.4 98.5
Imipenem >8 >8 0.5 to >8 0.4 1.1 98.5
Meropenem >8 >8 2 to >8 0.0 3.6 96.4
Amikacin 32 >32 ≤0.25 to >32 44.6 40.1 15.3
Colistin ≤0.12 >4 ≤0.015 to >4 82.6 17.4
Tigecycline 1 2 0.06 to 8 91.6 7.4 1.0
Levofloxacin >4 >4 ≤0.03 to >4 12.8 3.2 84.0
OXA-48-like positive, MBL negative (134)h Ceftazidime-avibactam 0.5 2 0.03 to 64 98.5 1.5
Ceftazidime 64 >128 0.06 to >128 17.9 1.5 80.6
Cefepime >16 >16 ≤0.12 to >16 12.7 4.5 82.8
Aztreonam 128 >128 ≤0.015 to >128 14.9 0.8 84.3
Piperacillin-tazobactam >128 >128 32 to >128 0.0 1.5 98.5
Doripenem >4 >4 0.5 to >4 14.9 16.4 68.7
Ertapenem (n = 62) >1 >1 >1 0.0 0.0 100
Imipenem 8 >8 1 to >8 2.2 12.0 85.8
Meropenem >8 >8 2 to >8 0.0 26.1 73.9
Amikacin 8 >32 0.5 to >32 78.4 3.0 18.6
Colistin ≤0.12 >4 ≤0.015 to >4 75.4 24.6
Tigecycline 1 2 0.12 to 4 91.0 9.0 0.0
Levofloxacin >4 >4 ≤0.03 to >4 20.9 6.7 72.4
Meropenem nonsusceptible, MBL negative, carbapenemase negative (207) Ceftazidime-avibactam 2 4 0.06 to >128 94.7 5.3
Ceftazidime 128 >128 0.12 to >128 6.8 1.9 91.3
Cefepime >16 >16 ≤0.12 to >16 7.2 9.7 83.1
Aztreonam 128 >128 0.03 to >128 7.7 1.5 90.8
Piperacillin-tazobactam >128 >128 0.5 to >128 8.7 9.2 82.1
Doripenem 2 >4 0.03 to >4 30.4 29.0 40.6
Ertapenem (n = 123) >1 >1 0.015 to >1 6.5 0.8 92.7
Imipenem 1 >8 0.06 to >8 53.1 11.6 35.3
Meropenem 4 >8 2 to >8 0.0 44.0 56.0
Amikacin 8 >32 ≤0.25 to >32 74.9 5.3 19.8
Colistin ≤0.12 >4 ≤0.015 to >4 83.1 16.9
Tigecycline 1 2 0.06 to 8 91.3 6.3 2.4
Levofloxacin >4 >4 ≤0.03 to >4 21.7 2.9 75.4
MBL positive (145)i Ceftazidime-avibactam >128 >128 0.5 to >128 3.4 96.6
Ceftazidime >128 >128 0.5 to >128 2.1 0.0 97.9
Cefepime >16 >16 ≤0.12 to >16 4.1 6.2 89.7
Aztreonam 64 >128 ≤0.015 to >128 32.4 2.1 65.5
Piperacillin-tazobactam >128 >128 0.5 to >128 6.9 2.8 90.3
Doripenem >4 >4 2 to >4 0.0 3.4 96.6
Ertapenem (n = 82) >1 >1 0.12 to >1 4.9 2.4 92.7
Imipenem >8 >8 0.5 to >8 1.4 3.4 95.2
Meropenem >8 >8 2 to >8 0.0 9.7 90.3
Amikacin 16 >32 1 to >32 59.3 4.1 36.6
Colistin ≤0.12 >4 ≤0.015 to >4 80.7 19.3
Tigecycline 1 4 0.06 to >8 88.3 8.3 3.4
Levofloxacin >4 >4 0.06 to >4 30.3 7.6 62.1
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a

MICs were interpreted according to CLSI breakpoints (16), with the exception of the following: ceftazidime-avibactam, for which MICs were interpreted using the MIC interpretative criteria according to the FDA (18); tigecycline, for which MICs were interpreted using the MIC interpretative criteria according to the FDA (19); and colistin, for which EUCAST breakpoints were applied (20).

