Abstract
Background:
Carbapenem-resistant Pseudomonas aeruginosa (CRPA) and Acinetobacter baumannii (CRAB) are major hospital pathogens associated with high mortality. Resistance is multifactorial, involving carbapenemase production, efflux pump overactivity, and biofilm formation; however, their phenotypic interrelations remain unclear in resource-limited settings such as Libya.

Aim:
This study aimed to investigate the correlation between efflux pump activity, biofilm formation, and carbapenem resistance among Pseudomonas aeruginosa (P. aeruginosa) and Acinetobacter baumannii (A. baumannii) isolates from Libyan hospitals.

Methods:
A cross-sectional study (January–June 2024) analyzed 150 nonduplicate isolates (75 P. aeruginosa and 75 A. baumannii) identified by API 20 NE. Antimicrobial susceptibility testing was performed following the CLSI 2024 guidelines. Carbapenemase activity was assessed using the Carba NP and imipenem–EDTA synergy tests. Efflux activity was detected using the ethidium bromide agar cartwheel and confirmed with CCCP inhibition. Biofilm formation was quantified using a crystal violet microtiter assay. Data were analyzed using sχ², Pearson correlation, and logistic regression; the multidrug resistance (MAR) index summarized.

Results:
Both species showed high resistance: imipenem 69–71%, meropenem 67–73%, and ceftazidime (65%–72%), with no significant species difference. Carba NP positivity was 63% in P. aeruginosa and 68% in A. baumannii. Active efflux pump activity occurred in 63% and 68% of P. aeruginosa and A. baumannii isolates, respectively. Strong biofilm formation was recorded in 27% of P. aeruginosa and 35% of A. baumannii isolates. Efflux activity correlated strongly with carbapenem resistance (r=0.71, p<0.001), and biofilm strength moderately (r=0.66, p<0.001). Logistic regression confirmed efflux-active (OR = 3.14) and strong-biofilm (OR = 2.69) isolates as independent resistance predictors. The MAR index was higher in A. baumannii (0.61) than in P. aeruginosa (0.53; p<0.001).

Conclusion:
Pseudomonas aeruginosa and A. baumannii in Libyan hospitals exhibit high multidrug and carbapenem resistance. Efflux activity and biofilm strength independently predict carbapenem resistance, highlighting the need for rapid carbapenemase screening and infection control.

Key words: Pseudomonas aeruginosa; Acinetobacter baumannii; carbapenem resistance; efflux pump; biofilm; Libya; MAR index.