Aztreonam/avibactam activity against a large collection of carbapenem-resistant Enterobacterales (CRE) collected in hospitals from Europe, Asia and Latin America (2019–21)

Abstract

Background

Aztreonam/avibactam is under development to treat infections caused by Gram-negative bacteria. We evaluated the in vitro activities of aztreonam/avibactam and comparators against a global collection of carbapenem-resistant Enterobacterales (CRE), including ceftazidime/avibactam-resistant isolates.

Methods

Isolates were consecutively collected (24 924; 1/patient) from 69 medical centres in 36 countries during 2019–21. Isolates were susceptibility tested by CLSI broth microdilution. All CRE isolates (n = 1098; 4.4%) were in silico screened for carbapenemase (CPE) genes after genome sequencing. CRE susceptibility results were stratified by CPE, geography and resistance phenotype.

Results

Aztreonam/avibactam inhibited 99.6% of CREs at ≤8 mg/L (MIC_50/90, 0.25/0.5 mg/L), including 98.9% (345/349) of ceftazidime/avibactam-resistant isolates. Aztreonam/avibactam activity was consistent across geographical regions (98.9%–100.0% inhibited at ≤8 mg/L), but susceptibility to comparators varied markedly. Susceptibility (CLSI criteria) for ceftazidime/avibactam and meropenem/vaborbactam ranged from 80.2% and 77.5% in Western Europe to 39.5% and 40.3% in the Asia-Pacific region, respectively. Aztreonam/avibactam retained activity against isolates non-susceptible to colistin (99.7% inhibited at ≤8 mg/L) or tigecycline (98.6% inhibited at ≤8 mg/L). A CPE gene was identified in 972 CRE isolates (88.5%). The most common CPEs were KPC (43.1% of CREs), NDM (26.6%) and OXA-48–like (18.7%); 57 isolates (5.2%) had >1 CPE gene. Aztreonam/avibactam inhibited 99.9% of CPE producers at ≤8 mg/L, whereas ceftazidime/avibactam and meropenem/vaborbactam exhibited limited activity against isolates producing MBL and/or OXA-48-like enzymes.

Conclusions

Aztreonam/avibactam activity was not adversely affected by clinically relevant CPEs. Our results support aztreonam/avibactam development to treat infections caused by CRE, including MBL producers.

Introduction

Among most Enterobacterales species, resistance to carbapenems is primarily due to the production of carbapenemases. For therapeutic purposes, the carbapenemases produced by Enterobacterales can be divided in two groups, the serine carbapenemases, such as KPCs and oxacillinase-48 (OXA-48), and the MBLs.¹

The main strategy to counteract carbapenem resistance in the Enterobacterales has been the development of β-lactamase inhibitors (BLIs). Many BLIs have been approved recently by the US FDA to be used in combination with various β-lactams (BLs). Some of these new BL/BLI combinations, particularly ceftazidime/avibactam, meropenem/vaborbactam and imipenem/relebactam, have shown potent in vitro activity and clinical efficacy against carbapenem-resistant Enterobacterales (CRE) producing class A carbapenemase; however, the current clinically available BL/BLI combinations are not active against MBL-producing Enterobacterales.²

MBLs are of particular concern due to their ability to hydrolyse virtually all β-lactams, their rapid development of new variants, the horizontal transferability of their encoding genes and, most importantly, the lack of clinically useful MBL inhibitors.^1,3 One strategy to overcome MBL-derived resistance is to combine an MBL-stable β-lactam with a serine carbapenemase inhibitor. A serine inhibitor would protect the MBL-stable β-lactam against ESBLs, AmpC β-lactamases and/or serine carbapenemases, often present in MBL-producing Enterobacterales.⁴ Thus, the combination of aztreonam with avibactam has showed great potential for successfully treating infections caused by Enterobacterales by broadening the spectrum of this combination against most classes of β-lactamases, including MBLs.⁵

Aztreonam was approved by the US FDA and the EMA in 1986, and remains the only clinically available member of the monobactam class.⁶ Aztreonam is stable to hydrolysis by MBLs, but it is hydrolysed by most clinically relevant β-lactamases, such as ESBLs, AmpCs and KPCs. Avibactam is a non-β-lactam BLI that inhibits most clinically relevant serine β-lactamases and its clinical utility has been demonstrated by the efficacy of ceftazidime/avibactam in the treatment of infections caused by serine carbapenemase-producing CRE.⁷ In the present study, we evaluated the in vitro activities of aztreonam/avibactam and comparators against a global (ex-USA) collection of CRE, including ceftazidime/avibactam-resistant isolates. We also evaluated the resistance mechanisms associated with decreased susceptibility to aztreonam/avibactam.

Materials and methods

Organism collection

Bacterial isolates were collected via a network of medical sites participating in the SENTRY Antimicrobial Surveillance Program, and sent to JMI Laboratories (North Liberty, IA, USA) for susceptibility testing.⁸ Each participating centre was asked to collect consecutive bacterial isolates from patients hospitalized with the following infection types: bloodstream infection, pneumonia, skin and skin structure infection, urinary tract infection and intra-abdominal infection. The number of isolates per infection type and portion of the year varied slightly by region. Isolates could be from any specimen type associated with the infections above, if determined to be significant by local criteria as the reported probable cause of infection.

A total of 24 924 Enterobacterales isolates were collected consecutively during 2019–21 from 69 medical centres located in Western Europe [W-EU; n = 13 125; 26 centres in 10 countries (1 in Belgium, 4 in France, 6 in Germany, 1 in Ireland, 4 in Italy, 1 in Portugal, 3 in Spain, 2 in Sweden, 1 in Switzerland and 3 in the UK)], Eastern Europe and the Mediterranean [E-EU; n = 4404; 15 centres in 10 countries (1 in Belarus, 1 in Czech Republic, 1 in Greece, 1 in Hungary, 2 in Israel, 1 in Poland, 1 in Romania, 3 in Russia, 2 in Slovenia and 2 in Turkey)], Asia-Pacific [APAC; n = 4270; 17 centres in 9 countries (5 in Australia, 4 in Japan, 1 in Malaysia, 1 in New Zealand, 1 in Philippines, 2 in South Korea, 1 in Taiwan, 1 in Thailand and 1 in Vietnam) and Latin America (LATAM; n = 3035; 11 centres in 7 countries (2 in Argentina, 3 in Brazil, 1 in Chile, 1 in Colombia, 1 in Costa Rica, 2 in Mexico and 1 in Panama]). Species identification was confirmed by using standard biochemical tests and/or a MALDI Biotyper (Bruker Daltonics, Billerica, MA, USA) and/or genome sequencing, when necessary.

