The in-vitro activity of ceftazidime avibactum against multidrug resistant gram-negative isolates.

The global rise of multidrug-resistant (MDR) Gram-negative pathogens has become a pressing public health concern(1-3). Resistance to β-lactam antibiotics, driven largely by β-lactamase production, severely restricts therapeutic options for life-threatening infections. Globally, ESBL-producing organisms have escalated the dependence on carbapenems (3,4), however, the emergence of carbapenemase-producing bacteria—such as carbapenem-resistant Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae—poses an even greater challenge. This trend underscores the urgent need for novel agents to combat resistant pathogens(1,5).

Combining β-lactam antibiotics with β-lactamase inhibitors has been one strategy, but traditional inhibitors (clavulanic acid, tazobactam, sulbactam) show limited activity against many clinically important enzymes. Consequently, first-generation combinations oftenfail against MDR organisms(6). Avibactam, a novel non-β-lactam inhibitor, expands the spectrum by targeting Ambler class A, class C, and selected class D enzymes(7,8). Studies demonstrate that avibactam restores ceftazidime activity against ESBL-, AmpC-, Klebsiella pneumoniae carbapenemase (KPC)-, and OXA-48-producing Enterobacteriaceae, as well as resistant P. aeruginosa(9-11 )

Ceftazidime, a third-generation cephalosporin, acts by binding penicillin-binding proteins, disrupting cell wall synthesis, and causing bacterial lysis(12,13). Avibactam itself lacks antibacterial activity but protects ceftazidime from enzymatic degradation via reversible covalent acylation(8,14). The fixed 4:1 ceftazidime–avibactam combination is administered intravenously and approved for complicated urinary tract infections, intra-abdominal infections, and hospital-acquired pneumonia caused by Gram-negative bacteria(12).

This study evaluates the susceptibility profile of ceftazidime–avibactam against Gram-negative bacilli from clinical specimens, aiming to generate data on its therapeutic potential. With rising ESBLs and carbapenem-resistant Enterobacterales, assessing novel agents is imperative.

This cross-sectional observational study was conducted in the Department of Microbiology, Acharya Shri Chander College of Medical Sciences & Hospital, Jammu (J&K), India, from January–December 2025, following Institutional Ethics Committee approval. Clinical samples were processed using standard microbiological techniques. Antimicrobial susceptibility testing was performed by Kirby–Bauer disc diffusion and interpreted as per CLSI M100, 2025(15). Antibiotics tested included β-lactams, β-lactam/β-lactamase inhibitor combinations, aminoglycosides, fluoroquinolones, carbapenems, polymyxins, tigecycline, tetracycline, and fosfomycin. The isolates were categorized as MDR (resistant to ≥3 classes), XDR (resistant to all but one or two classes), and PDR (resistant to all available agents). ESBL detection was performed by Double Disk Synergy Test, carbapenemase production confirmed by mCIM, and MDR Pseudomonas aeruginosa which is non-susceptibility to at least one agent in three or more antimicrobial categories, in accordance with international consensus definitions(21) Confirmed ESBL/CRE Enterobacteriaceae and MDR P. aeruginosa underwent E strip testing for ceftazidime–avibactam, with MIC values interpreted per manufacturer’instructions.

Out of a total of 1800 isolates, 576 (32%) were culture positive. Among these, 329 (57%) were identified as gram-negative bacteria.The prevalence of multidrug resistance (MDR) among gram-negative isolates was found to be 78%(257).The most common gram negative organisms isolated as MDR were E.coli 84(33%),Pseudomonas 63(25%),Klebsiella 58(23%),Acinetobacter 30(12%) ,Enterobacter11(4.2%), Proteus 7(3%) and Citrobacter 4(2%).(Figure 1)

Figure 1: Species wise distribution of MDR Gram Negative bacterial isolates

The antimicrobial susceptibility profile of 257 Gram-negative isolates revealed widespread resistance across multiple drug classes. Ampicillin and Cotrimoxazole  demonstrated near-universal resistance (94–100%) &(88–100%) respectively among all species tested, confirming its limited clinical utility. Fluoroquinolones & second- and third-generation cephalosporins  exhibited high resistance rates (87–100%)&(80-100%) particularly in Pseudomonas and Acinetobacter & Klebsiella respectively , while Fosfomycin and Nitrofurantoin retained activity against Enterobacteriaceae but were not applicable to non-fermenters.