b

The 34,062 Enterobacteriaceae isolates were composed of the following: Citrobacter amalonaticus (n = 21), Citrobacter braakii (n = 109), Citrobacter diversus (n = 1), Citrobacter farmeri (n = 4), Citrobacter freundii (n = 1,033), Citrobacter gillenii (n = 1), Citrobacter koseri (n = 707), Citrobacter murliniae (n = 5), Citrobacter sedlakii (n = 3), Citrobacter youngae (n = 1), Citrobacter, species not determined (n = 4), Enterobacter aerogenes (n = 1,350), Enterobacter amnigenus (n = 1), Enterobacter asburiae (n = 250), Enterobacter cancerogenus (n = 1), Enterobacter cloacae (n = 2,207), Enterobacter gergoviae (n = 4), Enterobacter hormaechei (n = 1), Enterobacter kobei (n = 93), Enterobacter ludwigii (n = 24), Escherichia coli (n = 11,770), Escherichia fergusonii (n = 1), Escherichia hermannii (n = 2), Escherichia vulneris (n = 1), Hafnia alvei (n = 4), Klebsiella oxytoca (n = 1,900), Klebsiella ozaenae (n = 1), Klebsiella pneumoniae (n = 9,098), Klebsiella variicola (n = 7), Kluyvera ascorbata (n = 2), Morganella morganii (n = 979), Pantoea agglomerans (n = 2), Proteus hauseri (n = 3), Proteus mirabilis (n = 2,235), Proteus penneri (n = 42), Proteus rettgeri (n = 2), Proteus vulgaris (n = 995), Providencia alcalifaciens (n = 14), Providencia rettgeri (n = 141), Providencia stuartii (n = 161), Raoultella ornithinolytica (n = 57), Raoultella planticola (n = 13), Raoultella terrigena (n = 2), Serratia liquefaciens (n = 16), Serratia marcescens (n = 785), Serratia odorifera (n = 1), Serratia rubidaea (n = 1), and Serratia ureilytica (n = 7).

c

For cefepime, the susceptible-dose-dependent (SDD) interpretive category replaced the intermediate category in 2014.

d

Ertapenem was not tested against isolates collected in 2014.

e

Colistin was tested with a final concentration of 0.002% polysorbate-80 in each panel well.

f

The 19 MBL-negative isolates that were resistant to ceftazidime-avibactam (MIC, >8 μg/ml) were composed of the following: C. braakii (n = 1), E. aerogenes (n = 1), E. coli (n = 2), K. oxytoca (n = 2), K. pneumoniae (n = 11), P. vulgaris (n = 1), and S. marcescens (n = 1).

g

Includes two isolates carrying KPC-2 and OXA-163 and one isolate carrying KPC-2 and GES-6; does not include 6 isolates carrying KPC and MBL.

h

Includes isolates carrying OXA-48 (n = 116), OXA-244 (n = 9), OXA-181 (n = 4), and OXA-163 (n = 5); two of these carry both OXA-163 and KPC-2. Does not include 7 isolates carrying OXA-48-like and MBL.

i

The 145 isolates that were MBL positive were composed of the following: C. freundii (n = 8), E. asburiae (n = 4), E. cloacae (n = 31), E. coli (n = 5), K. oxytoca (n = 7), K. pneumoniae (n = 73), P. mirabilis (n = 8), P. rettgeri (n = 1), P. stuartii (n = 3), and S. marcescens (n = 5). Also included are 13 isolates carrying MBLs and serine carbapenemases: VIM-1 and KPC-2 (n = 4), IMP-4 and KPC-2 (n = 2), VIM-4 and OXA-48 (n = 2), VIM-31 and OXA-48 (n = 1), NDM-1 and OXA-48 (n = 3), and NDM-1 and OXA-232 (n = 1).

Of the 961 meropenem-nonsusceptible isolates identified, 754 isolates (78.5%) possessed one (n = 738) or more (n = 16) carbapenemase genes, whereas in 207 (21.5%) isolates, no carbapenemase gene could be identified (Table 2). Of the 754 carbapenemase-positive isolates, 132 isolates possessed only a metallo-β-lactamase (MBL), 13 carried an MBL and a KPC or OXA-48-like gene, and 609 harbored one or more serine carbapenemase genes (KPC, OXA-48-like, or GES), with 548 of the 609 (90%) isolates being K. pneumoniae. Against carbapenemase-positive, metallo-β-lactamase-negative isolates (n = 609), susceptibility to ceftazidime-avibactam was higher (98.7%) than that for any other agent tested, including tigecycline (91.5%) and colistin (81.0%) (Table 1). Ceftazidime-avibactam was very active against KPC-positive (98.7% susceptible), OXA-48-like (98.5% susceptible), and GES (100%) carbapenemase-producing isolates of Enterobacteriaceae (Tables 1 and 2). Ceftazidime-avibactam was also active against meropenem-nonsusceptible carbapenemase-negative isolates (94.7% susceptible) but did not demonstrate activity against isolates with metallo-β-lactamases, as expected (Table 1).