Susceptibility testing

All isolates were susceptibility tested by the reference broth microdilution method specified by CLSI standards. Aztreonam/avibactam was tested with avibactam at a fixed concentration of 4 mg/L. All tests were conducted in a central monitoring laboratory (JMI Laboratories). CRE was defined as displaying imipenem or meropenem MIC values of ≥4 mg/L. Imipenem was not applied to Proteus mirabilis or indole-positive Proteeae due to their intrinsically elevated MIC values. Overall, 1098 (4.4%) CRE isolates were identified for further molecular evaluation.⁹

A tentative aztreonam/avibactam pharmacokinetic/pharmacodynamic (PK/PD) susceptible breakpoint of ≤8 mg/L was applied for comparison.¹⁰ CLSI and EUCAST breakpoints were applied to the comparator agents where available.^11,12 US FDA breakpoints were applied for tigecycline, whereas tigecycline breakpoints published by EUCAST for Escherichia coli and Citrobacter koseri (≤0.5 mg/L) were applied to all Enterobacterales species for comparison. Concurrent quality control (QC) testing was performed to ensure proper test conditions and procedures.

β-Lactamase screening and molecular characterization of isolates with decreased susceptibility to aztreonam/avibactam

All CRE isolates (n = 1098) were genome sequenced for in silico screening of β-lactamase-encoding genes. Briefly, total genomic DNA was extracted using the fully automated ThermoScientific™ KingFisher™ Flex Magnetic Particle Processor (Cleveland, Ohio, USA). Libraries were normalized using the bead-based normalization procedure (Illumina) and sequenced on MiSeq. FASTQ format files for each sample set were assembled independently using the de novo assembler SPAdes 3.15.3 with K-values of 21, 33, 55, 77 and 99 plus careful mode on to reduce the number of mismatches. This process produced FASTA-format files of contiguous sequences with the best N50 value. An in-house proprietary bioinformatic pipeline and a JMI-curated resistance gene database (Version 3; uses Python v2.7.9, SPAdes v3.15.3 and BBMap v36.x) based on the NCBI Bacterial Antimicrobial Resistance Reference Gene Database (https://www.ncbi.nlm.nih.gov/bioproject/PRJNA313047) was used for the in silico analysis. This resistance gene database was used as queries to align β-lactamase resistance determinants against the target assembled sequences. Hits with identities greater than 94% and 40% minimum coverage length were selected for further analysis and final assignment of β-lactamase alleles.^13,14

Expression of acrA and ampC was performed by extracting total mRNA from cultures grown to mid-log phase in tryptic soy broth at 37°C with shaking followed by purification using the RNeasy Mini Kit in the Qiacube (QIAGEN) workstation according to the manufacturer’s instructions. Residual DNA was eliminated by treatment with RNAse-free DNase (Promega, Madison, WI, USA). Quantification of mRNA and sample quality were assessed using the RNA 6000 Nano kit on the 2100 Bioanalyzer (Agilent Technologies, Santa Clara, CA, USA) according to the manufacturer’s instructions. The relative transcription levels of acrA and ampC were determined by real-time PCR assays in the StepOne Plus instrument (Life Technologies, Foster City, CA, USA) using the Power SYBR Green RNA-to-C_T 1-Step qPCR master mix (Applied Biosystems, ThermoFisher) with 0.5 ng input RNA. Assays were performed in triplicate. Transcription levels of target genes were measured by the quantification of the target gene mRNA using a normalized expression analysis method relative to a housekeeping reference gene (rpsL in E. coli and Enterobacter cloacae species complex and gyrA in Klebsiella pneumoniae) compared with a susceptible control strain.^15,16 Oligonucleotides used in this study are listed in Table S1, available as Supplementary data at JAC-AMR Online.

Results

K. pneumoniae (n = 878) accounted for 80.0% of CRE isolates. The second most common CRE species/group was E. cloacae species complex (n = 65; 5.9%), followed by E. coli (n = 58; 5.3%) and Serratia marcescens (n = 36; 3.3%). The CRE isolates were collected from patients with bloodstream infections (n = 326; 29.7% of total), pneumonia (n = 321; 29.2%), urinary tract infections (n = 191; 17.4%), skin and soft tissue infections (n = 152; 13.8%), intra-abdominal infections (n = 86; 7.8%) and other infection types (n = 22; 2.0%). CRE rates were highest in E-EU (454/4404; 10.3%), followed by LATAM (240/3035; 7.9%), APAC (177/4270; 4.1%) and W-EU (227/13 125; 1.7%).

Aztreonam/avibactam (MIC_50/90, 0.25/0.5 mg/L) inhibited 99.6% of CRE isolates (1094/1098) at ≤8 mg/L (96.7% at ≤1 mg/L), including all (100.0%) isolates producing MBLs or OXA-48-like, 99.8% (472/473) of KPC producers and 97.6% (123/126) of non-carbapenemase-producing CRE (Table 1). Aztreonam/avibactam was also highly active against CRE isolates resistant to ceftazidime/avibactam (n = 349; 98.9% inhibited at ≤8 mg/L), colistin (colistin MIC, >2 mg/L; n = 292; 99.7% inhibited at ≤8 mg/L) or tigecycline (tigecycline MIC, >0.5 mg/L; n = 515; 99.6% inhibited at ≤8 mg/L; Table 1). Notably, an MBL was identified in 97.4% (340/349) of ceftazidime/avibactam-resistant isolates (data not shown).

Table 1.

