Subspecies Distribution, Polymicrobial Associations, and Antimicrobial Resistance in Clinical Acinetobacter Isolates: A Prospective Cross-Sectional Study From a Tertiary Care Hospital in India

Introduction The global rise of multidrug-resistant Acinetobacter (A.), particularly A. baumannii, is well recognized. However, real-world data linking subspecies distribution with resistance patterns and polymicrobial dynamics in real-world tertiary care settings remain limited. This study addresses this gap by providing a comprehensive and clinically relevant profile of Acinetobacter isolates, emphasizing species-specific resistance patterns and polymicrobial associations. Methods A prospective cross-sectional study was conducted over six months (July-December 2023) in a tertiary care teaching hospital in South India. Clinical specimens yielding Acinetobacter species, including polymicrobial cultures, were analyzed. Antimicrobial susceptibility testing (AST) was performed using VITEK 2 COMPACT (BioMérieux, Marcy-l'Étoile, France) and Kirby-Bauer according to Clinical and Laboratory Standards Institute (CLSI) 2022 guidelines. The minimum inhibitory concentration (MIC) for polymyxin B was determined using microbroth dilution. Data were analyzed using Statistical Package for the Social Sciences (SPSS) Statistics version 21 (IBM Inc., Armonk, New York). Results Among 181 isolates, A. baumannii was the most prevalent, 118 (65.2%), followed by A. lwoffii, 39 (21.5%), and A. haemolyticus, 24 (13.3%). A strong predominance was observed in the intensive care unit (ICU), which accounted for 143 (79.0%) isolates. A key finding was the clear variation in resistance profiles across subspecies. A. baumannii demonstrated very high multidrug‑resistant (MDR), 102 (86.4%), and extensively drug‑resistant (XDR) 16 (13.6%) rates, whereas non‑A. baumannii species exhibited substantially lower resistance levels. Carbapenem resistance remained alarmingly high, with meropenem resistance at 145 (80.1%) and imipenem resistance at 124 (68.5%) across both ICU and non‑ICU settings, suggesting significant institutional selection pressure. Despite this, susceptibility to polymyxin B, 168 (92.8%), and tigecycline, 172 (95.0%), remained largely preserved. Polymicrobial infections were identified in 28 (15.5%) cases and showed a significant association with species type, with A. lwoffii exhibiting higher co‑isolation rates than A. baumannii. The most common co‑pathogens were Pseudomonas and Klebsiella species. Conclusion This study demonstrates clear species-level differences in both resistance and polymicrobial behavior within Acinetobacter, emphasizing the clinical importance of routine subspecies identification. The combination of high carbapenem resistance across care settings and species-specific polymicrobial trends highlights the need for precision antimicrobial stewardship, early targeted therapy, and unit-specific antibiograms. Together, these findings offer practical evidence to guide improvements in empiric treatment strategies in high-risk hospital settings.