Molecular Epidemiology, Antimicrobial Resistance Mechanisms, and Virulence Profiling of Acinetobacter Species in Tertiary Care Hospitals: A Systematic Review

Abstract

Background

Acinetobacter baumannii has emerged as one of the most consequential nosocomial pathogens of the modern antibiotic era, distinguished by an extraordinary and continually expanding capacity for antimicrobial resistance acquisition, prolonged environmental persistence through biofilm formation, and repeated involvement in healthcare-associated outbreaks. The World Health Organization has designated carbapenem-resistant Acinetobacter baumannii (CRAB) a critical-priority pathogen for which the development of new antibacterial agents is urgently required, reflecting both the scale of the clinical threat and the narrowing range of effective therapeutic options.

Objective

To systematically synthesise the published evidence from January 2015 to July 2026 on the molecular epidemiology, antimicrobial resistance mechanisms, and virulence determinants of Acinetobacter species isolated from clinical specimens in tertiary care hospitals worldwide, with particular emphasis on the comparatively under-characterised evidence base from the Indian subcontinent and, specifically, North-East India.

Methods

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 Statement and the PRISMA-S extension for search reporting. Eight electronic databases — PubMed/MEDLINE, Scopus, Web of Science, Embase, ScienceDirect, SpringerLink, Wiley Online Library, and Taylor & Francis Online — were searched using Boolean strategies combining Medical Subject Headings (MeSH) and free-text keywords, supplemented by Google Scholar and manual reference-list screening. Two reviewers independently screened records against predefined eligibility criteria and extracted data using a standardised template; discrepancies were resolved by consensus or third-reviewer adjudication. Methodological quality was appraised independently by two reviewers using the Joanna Briggs Institute (JBI) Critical Appraisal Checklist for prevalence studies and the Newcastle-Ottawa Scale for observational studies. Given substantial clinical and methodological heterogeneity across included studies, findings were synthesised narratively, organised by thematic domain; pooled estimates are reported only where drawn directly from previously published meta-analyses identified within the search, with 95% confidence intervals reported where available.

Results

Of 4,287 records identified, 125 studies met the eligibility criteria and were included in the qualitative synthesis (68 with a global or multi-regional focus, 42 from India, and 15 specifically from North-East India). A previously published meta-analysis reported a pooled prevalence of carbapenemase-producing Acinetobacter baumannii of 56.97% across African clinical specimens. Globally, the international clone corresponding to sequence type STPas2 (formerly designated ST2 under the Pasteur multilocus sequence typing scheme) predominated among carbapenem-resistant isolates, with STOxf208 prominent among isolates from the United States and China and STOxf191 prevalent across East Asia. The class D carbapenemase OXA-23 remained the most widespread resistance determinant globally, and co-production of OXA-23 with New Delhi metallo-beta-lactamase (NDM) emerged as a concerning and increasingly reported trend, particularly across the Indian subcontinent. Within India, carbapenem susceptibility as low as approximately 12% was reported, with ST2 (international clone IC2) predominant in most tertiary care settings alongside extensive genomic-island-mediated horizontal resistance-gene acquisition, although multi-clonal populations were documented in South India. In North-East India, 91.27% of isolates were carbapenem-resistant, blaNDM (70.34%) and blaOXA-23 (100%) were the dominant resistance determinants, and 9.30% of isolates met criteria for pan-drug resistance. The biofilm-associated genes bap and ompA were detected in 90–95% of clinical isolates across included studies, with a pooled biofilm-production prevalence of 65.63% (95% CI: 56.70–74.56%) reported in a dedicated meta-analysis.

Conclusion

This review demonstrates the alarming global dissemination of multidrug- and extensively drug-resistant Acinetobacter baumannii in tertiary care settings, driven by the international spread of a small number of high-risk clones, horizontal transfer of carbapenemase-encoding genes, and biofilm-mediated environmental and host persistence. Critical knowledge gaps remain regarding the genomic-level molecular epidemiology of Acinetobacter in North-East India — where no multilocus sequence typing, whole-genome sequencing, or systematic virulence-gene profiling data were identified — and regarding the mechanistic relationship between resistance and virulence co-regulation; both warrant prioritised research investment.