Aztreonam/avibactam MIC distributions

	No. of isolates and cumulative % inhibited at aztreonam/avibactam MIC (mg/L) of:											MIC50	MIC90
Organism group (n)	≤0.03	0.06	0.12	0.25	0.5	1	2	4	8	16	>16
CRE (1098)	74	118	243	394	189	44	23	8	1	1	3	0.25	0.5
	6.7	17.5	39.6	75.5	92.7	96.7	98.8	99.5	99.6	99.7	100.0
KPC producers (473)	14	50	111	200	78	16	3	0	0	1		0.25	0.5
	3.0	13.5	37.0	79.3	95.8	99.2	99.8	99.8	99.8	100.0
MBL producers (347)	59	59	86	88	32	12	7	4				0.12	0.5
	17.0	34.0	58.8	84.1	93.4	96.8	98.8	100.0
OXA-48-like producers (205)	2	12	43	85	52	7	1	2	1			0.25	0.5
	1.0	6.8	27.8	69.3	94.6	98.0	98.5	99.5	100.0
≥2 carbapenemases (57)	3	5	13	23	10	1	0	2				0.25	0.5
	5.3	14.0	36.8	77.2	94.7	96.5	96.5	100.0
No carbapenemase (126)	1	1	15	42	38	10	12	4	0	0	3	0.5	2
	0.8	1.6	13.5	46.8	77.0	84.9	94.4	97.6	97.6	97.6	100.0
CZA-R (349)	53	59	86	91	33	12	7	4	0	1	3	0.12	0.5
	15.2	32.1	56.7	82.8	92.3	95.7	97.7	98.9	98.9	99.1	100.0
Colistin-R (292)	17	12	61	124	63	11	2	1	0	1		0.25	0.5
	5.8	9.9	30.8	73.3	94.9	98.6	99.3	99.7	99.7	100.0
Tigecycline-NS (515)a	27	34	116	202	101	27	5	1	0	0	2	0.25	0.5
	5.2	11.8	34.4	73.6	93.2	98.4	99.4	99.6	99.6	99.6	100.0

CZA, ceftazidime/avibactam; R, resistant; NS, non-susceptible.

^aIsolates with tigecycline MIC values >0.5 mg/L, which is the susceptible breakpoint published by EUCAST for E. coli and C. koseri.

The activities of aztreonam/avibactam and comparator agents against the CRE collection and the four most common Enterobacterales species are shown in Table 2. The most active comparator agents were ceftazidime/avibactam (MIC_50/90, 2/>32 mg/L; 68.2% susceptible per CLSI and EUCAST), meropenem/vaborbactam (MIC_50/90, 2/>32 mg/L; 60.5%/64.3% susceptible per CLSI/EUCAST), amikacin (MIC_50/90, 8/>32 mg/L; 64.0%/53.7% susceptible per CLSI/EUCAST), minocycline (MIC_50/90, 4/>32 mg/L; 57.8% susceptible per CLSI), tigecycline (MIC_50/90, 0.5/2 mg/L; 93.4%/53.1% susceptible per US FDA/EUCAST) and colistin (MIC_50/90, 0.25/>8 mg/L; 73.3% susceptible per EUCAST; Table 2). When comparing susceptibility rates for isolates collected in 2019 with those collected in 2021, we observed that susceptibility rates improved slightly for ceftazidime/avibactam [from 63.7% to 68.6% (CLSI and EUCAST)] and meropenem/vaborbactam [from 55.8%/59.6% to 59.9%/64.0% (CLSI/EUCAST)], decreased slightly for amikacin [from 63.0%/51.6% to 60.3%/50.7% (CLSI/EUCAST)] and decreased markedly for colistin [from 80.0% to 67.8% (EUCAST); data not shown]. Moreover, K. pneumoniae represented 69.2%, 82.4% and 85.5% of isolates resistant to ceftazidime/avibactam, meropenem/vaborbactam and amikacin, respectively, and 80.8% of isolates non-susceptible to colistin per CLSI criteria (data not shown).

Table 2.

Activity of aztreonam/avibactam and comparator antimicrobial agents tested against all 1098 CRE isolates combined and the four most common species (2019–21)

