Antimicrobial Sensitivity and Resistance Trends in Bacterial Respiratory Isolates from Adults with Lower Respiratory Tract Infections

Abstract

Objective: To characterize the bacteriological profile and antimicrobial resistance patterns of respiratory isolates from adults with LRTIs at a tertiary care hospital in Karachi, Pakistan. Methods: A retrospective observational study was conducted at Combined Military Hospital (CMH) Malir, Karachi, from October 2024 to March 2025. Of 112 symptomatic hospitalized adults evaluated, 67 provided adequate respiratory specimens. Of these, 24 (36%) were Tuberculosis (TB) positive and were excluded from bacterial culture analysis. The remaining 43 (64%) underwent bacterial culture. Isolates were identified using standard biochemical methods. Antimicrobial susceptibility testing followed CLSI guidelines (M100, 36th edition; M07, 12th edition), with broth microdilution exclusively for colistin. SPSS version 25 was used for statistical analysis. Results: Gram-negative bacteria accounted for 88% of isolates. Acinetobacter baumannii (35%), Klebsiella pneumoniae (21%), and Pseudomonas aeruginosa (14%) predominated. All Gram-positive isolates were methicillin-resistant Staphylococcus aureus (MRSA, 11%), susceptible to linezolid and vancomycin. A. baumannii showed 80% carbapenem resistance; K. pneumoniae showed 77% resistance to fluoroquinolones and co-trimoxazole, while 67% P. aeruginosa isolates were carbapenem-resistant. Prior antibiotic use (49%), smoking (46%), and COPD (19%) were common risk factors. Conclusion: High resistance among respiratory Gram-negative pathogens underscores the need for antimicrobial stewardship, culture-guided prescribing, and continuous local surveillance