Aminoglycosides showed moderate efficacy, with resistance ranging between (54-90%). Carbapenem resistance was alarmingly high, with Imipenem showing resistance rates of( 82–89%) and Meropenem( 66–84%). These findings indicate that carbapenems have limited therapeutic utility in our setting, reflecting widespread resistance among Gram-negative pathogens.  Resistance was notable in Enterobacter (89%) and Pseudomonas (85%), suggesting emerging carbapenem resistance.
Fosfomycin and Nitrofurantoin retained activity against Enterobacteriaceae but were not applicable to non-fermenters. Last-resort agents demonstrated resistance rates  as Tigecycline(46%), Polymyxin B(28%), and Colistin(27%)  raising concern about diminishing therapeutic options.(Table 1)

Table 1:  Resistance Pattern of MDR gram negative isolates

Ceftazidime-avibactam demonstrated high in-vitro activity against multidrug-resistant Enterobacteriaceae, with susceptibility rates of 95% in E. coli, 91% in Enterobacter spp, 86% in Klebsiella & Proteus spp., and 83% in Citrobacter spp.Activity against Pseudomonas aeruginosa was moderate, with 75% of isolates susceptible. In contrast, Acinetobacter spp. showed poor susceptibility (17%).(Table 2)

Table 2: Susceptibility of Ceftazidime-Avibactam Against MDR Gram-Negative Isolates

Our study demonstrated a 78% prevalence of MDR among Gram-negative isolates, markedly higher than the national average of 33% reported by CDDEP( Center for Disease Dynamics, Economics & Policy). This difference likely reflects hospital-based sampling, where selective antibiotic pressure and nosocomial transmission contribute to elevated resistance rates compared to community settings(16). Comparable findings have been reported in Indian multicenter studies. The ICMR AMR network reported MDR rates of 65–82% across E. coli, Klebsiella pneumoniae, and Acinetobacter baumannii, while multicenter ICU studies documented overall MDR prevalence of 68–74%(17,18) .

Ampicillin resistance was nearly universal (94–100%), confirming its limited clinical utility against Gram-negative infections. Our study also showed resistance to second- and third-generation cephalosporins exceeded 80–100% in Klebsiella, Pseudomonas, and Acinetobacter. Similar findings were seen in a study conducted by Chavan & Panwar in 2020(19) in which Ampicillin was largely ineffective & 55.2% were resistant to 3rd-gen cephalosporins . Mohamudha Parveen R, Harish BN, Parija SC(20) also had similar results in which high  resistance was noted against Ampicillin and  57% resistance to 3rd-generation cephalosporins .Both the studies reflected widespread dissemination of extended-spectrum β-lactamases (ESBLs) and plasmid mediated AmpC enzymes. Our isolates showed >80–100% resistance to fluoroquinolones, a pattern that closely parallels the description by Paterson & Bonomo (2005)(21), who emphasized that ESBL-producing strains often carry plasmid-mediated co-resistance mechanisms, severely limiting treatment options. Our findings can be  explained at molecular level by Hooper and Jacoby(22) who explained the basis of fluoroquinolone resistance was due to QRDR mutations,efflux pumps and plasmid mediated.

Our study demonstrated moderate efficacy among Aminoglycosides, with resistance rates of 54%-90% to Gentamicin and Amikacin respectively.  Cullen LBL et.al(23) demonstrated enzymatic modification and efflux pumps as key drivers, with gentamicin resistance rates surpassing 70%, consistent with our 54–90% range. Gupta V, Bansal N(24) reported Gentamicin resistance ~75% and Amikacin resistance ~69% among Gram-negative pathogens which resonates with our dataset.