Antimicrobial agent (no. of isolates)	mg/L			CLSI/US FDAa			EUCASTa
	MIC50	MIC90	MIC range	%S	%I	%R	%S	%I	%R
All Enterobacterales (1098)
Aztreonam/avibactamb	0.25	0.5	≤0.03 to >16	(99.6)b			(99.6)b
Ceftazidime/avibactam	2	>32	≤0.015 to >32	68.2		31.8	68.2		31.8
Meropenem/vaborbactam	2	>32	≤0.015 to >32	60.5	3.8	35.7	64.3		35.7
Ceftolozane/tazobactam	>16	>16	0.25 to >16	2.6c	1.5	95.9	2.6		97.4
Aztreonam	>16	>16	≤0.03 to >16	7.9	0.5	91.5	7.1	0.8	92.1
Ciprofloxacin	>4	>4	≤0.03 to >4	7.7	2.0	90.3	7.7	2.0	90.3
Levofloxacin	16	>32	≤0.015 to >32	10.4	6.7	82.9	10.4	6.7	82.9
Gentamicin	16	>16	≤0.12 to >16	44.1	3.7	52.1	41.9d		58.1
Amikacin	8	>32	0.5 to >32	64.0	9.8	26.1	53.7d		46.3
Minocycline	4	>32	0.5 to >32	57.8	16.0	26.2
Tigecycline	0.5	2	≤0.06 to >8	93.4	5.5	1.1	53.1e
TMP-SMX	>4	>4	≤0.12 to >4	16.5		83.5	16.5	3.1	80.4
Colistin	0.25	>8	≤0.06 to >8		73.3	26.7	73.3		26.7
K. pneumoniae (878)
Aztreonam/avibactamb	0.25	0.5	≤0.03 to >16	(99.8)b			(99.8)b
Ceftazidime/avibactam	2	>32	≤0.015 to >32	72.6		27.4	72.6		27.4
Meropenem/vaborbactam	2	>32	≤0.015 to >32	60.4	2.9	36.8	63.2		36.8
Ceftolozane/tazobactam	>16	>16	≤0.12 to >16	2.2	0.9	96.9	2.2		97.8
Aztreonam	>16	>16	≤0.03 to >16	5.1	0.1	94.8	4.9	0.2	94.9
Ciprofloxacin	>4	>4	≤0.03 to >4	5.0	0.8	94.2	5.0	0.8	94.2
Levofloxacin	32	>32	0.03 to >32	6.6	5.3	88.1	6.6	5.3	88.1
Gentamicin	>16	>16	≤0.12 to >16	43.5	3.0	53.5	41.3d		58.7
Amikacin	8	>32	0.5 to >32	61.8	10.6	27.6	52.5d		47.5
Minocycline	4	>32	0.5 to >32	56.9	17.3	25.8
Tigecycline	0.5	2	0.12 to >8	95.0	3.9	1.1	50.7e
TMP-SMX	>4	>4	≤0.12 to >4	14.6		85.4	14.6	2.5	82.9
Colistin	0.25	>8	≤0.06 to >8		73.2	26.8	73.2		26.8
E. cloacae complex (65)
Aztreonam/avibactamb	0.25	1	≤0.03 to >16	(98.4)b			(98.4)b
Ceftazidime/avibactam	>32	>32	0.25 to >32	42.2		57.8	42.2		57.8
Meropenem/vaborbactam	4	>32	0.03 to >32	57.8	10.9	31.2	68.8		31.2
Ceftolozane/tazobactam	>16	>16	0.25 to >16	4.7	0.0	95.3	4.7		95.3
Aztreonam	>16	>16	0.06 to >16	20.3	4.7	75.0	17.2	3.1	79.7
Ciprofloxacin	4	>4	≤0.03 to >4	18.8	15.6	65.6	18.8	15.6	65.6
Levofloxacin	2	32	0.03 to >32	34.4	12.5	53.1	34.4	12.5	53.1
Gentamicin	>16	>16	≤0.12 to >16	31.2	14.1	54.7	31.2d		68.8
Amikacin	8	>32	0.5 to >32	68.8	7.8	23.4	56.2d		43.8
Minocycline	4	32	0.5 to >32	70.3	6.2	23.4
Tigecycline	0.5	2	0.12 to 4	90.6	9.4	0.0	76.6e
TMP-SMX	>4	>4	≤0.12 to >4	17.2		82.8	17.2	1.6	81.2
Colistin	0.25	>8	0.12 to >8		82.5	17.5	82.5		17.5
E. coli (58)
Aztreonam/avibactamb	0.25	4	≤0.03 to >16	(98.3)b			(98.3)b
Ceftazidime/avibactam	>32	>32	≤0.015 to >32	49.2		50.8	49.2		50.8
Meropenem/vaborbactam	8	>32	≤0.015 to >32	47.5	8.5	44.1	55.9		44.1
Ceftolozane/tazobactam	>16	>16	≤0.12 to >16	6.8	5.1	88.1	6.8		93.2
Aztreonam	>16	>16	0.06 to >16	10.2	1.7	88.1	5.1	5.1	89.8
Ciprofloxacin	>4	>4	≤0.008 to >4	13.6	1.7	84.7	13.6	1.7	84.7
Levofloxacin	16	>32	≤0.015 to >32	18.6	8.5	72.9	18.6	8.5	72.9
Gentamicin	2	>16	0.5 to >16	57.6	0.0	42.4	54.2d		45.8
Amikacin	4	>32	1 to >32	86.4	1.7	11.9	76.3d		23.7
Minocycline	2	32	0.5 to >32	72.9	5.1	22.0
Tigecycline	0.25	0.25	≤0.06 to 1	100.0	0.0	0.0	98.3		1.7
TMP-SMX	>4	>4	≤0.12 to >4	18.6		81.4	18.6	0.0	81.4
Colistin	0.25	0.25	0.12 to >8		98.3	1.7	98.3		1.7
S. marcescens (36)
Aztreonam/avibactamb	0.25	1	0.12 to 2	(100.0)b			(100.0)b
Ceftazidime/avibactam	1	>32	0.12 to >32	88.9		11.1	88.9		11.1
Meropenem/vaborbactam	1	16	0.03 to >32	88.9	0.0	11.1	88.9		11.1
Ceftolozane/tazobactam	>16	>16	0.5 to >16	11.1	5.6	83.3	11.1		88.9
Aztreonam	>16	>16	0.06 to >16	16.7	2.8	80.6	16.7	0.0	83.3
Ciprofloxacin	4	>4	0.03 to >4	13.9	8.3	77.8	13.9	8.3	77.8
Levofloxacin	2	32	0.06 to 32	13.9	25.0	61.1	13.9	25.0	61.1
Gentamicin	2	>16	0.25 to >16	55.6	2.8	41.7	50.0b		50.0
Amikacin	16	>32	1 to >32	72.2	11.1	16.7	44.4b		55.6
Minocycline	8	32	1 to >32	34.3	31.4	34.3
Tigecycline	2	4	0.5 to 8	55.6	41.7	2.8	5.6e
TMP-SMX	4	>4	≤0.12 to >4	38.9		61.1	38.9	22.2	38.9
Colistin	>8	>8	0.25 to >8		16.7	83.3	16.7		83.3

TMP-SMX, trimethoprim/sulfamethoxazole.

^aCriteria as published by CLSI,¹¹ US FDA¹⁷ and EUCAST.¹²

^bValues in parentheses indicate percentage inhibited at ≤8 mg/L aztreonam/avibactam.

^cThe vast majority (89.3%) of ceftolozane/tazobactam-susceptible isolates harboured a class D β-lactamase, mainly OXA-48-like (75.0%). Only two isolates had a class A carbapenemase: one KPC-2 and one IMI-4.

^dFor infections originating from the urinary tract. For systemic infections, aminoglycosides must be used in combination with other active therapy.¹²

^eFor comparison, EUCAST breakpoints published for E. coli and C. koseri (≤0.5 mg/L) were applied for all Enterobacterales species.¹²

Although the main mechanism of carbapenem resistance in all four geographical regions was the production of a carbapenemase, carbapenemase genes varied substantially by region, as shown in Table 3. Genes encoding KPC predominated in W-EU (66.5% of CRE) and LATAM (70.0% of CRE). KPC-3 predominated in W-EU (86.8% of KPCs) and KPC-2 predominated in LATAM (94.0% of KPCs). MBL genes were detected in 61.6% of CRE isolates from the APAC region; NDM represented 92.7% (101/109) of these MBLs. A variety of carbapenemase genes were observed among E-EU countries. Overall, KPC, MBL and OXA-48-like represented 25.6%, 29.5% and 31.7% of CRE isolates from E-EU, respectively. KPC predominated in Greece and Romania, while OXA-48-like predominated in Russia and Turkey. MBLs, mainly NDM, were common in Belarus, Greece, Poland, Russia and Turkey.

Table 3.