TABLE 2.

Ceftazidime-avibactam and ceftazidime MIC distributions for meropenem-nonsusceptible Enterobacteriaceae that were carbapenemase negative, KPC positive, OXA-48 positive, GES positive, or metallo-β-lactamase positive and ceftazidime-avibactam distributions stratified by the presence or absence of an ESBL or AmpC β-lactamase gene

Genotype/organism (n) Antimicrobial agent No. (%) of isolates inhibited at MIC (μg/ml) ofa:
≤0.015 0.03 0.06 0.12 0.25 0.5 1 2 4 8 16 32 64 128 >128
Carbapenemase negative
    All (207) Ceftazidime 4 (1.9) 2 (2.9) 4 (4.8) 1 (5.3) 1 (5.8) 2 (6.8) 4 (8.7) 6 (11.6) 19 (20.8) 34 (37.2) 43 (58.0) 87 (100)
Ceftazidime-avibactam 4 (1.9) 15 (9.2) 17 (17.4) 24 (29.0) 36 (46.4) 60 (75.4) 36 (92.8) 4 (94.7) 4 (96.6) 1 (97.1) 0 (97.1) 1 (97.6) 5 (100)
    AmpC positive,b ESBL positivec (16) Ceftazidime-avibactam 1 (6.3) 1 (12.5) 2 (25.0) 3 (43.8) 5 (75.0) 2 (87.5) 1 (93.8) 0 (93.8) 0 (93.8) 0 (93.8) 1 (100)
    AmpC positive, ESBL negative (32) Ceftazidime-avibactam 2 (6.3) 2 (12.5) 5 (28.1) 3 (37.5) 8 (62.5) 5 (78.1) 5 (93.8) 0 (93.8) 2 (100)
    AmpC negative, ESBL positive (147) Ceftazidime-avibactam 1 (0.7) 9 (6.8) 10 (13.6) 17 (25.2) 24 (41.5) 50 (75.5) 29 (95.2) 3 (97.3) 0 (97.3) 0 (97.3) 0 (97.3) 0 (97.3) 4 (100)
    AmpC negative, ESBL negative (12) Ceftazidime-avibactam 1 (8.3) 3 (33.3) 1 (41.7) 2 (58.3) 1 (66.7) 0 (66.7) 0 (66.7) 0 (66.7) 2 (83.3) 1 (91.7) 0 (91.7) 0 (91.7) 1 (100)
KPC positive, MBL negative
    All (476)d Ceftazidime 1 (0.2) 5 (1.3) 4 (2.1) 16 (5.5) 37 (13.2) 38 (21.2) 70 (35.9) 54 (47.3) 251 (100)
Ceftazidime-avibactam 2 (0.4) 12 (2.9) 12 (5.5) 17 (9.0) 34 (16.2) 83 (33.6) 150 (65.1) 109 (88.0) 41 (96.6) 10 (98.7) 4 (99.6) 1 (99.8) 0 (99.8) 1 (100)
    AmpC positive, ESBL positive (11) Ceftazidime-avibactam 1 (9.1) 3 (36.4) 5 (81.8) 1 (90.9) 1 (100)
    AmpC positive, ESBL negative (19) Ceftazidime-avibactam 1 (5.3) 2 (15.8) 0 (15.8) 5 (42.1) 9 (89.5) 2 (100)
    AmpC negative, ESBL positive (205) Ceftazidime-avibactam 1 (0.5) 5 (2.9) 6 (5.9) 6 (8.8) 13 (15.1) 30 (29.8) 73 (65.4) 50 (89.8) 18 (98.5) 1 (99.0) 2 (100)
    AmpC negative, ESBL negative (241) Ceftazidime-avibactam 1 (0.4) 7 (3.3) 5 (5.4) 9 (9.1) 20 (17.4) 45 (36.1) 63 (62.2) 56 (85.5) 22 (94.6) 9 (98.3) 2 (99.2) 1 (99.6) 0 (99.6) 1 (100)
OXA-48-like positive, MBL negative
    All (134)e Ceftazidime 1 (0.7) 0 (0.7) 3 (3.0) 6 (7.5) 4 (10.4) 3 (12.7) 7 (17.9) 2 (19.4) 6 (23.9) 11 (32.1) 26 (51.5) 28 (72.4) 37 (100)
Ceftazidime-avibactam 1 (0.7) 1 (1.5) 6 (6.0) 25 (24.6) 46 (59.0) 37 (86.6) 12 (95.5) 3 (97.8) 1 (98.5) 1 (99.3) 0 (99.3) 1 (100)
    AmpC positive, ESBL positive (4) Ceftazidime-avibactam 2 (50.0) 1 (75.0) 0 (75.0) 0 (75.0) 0 (75.0) 0 (75.0) 0 (75.0) 1 (100)
    AmpC positive, ESBL negative (6) Ceftazidime-avibactam 2 (33.3) 2 (66.7) 2 (100)
    AmpC negative, ESBL positive (105) Ceftazidime-avibactam 3 (2.9) 19 (21.0) 37 (56.2) 31 (85.7) 11 (96.2) 3 (99.0) 1 (100)
    AmpC negative, ESBL negative (19) Ceftazidime-avibactam 1 (5.3) 1 (10.5) 3 (26.3) 4 (47.4) 5 (73.7) 3 (89.5) 1 (94.7) 0 (94.7) 0 (94.7) 1 (100)
GES positive, MBL negative
    All (2)f Ceftazidime 1 (50.0) 0 (50.0) 1 (100)
Ceftazidime-avibactam 1 (50.0) 0 (50.0) 1 (100)
    AmpC negative, ESBL positive (1) Ceftazidime-avibactam 1 (100)
    AmpC negative, ESBL negative (1) Ceftazidime-avibactam 1 (100)
MBL positive
    All (145)g Ceftazidime 3 (2.1) 0 (2.1) 0 (2.1) 0 (2.1) 0 (2.1) 1 (2.8) 4 (5.5) 4 (8.3) 9 (14.5) 124 (100)
Ceftazidime-avibactam 4 (2.8) 0 (2.8) 0 (2.8) 0 (2.8) 1 (3.4) 2 (4.8) 8 (10.3) 8 (15.9) 10 (22.8) 112 (100)
    AmpC positive, ESBL positive (28) Ceftazidime-avibactam 2 (7.1) 2 (14.3) 2 (21.4) 22 (100)
    AmpC positive, ESBL negative (37) Ceftazidime-avibactam 1 (2.7) 1 (5.4) 4 (16.2) 4 (27.0) 2 (32.4) 25 (100)
    AmpC negative, ESBL positive (56) Ceftazidime-avibactam 1 (1.8) 2 (5.4) 1 (7.1) 4 (14.3) 48 (100)
    AmpC negative, ESBL negative (24) Ceftazidime-avibactam 4 (16.7) 0 (16.7) 0 (16.7) 0 (16.7) 0 (16.7) 0 (16.7) 0 (16.7) 1 (20.8) 2 (29.2) 17 (100)
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a

