Comparison between the antimicrobial activity of colistin and ceftazidime-avibactam against multidrug-resistant Pseudomonas aeruginosa isolated from patients with ventilator-associated pneumonia in intensive care units of Kasr Al-Ainy hospitals

With the growing challenge of antibiotic resistance, evaluating the effectiveness of existing antimicrobials is crucial. This work aimed to compare the antimicrobial activity of colistin versus ceftazidime/avibactam (CAZ/AVI) against MDR P. aeruginosa. It also aimed to estimate the prevalence of P. aeruginosa as a causative agent for VAP and the rate of its multi-drug resistance. This observational cross-sectional study was conducted from January to June 2025 on ICU patients with VAP at Kasr Alainy Hospitals. A total of 120 endotracheal aspirates were collected from patients and cultured. P. aeruginosa was identified, tested for antibiotic susceptibility by the Kirby-Bauer disc diffusion method, and underwent colistin minimal inhibitory concentration (MIC) testing via broth microdilution and CAZ/AVI MIC determination using E-test, following the Clinical and Laboratory Standards Institute 2025 guidelines. Of 120 ICU patients diagnosed with VAP, P. aeruginosa was isolated from 47 (39.2%). Among these, 40 isolates (85.1%) were identified as MDR. Of 40 MDR P. aeruginosa isolates, 32 (80%) were susceptible to colistin, with MIC₅₀ and MIC₉₀ values of 2 µg/mL and 4 µg/mL, respectively. In contrast, only 5 (12.5%) isolates were susceptible to CAZ/AVI, with both MIC₅₀ and MIC₉₀ exceeding 16 µg/ml, indicating widespread resistance. The difference in susceptibility between the two antibiotics was statistically significant (p < 0.01). Colistin showed significantly greater in vitro effectiveness than CAZ/AVI against MDR P. aeruginosa from VAP patients in the ICU, supporting its continued use despite nephrotoxicity concerns. The limited activity of CAZ/AVI underscores the need for ongoing local antimicrobial surveillance and strict stewardship practices.