Frequency of carbapenemase genes stratified by geographical region

β-Lactamase	No. of isolates (% of CREs for the region)
	W-EU	E-EU	APAC	LATAM	All regions
KPC type	151 (66.5)	116 (25.6)	38 (21.5)	168 (70.0)	473 (43.1)
KPC-2	20 (8.8)	81 (17.8)	37 (20.9)	158 (65.8)	296 (27.0)
KPC-3	131 (57.7)	35 (7.7)	—	10 (4.2)	176 (16.0)
KPC-4	—	–	1 (0.6)	—	1 (0.1)
MBL	44 (19.4)	134a (29.5)	109 (61.6)	60 (25.0)	347 (31.6)
NDM type	26 (11.5)	109a (24.0)	101 (57.1)	56b (23.3)	292 (26.6)
VIM type	18 (7.9)	25a (5.5)	3 (1.7)	3 (1.3)	48 (4.4)
IMP type	—	1 (0.2)	5 (2.8)	1 (0.4)	7 (0.6)
OXA-48 type	31 (13.7)	144 (31.7)	29 (16.4)	1 (0.4)	205 (18.7)
≥2 β-lactamase	12 (5.3)	22 (4.8)	21 (11.9)	2 (0.8)	57 (5.2)
Total	214 (94.3)	374 (82.4)c	155 (87.6)	229 (95.4)	972 (88.5)
No carbapenemase	13 (5.7)	80 (17.6)	22 (12.4)	11 (4.6)	126 (11.5)

Values in bold indicates the frequency of the most common carbapenemase in the region.

^aOne isolate had an NDM-1 and a VIM-1.

^bOne isolate had an NDM-1 and an NDM-5.

^c86.3% (69/80) of carbapenemase-negative CRE were from Poland.

Notably, combinations of two carbapenemase genes were identified in 57 (5.2%) isolates and three carbapenemase genes were identified in 1 (0.1%) isolate (Table 3). Isolates with two carbapenemase genes were mainly K. pneumoniae (84.2%; 48/57) and were isolated in E-EU (n = 23; 40.4%), APAC (n = 20; 35.0%), W-EU (n = 12; 21.1%) and LATAM (n = 2; 3.5%). The countries with the highest percentages of CRE isolates harbouring two carbapenemases genes were Thailand (38.2%; 13/34), Vietnam (29.4%; 5/17), South Korea (20.0%; 2/10), Romania (15.3%; 2/13), Spain (10.2%; 4/39) and Turkey (9.6%; 14/146).

A carbapenemase gene was not identified in 126 (11.5%) CRE isolates. Carbapenemase-negative CRE isolates were mostly K. pneumoniae (66.7%; 84/126), followed by E. coli (11.9%), Klebsiella aerogenes (9.5%), E. cloacae complex (8.7%) and S. marcescens (3.2%). Interestingly, the percentage of carbapenem-resistant isolates that were carbapenemase negative were markedly higher among K. aerogenes (80.0%) compared with the other species listed above (9.6% to 25.9%). Moreover, most carbapenemase-negative CRE isolates were from E-EU (80 of 126), mainly from Poland (n = 69); 74.2% (69/93) of CRE isolates from Poland were carbapenemase negative.

Aztreonam/avibactam activity was very consistent across the geographical regions evaluated (Table 4). Aztreonam/avibactam MIC_50/90 values were 0.25/0.5 mg/L in W-EU, E-EU and LATAM, and 0.12/0.5 mg/L in APAC. All isolates from W-EU and LATAM were inhibited at an aztreonam/avibactam MIC of ≤8 mg/L (Table 4). Notably, aztreonam/avibactam inhibited 100.0% of MBL producers (MIC_50/90, 0.12/0.5 mg/L) and OXA-48-like producers (MIC_50/90, 0.25/0.5 mg/L) at ≤4 and ≤8 mg/L, respectively (Table 1 and 4). Of the 57 isolates that carried more than one carbapenemase gene, 97.6% were inhibited at an aztreonam/avibactam MIC of ≤8 mg/L (MIC_50/90, 0.25/0.5 mg/L; Table 1). CRE isolates with aztreonam/avibactam MICs >8 mg/L were observed only in Poland [an E. coli and an Enterobacter hormaechei (E. cloacae complex), both collected in 2020), Taiwan (a K. pneumoniae from 2021) and Thailand (a K. pneumoniae from 2019; Table S2).

Table 4.

Activity of aztreonam/avibactam and comparator antimicrobial agents tested against 1098 CRE isolates collected worldwide (ex-USA) in 2019–21 stratified by region and β-lactamase type