The MIC90 is in bold type for each MIC distribution. The MIC90 was not calculated for <10 isolates.

b

AmpC-positive isolates include organisms that carry a plasmid-mediated AmpC or are presumed to harbor an intrinsic AmpC (Citrobacter spp., Enterobacter spp., Providencia spp., and S. marcescens).

c

ESBL-positive isolates include organisms that carry a plasmid-mediated ESBL or are presumed to harbor an intrinsic ESBL (K. oxytoca).

d

Includes two isolates carrying KPC-2 and OXA-163 and one isolate carrying KPC-2 and GES-6; does not include six isolates carrying KPC and MBL.

e

Includes two isolates carrying OXA-163 and KPC-2; does not include seven isolates carrying OXA-48-like β-lactamases and MBL.

f

Includes one isolate carrying KPC-2 and GES-6.

g

Includes 13 isolates carrying MBLs and serine carbapenemases.

Many of the carbapenemase-containing carbapenem-nonsusceptible isolates carried additional β-lactamase genes. Approximately 50% of isolates with a KPC also possessed an ESBL and/or plasmid-encoded AmpC β-lactamase. Similarly, 83%, and 86% of MBL-positive and OXA-48-like-positive isolates, respectively, possessed an ESBL gene, an AmpC gene, or both. The presence of these ESBL and/or AmpC β-lactamases in these carbapenemase-containing isolates did not significantly affect the susceptibility for ceftazidime-avibactam, as shown by the MIC distributions of these subsets of isolates (Table 2). The majority (94%) of the carbapenem-nonsusceptible Enterobacteriaceae that did not contain a carbapenemase harbored ESBLs and/or Ambler class C β-lactamases that were encoded by chromosomally located or plasmid-carried genes. The presence of these β-lactamases, coupled with reduced expression or loss of function of one or more outer membrane pore-forming proteins, is most likely the reason for the reduced susceptibility to carbapenems observed among these isolates (6–8). Whereas susceptibility to carbapenems was lost in these isolates, this resistance mechanism did not impact the activity of ceftazidime-avibactam, with 94.7% of the isolates remaining susceptible to this combination.

