Antimicrobial resistance (AMR) is a significant public health concern in the 21st century, driven by changes in bacteria that diminish the potency of antibiotics [1]. By 2050, infections due to AMR are predicted to be the leading cause of mortality, contributing up to 10 million fatalities annually. Asia and Africa are expected to account for 90 % of these deaths [2]. In Bangladesh, from 2015 to 2018, multidrug resistant (MDR) organisms gradually increased year by year. There were 33 % MDR organisms in 2015, which climbed significantly, reaching 62 % in 2019 [3].

The rise in carbapenem resistance has increased, posing a serious threat by the introduction and broad dissemination of carbapenem-resistant Enterobacterales (CRE) and non-fermenters such as Pseudomonas aeruginosa and Acinetobacter baumannii [4]. According to a recent study, 31.3 % of organisms in Asia were resistant to carbapenem. The highest occurrence was noted in Southeast Asia (42.8 %), succeeded by South Asia (32.3 %) [5]. A study in Bangladesh reported that 70.28 % of Gram negative bacteria were carbapenem resistant [6].

Carbapenem resistance can arise via modifications to the penicillin-binding protein in the cell wall of bacteria, an upsurge in efflux pumps, a reduction in membrane permeability, and the emergence of carbapenemase enzymes [7,8]. According to a recent global assessment, carbapenemases have been identified in approximately 85 % of CRE worldwide, with significant variation between regions, ranging from 76 % in Latin America to 90 % in the Middle East and Africa [9].

The Ambler scheme of classification categorizes carbapenemases as A, B, or D β-lactamases [10]. Ambler class A carbapenemase encompass Klebsiella pneumoniae carbapenemases (KPCs), Serratia marcescens enzyme (Sme), non-metalloenzyme carbapenemase (NMC)-A, imipenem-hydrolyzing β-lactamase (IMI), and certain allelic variations of Guiana extended spectrum (GES)/integron-borne cephalosporinase (IBC) [11] .

Class B metallo-β-lactamases (MBLs) contain the blaNDM, blaVIM, and blaIMP genes [12]. Metallo-β-lactamases can hydrolyze several kinds of beta-lactams, but not monobactam like aztreonam [13]. Asia constitutes 58.15 % of the worldwide prevalence of NDM-producing organisms, with China and India leading the way, followed by Europe (16.8 %) and the American continents (10.8 %) [14]. Bangladesh is facing a growing challenge from NDM-1 producing bacteria, as studies have identified high levels of these resistant pathogens in wastewater linked to hospital settings [15]. A study revealed that around 3.5 % of Gram-negative clinical isolates were NDM-1 producing, underscoring the growing prevalence of this resistance mechanism in clinical surroundings [16]. Another study in Bangladesh detected 83 % of imipenem resistant Pseudomonas spp as metallo-β-lactamase (MBL) producers [17].

Noncarbapenemase mechanisms, such as upregulation of the MexAB-OprM efflux pump, increased synthesis of AmpC β-lactamase, and activation of the OprD outer membrane protein, mostly contribute to carbapenem resistance in P. aeruginosa [10].

However, ceftazidime-avibactam (CZA) has an effectiveness of >70 % in curing CR-GNB infection [18]. When used with ceftazidime, avibactam successfully inhibits class A and class C β-lactamase enzymes. Yet, CZA is not efficacious against strains containing class B metallo-β-lactamases (MBLs) such NDM, VIM, and IMP [19,20]. Even before extensive clinical application of CZA, there were reports of resistance to CZA [21]. Aztreonam (ATM) remains stable against MBLs, but it is frequently ineffective due to its inactivation by ESBLs, KPCs, and other cephalosporinases that are typically found in the background of bacteria that produce MBLs [19,20]. As avibactam secures aztreonam from being hydrolyzed by serine β-lactamases and aztreonam is stable to the hydrolysis of MBLs like NDM, the combination of CZA and ATM is a helpful treatment for infections caused by beta-lactamase-producing Gram-negative organisms that produce serine β-lactamases, MBLs, or both [22]. Additionally, the Infectious Diseases Society of America proposed CZA-ATM as an alternate for infections caused by MBL producing organisms [23].

Various susceptibility testing methods were employed for CZA-ATM susceptibility testing, including disk stacking, gradient strip stacking, gradient strip crossing, Broth Disk Elution method [24]. However a study in UK described E- strip disk diffusion as rapid, easy to perform and practical method to test CZA-ATM combination [25]. According to Harris et al., the CLSI in 2024 validated the broth disk elution (BDE) as an acceptable procedure for assessing Enterobacterales' sensitivity to aztreonam with ceftazidime-avibactam [26,27]. Moreover, a study in China described Disk stacking plus micro-elution (DSE) method for testing CZA-ATM combination [28].

This study is distinctive, being the very first report on CZA-ATM susceptibility from Bangladesh. But our neighbor India has already explored CZA-ATM susceptibility through a number of studies [[29], [30], [31]]. To address the urgent need for effective treatments against serine and MBL producing CR-GNB in Bangladesh, the in vitro activity of the CZA-ATM was evaluated against carbapenem resistant Klebsiella spp., E. coli, and Pseudomonas spp., using E-strip disk diffusion, BDE, and DSE methods.