Antimicrobial agent	% Susceptible per CLSI/EUCASTa
	W-EU		E-EU		APAC		LATAM		All regions
CRE, n	227		454		177		240		1098
Aztreonam/avibactamb	(100.0)b		(99.6)b		(98.9)b		(100.0)b		(99.6)b
Ceftazidime/avibactam	80.2/	80.2	70.3/	70.3	39.5/	39.5	74.1/	74.1	68.2/	68.2
Meropenem/vaborbactam	77.5/	81.1	50.1/	53.0	40.3/	46.0	78.8/	83.3	60.5/	64.3
Aztreonam	8.8/	7.9	8.8/	8.1	5.6/	5.1	7.1/	5.8	7.9	7.1
Amikacin	70.5/	65.6c	50.4/	37.4c	89.8/	80.8c	64.6/	53.3c	64.0/	53.7c
Gentamicin	54.9/	54.0c	40.7/	38.1c	48.6/	47.5c	37.1/	33.8 c	44.1/	41.9c
Levofloxacin	13.3/	13.3	7.3/	7.3	9.6/	9.6	14.2/	14.2	10.4/	10.4
Minocycline	64.8/	NAd	52.2/	NAd	55.1/	NAd	64.9/	NAd	58.1/	NAd
Tigecyclinee	94.7/	55.9	93.6/	47.6	93.2/	59.3	92.1/	56.2	93.4/	53.1
Colistin	NAd/	90.3	NAd/	69.1	NAd/	79.7	NAd/	60.5	NAd/	73.3
KPC producers, n	151		116		38		168		473
Aztreonam/avibactamb	(100.0)b		(100.0)b		(97.4)b		(100.0)b		(99.8)b
Ceftazidime/avibactam	94.7/	94.7	94.8/	94.8	89.5/	89.5	99.4/	99.4	96.0/	96.0
Meropenem/vaborbactam	92.1/	94.7	95.7/	96.6	92.1/	92.1	98.8/	99.4	95.3/	96.6
Aztreonam	0.0/	0.0	0.0/	0.0	0.0/	0.0	0.0/	0.0	0.0/	0.0
Amikacin	67.5/	64.2c	42.2/	33.6c	92.1/	89.5c	70.2/	59.5c	64.3/	57.1c
Gentamicin	54.0/	52.7c	58.6/	54.3c	39.5/	39.5c	38.7/	36.3c	48.5/	46.2c
Levofloxacin	10.7/	10.7	1.7/	1.7	5.3/	5.3	9.5/	9.5	7.6/	7.6
Minocycline	68.9/	NAd	59.5/	NAd	63.2/	NAd	71.9/	NAd	67.2/	NAd
Tigecyclinee	96.7/	55.6	94.0/	49.1	100.0/	42.1	91.7/	58.9	94.5/	54.1
Colistin	NAd/	91.3	NAd/	70.7	NAd/	55.3	NAd/	53.6	NAd/	70.0
MBL producers, n	44		134		109		60		347
Aztreonam/avibactamb	(100.0)b		(100.0)b		(100.0)b		(100.0)b		(100.0)b
Ceftazidime/avibactam	0.0/	0.0	0.7/	0.7	5.5/	5.5	0.0/	0.0	2.0/	2.0
Meropenem/vaborbactam	27.3/	29.5	11.2/	16.4	12.0/	21.3	21.7/	38.3	15.3/	23.4
Aztreonam	20.5/	18.2	14.9/	14.2	8.3/	7.3	26.7/	23.3	15.6/	14.1
Amikacin	65.9/	59.1 c	50.0/	23.9c	87.2/	77.1c	48.3/	33.3 c	63.4/	46.7c
Gentamicin	40.9/	40.9 c	31.3/	26.9c	48.6/	46.8c	30.0/	23.3c	37.8/	34.3c
Levofloxacin	11.4/	11.4	9.0/	9.0	8.3/	8.3	26.7/	26.7	12.1/	12.1
Minocycline	63.6/	NAd	65.7/	NAd	59.3/	NAd	50.0/	NAd	60.7/	NAd
Tigecyclinee	90.9/	56.8	93.3/	51.5	93.6/	70.6	95.0/	55.0	93.4/	58.8
Colistin	NAd/	90.9	NAd/	76.7	NAd/	86.2	NAd/	76.7	NAd/	81.5
OXA-48-like producers, n	31		144		29		1		205
Aztreonam/avibactamb	(100.0)b		(100.0)b		(100.0)b		(100.0)b		(100.0)b
Ceftazidime/avibactam	87.1/	87.1	91.0/	91.0	37.9/	37.9	100.0/	100.0	82.9/	82.9
Meropenem/vaborbactam	54.8/	61.3	22.2/	22.9	6.9/	6.9	0.0/	0.0	24.9/	26.3
Aztreonam	29.0/	25.8	13.9/	13.9	10.3/	10.3	0.0/	0.0	15.6/	15.6
Amikacin	83.9/	80.6c	36.8/	30.6c	82.8/	62.1c	100.0/	100.0c	50.7/	42.9c
Gentamicin	61.3/	61.3c	29.2/	27.8c	72.4/	72.4c	0.0/	0.0c	40.0/	39.0c
Levofloxacin	16.1/	16.1	9.0/	9.0	0.0/	0.0	0.0/	0.0	8.8/	8.8
Minocycline	61.3/	NAd	34.7/	NAd	20.7/	NAd	0.0/	NAd	36.6/	NAd
Tigecyclinee	87.1/	61.3	96.5/	35.4	96.6/	55.2	0.0/	0.0	94.6/	42.0
Colistin	NAd/	83.9	NAd/	60.4	NAd/	72.4	NAd/	0.0	NAd/	65.4
No carbapenemase, n	13		80		22		11		126
Aztreonam/avibactamb	(100.0)b		(97.5)b		(95.5)b		(100.0)b		(97.6)b
Ceftazidime/avibactam	92.3/	92.3	97.5/	97.5	86.4/	86.4	81.8/	81.8	93.7/	93.7
Meropenem/vaborbactam	92.3/	100.0	91.1/	96.2	95.5/	95.5	90.9/	90.9	92.0/	96.0
Aztreonam	15.4/	15.4	2.5/	0.0	0.0/	0.0	0.0/	0.0	3.2	1.6
Amikacin	86.4/	69.2c	78.8/	70.0c	90.9/	81.8c	63.6/	63.6c	80.2/	71.4c
Gentamicin	69.2/	69.2c	46.2/	46.2c	54.5/	54.5c	45.5/	45.5c	50.0/	50.0c
Levofloxacin	38.5/	38.5	7.5/	7.5	27.3/	27.3	18.2/	18.2	15.1/	15.1
Minocycline	38.5/	NAd	48.8/	NAd	40.9/	NAd	45.5/	NAd	46.0/	NAd
Tigecyclinee	100.0/	53.8	88.8/	58.8	77.3/	50.0	90.9/	27.3	88.1/	54.0
Colistin	NAd/	92.3	NAd/	72.5	NAd/	90.9	NAd/	81.8	NAd/	78.6
CZA-R (>8 mg/L), n	45		135		107		62		349
Aztreonam/avibactamb	(100.0)b		(98.5)b		(98.1)b		(100.0)b		(98.9)b
Meropenem/vaborbactam	27.3/	29.5	11.1/	16.3	13.1/	20.6	23.0/	39.3	15.9/	23.3
Aztreonam	20.0/	17.8	14.8/	14.1	6.5/	5.6	25.8/	22.6	14.9/	13.5
Amikacin	64.4/	57.8c	51.1/	25.2c	87.9/	77.6c	48.4/	33.9c	63.6/	47.0c
Gentamicin	40.0/	40.0c	31.9/	27.4c	52.3/	50.5c	30.6/	24.2c	39.0/	35.5c
Levofloxacin	11.1/	11.1	9.0/	9.0	7.5/	7.5	27.4/	27.4	12.1/	12.1
Minocycline	62.2/	NAd	65.9/	NAd	57.9/	NAd	50.0/	NAd	60.2/	NAd
Tigecyclinee	91.1/	55.6	93.3/	51.9	91.6/	70.1	95.2/	56.5	92.8/	58.7
Colistin	NAd/	91.1	NAd/	76.9	NAd/	86.9	NAd/	77.4	NAd/	81.9
Colistin-R (>2 mg/L), n	22		140		36		94		292
Aztreonam/avibactamb	(100.0)b		(100.0)b		(97.2)b		(100.0)b		(99.7)b
Ceftazidime/avibactam	81.8/	81.8	77.9/	77.9	61.1/	61.1	85.1/	85.1	78.4/	78.4
Meropenem/vaborbactam	68.2/	72.7	43.6/	46.4	68.6/	68.6	84.9/	89.2	61.7/	64.8
Aztreonam	18.2/	18.2	8.6/	8.6	2.8/	2.8	7.4/	7.4	8.2/	8.2
Amikacin	36.4/	36.4c	42.1/	31.4c	83.3	72.2c	43.6/	30.9c	47.3/	36.6c
Gentamicin	63.6/	59.1c	30.0/	30.0c	38.9/	38.9c	30.9/	26.6c	33.9/	32.2c
Levofloxacin	4.5/	4.5	5.7/	5.7	5.6/	5.6	8.5/	8.5	6.5/	6.5
Minocycline	27.3/	NAd	40.7/	NAd	40.0/	NAd	62.4/	NAd	46.6/	NAd
Tigecyclinee	86.4/	13.6	92.1/	32.1	91.7/	33.3	83.0/	41.5	88.7/	33.9
Tigecycline-NS (>0.5 mg/L)e, n	100		238		72		105		515
Aztreonam/avibactamb	(100.0)b		(99.6)b		(98.6)b		(100.0)b		(99.6)b
Ceftazidime/avibactam	80.0/	80.0	72.7/	72.7	55.6/	55.6	74.3/	74.3	72.0/	72.0
Meropenem/vaborbactam	75.8/	79.8	42.4/	46.2	50.7/	52.2	72.1/	77.9	56.1/	60.0
Aztreonam	7.0/	6.0	10.5/	10.5	5.6/	4.2	5.7/	5.7	8.2/	7.8
Amikacin	63.0/	59.0c	40.8/	29.0c	87.5/	80.6c	61.9/	48.6c	55.9/	46.0c
Gentamicin	57.0/	56.0c	30.7/	28.6c	45.8/	45.8c	36.2/	31.4c	39.0/	36.9c
Levofloxacin	6.0/	6.0	3.8/	3.8	6.9/	6.9	1.9/	1.9	4.3/	4.3
Minocycline	30.0/	NAd	27.3/	NAd	23.2/	NAd	39.4/	NAd	29.7/	NAd
Colistin	NAd/	80.8	NAd/	59.9	NAd/	66.7	NAd/	47.6	NAd/	62.4