Carbapenems(63-89%) retained partial activity, with resistance to Imipenem ranging from 82–89% and Meropenem from 66-84%. These findings are consistent with national surveillance data from the Kaur J et.al,(25), which reported Klebsiella pneumoniae resistance to Imipenem at 68–69% and Meropenem at 62–65%, while Acinetobacter baumannii exhibited resistance exceeding 80%. Similarly, a systematic review by Kapoor et al. (2026)(26) highlighted carbapenem resistance rates of ~59% in A. baumannii and ~42% in K. pneumoniae, with NDM and OXA-48-like carbapenemases  predominating. A meta-analysis by Khan et al. (2026)(27) further confirmed pooled prevalence of carbapenem-resistant Enterobacteriaceae (~9%) and non-Enterobacteriaceae (~16%), underscoring the widespread dissemination of resistance genes across India. Taken together, these studies corroborate the present findings, suggesting that carbapenem resistance in India has reached critical levels, particularly for Imipenem, where resistance rates above 80% are now frequently observed. The convergence of local data with national and systematic reviews emphasizes the urgent need for strengthened antimicrobial stewardship and surveillance programs to mitigate the therapeutic challenges posed by carbapenem-resistant  pathogens.

In our study, resistance to Polymyxin B, and Colistin ranged from 18-28% respectively, underscoring the narrowing therapeutic options for multidrug-resistant Gram-negative infections. Comparable findings have been reported in Indian settings. Kalaivani et al. (2025)(28) documented resistance rates of 14.7% to colistin and 10.1% to polymyxin B among XDR Enterobacterales in Puducherry. Similarly, Singh et al. (2026)(29)  highlighted the rapid emergence of colistin resistance in India, driven by plasmid-mediated mcr genes and chromosomal mutations. These reports, though showing slightly lower percentages than ours, confirm the national trend of rising resistance to last-line agents, reinforcing the urgent need for antimicrobial stewardship and novel therapeutic strategies.The resistance of Tigecycline ranged from(30-46%) in our study,comparable findings have been reported in Indian settings. Jyoti et al. (2024)(30) documented tigecycline resistance among carbapenem-resistant isolates in Lucknow& Acharya et al. (2026)(31) provided genomic evidence of efflux pump mutations driving tigecycline resistance in Acinetobacter baumannii.

Ceftazidime–avibactam (CZA) demonstrated high in vitro activity against multidrug resistant Enterobacteriaceae in our study, with susceptibility rates of 95% in E. coli, 91% in Enterobacter spp., 86% in Klebsiella and Proteus spp., and 83% in Citrobacter spp.. Activity against Pseudomonas aeruginosa was moderate, with 75% of isolates susceptible, whereas Acinetobacter spp. showed poor susceptibility. These findings emphasize the therapeutic potential of CZA against carbapenem resistant Enterobacterales, while highlighting its limitations against non fermenters such as Acinetobacter.

 Priyadarshi et al. (2023)(32) documented >90% susceptibility among Enterobacterales isolates, moderate activity (~72%) against Pseudomonas aeruginosa, and poor activity against Acinetobacter spp. in bloodstream  infections which matches our data. Todi et al. (2024)(33) observed microbiological success in ~76% of patients treated with CZA, with outcomes consistent with its strong activity against Enterobacterales and moderate efficacy against Pseudomonas. Taken together, these Indian studies corroborate our findings, confirming that CZA retains excellent activity against Enterobacterales, moderate efficacy against Pseudomonas aeruginosa, and poor activity against Acinetobacter spp. Similarly, the ATLAS India surveillance program (2018–2019)(34) reported CZA activity against 85–95% of carbapenem resistant Enterobacterales, ~70% of Pseudomonas aeruginosa, and <20% of Acinetobacter spp. This concordance highlights the regional relevance of our data and reinforces the role of CZA as a valuable therapeutic option in India for multidrug resistant Enterobacterales infections, while cautioning against its use as monotherapy for Acinetobacter infections.

Our findings reinforce the urgent need for antimicrobial stewardship, surveillance, and the development of novel therapeutic agents. The high prevalence of MDR Gram-negative pathogens, coupled with emerging resistance to carbapenems and last-resort agents, poses a significant public health challenge. Ceftazidime-avibactam offers promise against Enterobacteriaceae but remains ineffective against Acinetobacter, necessitating alternative strategies for these infections.

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