If future clinical results confirm the in vitro data described here, ceftazidime-avibactam might be useful in the chemotherapy of infections caused by carbapenem-nonsusceptible Enterobacteriaceae. To understand this potential, it is helpful to summarize susceptibility to ceftazidime-avibactam by the carbapenem resistance mechanism. In general, only the presence of an MBL gene was associated with resistance to ceftazidime-avibactam (this study and references 12, 23, and 24). Isolates nonsusceptible to carbapenems by other mechanisms, such as KPC (this study and references 10, 12, and 23–28) and OXA-48 (this study and references 12, 14, and 24) carbapenemases were susceptible to ceftazidime-avibactam. Additionally, carbapenem-nonsusceptible Enterobacteriaceae that carried noncarbapenemase β-lactamase genes, such as those encoding an ESBL or AmpC enzyme, were susceptible to ceftazidime-avibactam (this study and reference 12). This is in agreement with a recent surveillance analysis of Enterobacteriaceae isolates from the United States (25). A further noteworthy point is that intrinsic imipenem resistance among Proteeae species (16) did not affect susceptibility to ceftazidime-avibactam (Table 1).

This study identified only a few isolates (n = 19; <0.1% of Enterobacteriaceae) for which the reduced ceftazidime-avibactam susceptibility could not be explained by the presence of avibactam-insensitive β-lactamases, i.e., MBLs. The mechanisms of reduced susceptibility in those isolates remain to be investigated but might be attributable to target modifications (29). Additionally, the presence of avibactam-insensitive β-lactamases (e.g., other MBLs) that were not detected with the PCR assays cannot be excluded. Upregulated efflux is a less-likely mechanism, as it was not implicated in reduced susceptibility to ceftazidime-avibactam in a direct test of that hypothesis (30). Interestingly, a carbapenem-resistant clinical isolate of K. pneumoniae carrying blaKPC-3 was recently described that was resistant to ceftazidime-avibactam (31). The inferred amino acid sequence encoded by the blaKPC-3 in that isolate was unaltered, similarly implying a non-β-lactamase-mediated mechanism of ceftazidime-avibactam resistance.

A noteworthy feature of the study reported here was that a substantial proportion of the meropenem-nonsusceptible isolates did not contain any gene related to currently known carbapenemases but rather contained an ESBL gene, AmpC gene, or both. Ceftazidime-avibactam showed good activity against these isolates (96.4% susceptible [188 of the 195 isolates]). The proportion of noncarbapenemase-mediated meropenem-nonsusceptible isolates of Enterobacteriaceae (207/961) should not be taken as an estimate of prevalence, because INFORM is not a prevalence-based surveillance program. However, prevalence-based studies have been performed, and the frequency of noncarbapenemase-mediated mechanisms among carbapenem-nonsusceptible Enterobacteriaceae has been found to be 77.2 to 98.6% (13, 32–34). These estimates exceeded the proportion observed here, but that is likely because those collections included ertapenem-nonsusceptible isolates, against which meropenem can retain some activity, whereas that was not the case in the present work (35). In one of these examples, consisting of carbapenem-nonsusceptible but carbapenemase-negative isolates collected in France (13), the ceftazidime-avibactam MICs were ≤4 μg/ml (MIC90, 1 μg/ml), in good agreement with the global surveillance data reported here.

In conclusion, the analyses presented here of global surveillance data for ceftazidime-avibactam and comparator agents tested against carbapenem-nonsusceptible Enterobacteriaceae should prove helpful in identifying potential anti-Enterobacteriaceae therapies when therapeutic options are limited through reduced susceptibility to currently available agents. Ceftazidime-avibactam is a potent agent in vitro against meropenem-nonsusceptible Enterobacteriaceae, except for isolates in which carbapenem resistance is mediated through MBLs.

ACKNOWLEDGMENTS

We thank all INFORM participants for their contributions to the program.

This study at IHMA was supported by AstraZeneca Pharmaceuticals LP, which also included compensation fees for services in relation to preparation of the manuscript. B. L. M. de Jonge and W. W. Nichols are, respectively, current and former employees of AstraZeneca. K. M. Kazmierczak, J. A. Karlowsky, D. J. Biedenbach, and D. F. Sahm are employees of IHMA.

Funding Statement

This investigation was funded by AstraZeneca Pharmaceuticals as part of the sponsored INFORM global surveillance program. The sponsor approved the overall study design. All investigative sites were recruited and study supplies were provided by IHMA, Inc. Analysis of the final MIC and molecular data was performed by IHMA.

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