CZA, ceftazidime/avibactam; R, resistant; NS, non-susceptible.

^aCriteria as published by CLSI,¹¹ EUCAST¹² and US FDA.¹⁷

^bValues in brackets indicate percentage inhibited at ≤8 mg/L aztreonam/avibactam.

^cFor infections originating from the urinary tract. For systemic infections, aminoglycosides must be used in combination with other active therapy.¹²

^dNA, not applicable; breakpoint has not been published.^11,12

^eUS FDA breakpoints were applied due to the absence of CLSI breakpoints for this compound.¹¹ Moreover, EUCAST breakpoints published for E. coli and C. koseri (≤0.5 mg/L) were applied for all Enterobacterales species for comparison.¹²

Ceftazidime/avibactam exhibited good activity (70.3%–80.2% susceptibility) against CRE isolates from W-EU, E-EU and LATAM, but limited activity against CRE isolates from APAC (39.5% susceptibility; Table 4). Moreover, ceftazidime/avibactam exhibited good activity against KPC producers from all regions (89.5%–99.4% susceptible) as well as OXA-48-like producers from W-EU (87.1% susceptible), E-EU (91.0% susceptible) and LATAM (100% susceptible; Table 4). The limited activity of ceftazidime/avibactam against OXA-48-like producers from APAC (37.9% susceptible) was mainly because 62.1% (18/29) of these isolates co-produced an NDM (data not shown).

Meropenem/vaborbactam exhibited good activity against CRE isolates from W-EU (77.5%/81.1% susceptible per CLSI/EUCAST) and LATAM (78.8%/83.3% susceptible per CLSI/EUCAST), but limited activity against CRE isolates from both E-EU (50.1%/53.0% susceptible per CLSI/EUCAST) and APAC (40.3%/46.0% susceptible per CLSI/EUCAST; Table 4). The limited activity of meropenem/vaborbactam in E-EU and APAC can be justified by the high prevalence of OXA-48-like in E-EU and MBLs in APAC (Table 3). It is also important to note that 15.3%/23.4% of MBL producers were categorized as susceptible to meropenem/vaborbactam per CLSI/EUCAST criteria, and susceptibility rates were higher in W-EU (27.3%/29.5%) and LATAM (21.7/38.3%; Table 4). Since vaborbactam does not effectively inhibit MBLs, meropenem/vaborbactam susceptibility among MBL producers can be explained mainly by low expression of the MBL gene, low hydrolysis of meropenem by the MBL, or a combination of these two factors. Another important point to be considered is the higher breakpoint for meropenem/vaborbactam compared with meropenem alone. Among the MBL-producing, meropenem/vaborbactam-susceptible isolates evaluated in this study, 88.8% and 83.8% had MIC values ≤8 mg/L for meropenem and imipenem, respectively (data not shown).

Tigecycline susceptibility rates ranged from 92.1% to 94.7% when US FDA breakpoints were applied but varied from 47.6% to 59.3% when EUCAST breakpoints for E. coli and C. koseri were applied.¹² Colistin susceptibility rates per EUCAST criteria varied widely from 90.3% in W-EU to only 60.5% in LATAM. Moreover, the in vitro activity of comparator agents also varied broadly according to the carbapenemase produced and the antimicrobial resistance phenotype (Table 4).

A summary of the MIC results from the characterization of the four isolates exhibiting aztreonam/avibactam MIC values >8 mg/L is shown in Table S2. A four amino acid insertion (YRIK) in the PBP3 and a CMY-141-encoding gene were detected in the E. coli strain 1183154 from Poland. The E. cloacae species complex isolate 1183311, also from Poland, was identified as E. hormaechei; it overexpressed act-17 and carried bla_CTX-M-15. This isolate had a premature stop codon within OmpF and alterations in OmpC and PBP3.

The K. pneumoniae isolate 1116221 from Thailand carried bla_DHA-1 and had a premature stop codon at position 43 of OmpK36, whereas a WT sequence was observed for PBP3. Lastly, the K. pneumoniae isolate 1215485 from Taiwan carried KPC-2 and a VEB variant with a single amino acid substitution at residue 75 when compared with VEB-1 (VEB-1_P75L), designated bla_VEB-31. This isolate also had multiple alterations in the OmpK36, whereas WT sequences were observed for PBP3. Finally, the main β-lactamase genes (i.e., bla_CMY141, bla_CTX-M-15, bla_DHA-1 and bla_KPC-2) detected in each of these four isolates with elevated aztreonam/avibactam MICs were present in single copies (Table S2).

Discussion

The spread of CRE represents a major challenge for antimicrobial treatment worldwide. While the approval of new BL/BLI compounds in the last decade, such as ceftazidime/avibactam, meropenem/vaborbactam and imipenem/relebactam, represented remarkable progress for the treatment of infections caused by CRE, their spectrum of activity is reduced in regions where MBL-producing Enterobacterales are prevalent.¹⁸ Furthermore, because these newer BL/BLIs lack activity against MBL-producing organisms, the increased use of these compounds, especially in regions where serine carbapenemases represent the main mechanisms of carbapenem resistance among CREs, may increase the occurrence of MBL-producing and non-carbapenemase-producing CREs.¹⁹

Cefiderocol is the only β-lactam compound with activity against MBL-producing strains, including those that co-produce ESBLs and/or serine carbapenemases, currently approved by the US FDA and EMA.²⁰ The absence of cefiderocol in this investigation is a limitation of the study. Other compounds with in vitro activity against MBL-producing CREs include colistin, tigecycline, fosfomycin, minocycline and some aminoglycosides; however, these compounds have significant toxicity issues and/or spectrum deficiencies that prevent their use as empirical or guided treatment for life-threatening infections.²

Decreased susceptibility to aztreonam/avibactam (MIC, >8 mg/L) was observed in only four CRE isolates, two K. pneumoniae, one E. coli and one E. hormaechei (E. cloacae complex), which showed an MDR profile (Table S2). The BL and BL/BLI MIC results obtained against K. pneumoniae 1116221 were likely due to the production of DHA-1 combined with the lack of OmpK36, as previously described.²¹ Decreased susceptibility to aztreonam/avibactam in E. coli isolates was reported by other investigators and seemed to be caused mostly by the association of PBP3 alterations with the production of CMY β-lactamases, similar to what was observed here with E. coli 1183154.^22,23 Furthermore, E. coli 1183154 produced altered OmpC and OmpF, which compromised the entry of BLs, including carbapenems.²⁴ The decreased susceptibility of E. hormaechei 1183311 to aztreonam/avibactam could be due to the absence of OmpF and hyperproduction of AmpC (ACT-17). Although the mutations observed in the OmpC and PBP3 could also provide a deleterious effect on the activity of this combination, this remains to be elucidated.

K. pneumoniae 1215485 lacked OmpK35 and showed numerous mutations within OmpK36, but their effect on aztreonam/avibactam MIC is not well understood. However, the lack of OmpK35 and alterations in OmpK36 could compromise the access of BL agents other than carbapenems to the periplasmic space and act synergistically with the presence of both bla_KPC-2 and the novel bla_VEB-31 on the decreased susceptibility to various BL agents. However, these resistance mechanisms could not completely explain the elevated MIC for aztreonam/avibactam and ceftazidime/avibactam, as avibactam should inhibit both KPC-2 and VEB-31. The VEB-31 presented here was fully conserved at 8 of 12 key residues that form the avibactam-binding pocket in class A β-lactamases based on the CTX-M-15 crystal structure.²⁵ Compared with VEB-1, VEB-31 had an altered amino acid that resided close to the serine active site (S⁷⁰XXK). Studies are underway to help elucidate whether this alteration could jeopardize avibactam binding. Nevertheless, a synergistic effect of multiple resistance mechanisms seems to be necessary to increase the aztreonam/avibactam MIC, but these mechanisms vary markedly by bacterial species, as shown here and elsewhere.^21,22,26

In the present study, we evaluated the in vitro activity of aztreonam/avibactam against a large collection of CRE isolates (n = 1098) obtained from a contemporary (2019–21) collection of 24 924 consecutively collected Enterobacterales isolates from 69 medical centres located in 36 countries. Aztreonam/avibactam was active against 99.6% of the CRE isolates, including 100.0% of the MBL producers and 98.9% of the ceftazidime/avibactam-resistant isolates. Our results corroborate previous publications by other investigators.^27–29 Moreover, although we did not include US CRE isolates in our study, the activity of aztreonam/avibactam was recently evaluated against MBL-producing CRE isolates from the USA by investigators from the CDC.³⁰ Bhatnagar et al.³⁰ evaluated 64 MBL-producing, ceftazidime/avibactam-resistant isolates collected from 24 states by four Antimicrobial Resistance Laboratory Network regional laboratories. Aztreonam/avibactam MIC_50/90 values were 0.5/8 mg/L and 96.9% (62/64) of isolates were inhibited at ≤8 mg/L aztreonam/avibactam. Thus, these results, coupled with results published by other investigators, indicate that aztreonam/avibactam provides worldwide coverage of CRE.

In summary, aztreonam/avibactam demonstrated potent activity against a large collection of contemporary CRE isolates, including MBL producers and ceftazidime/avibactam-resistant isolates. The results of this large international investigation support the clinical development of aztreonam/avibactam.

Funding

This study at JMI Laboratories was supported by Pfizer, Inc (New York, NY, USA). Pfizer was involved in the decision to present these results. H. S. Sader, V. Kantro, J. H. Kimbrough, M. Castanheira and R. E. Mendes are employees of JMI Laboratories, which was a paid consultant to Pfizer in connection with the development of this manuscript. Pfizer had no involvement in the collection, analysis and interpretation of data.

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Supplementary data

Tables S1 and S2 are available as Supplementary data at JAC-AMR Online.