Exploratory Evaluation of Susceptibility Patterns of Clinical Isolates of Acinetobacter baumannii Towards Polymyxins and Different Cationic Antimicrobials.

In vitro susceptibility study using broth microdilution to determine MICs of cationic antimicrobials against A. baumannii isolates.

“Minimum inhibitory concentrations (MICs) of polymyxin B, colistin, three cationic disinfectants (polyquaternium-1; PQ-1), polyhexamethylene biguanide (PHMB), and chlorhexidine (CHX), and four Antimicrobial peptides (AMPS: LL-37, melimine, Mel4, and lactoferricin) were determined using broth microdilution.”

Clinical isolates of Acinetobacter baumannii from Australia, Pakistan, and Switzerland, along with standard strains and polymyxin-resistant mutant strains.

“A diverse panel of clinical A. baumannii isolates from Australia (n = 9), Pakistan (n = 15), and Switzerland (n = 3), and standard strains (n = 3), including polymyxin-resistant laboratory and clinical mutant strains (n = 4), was used.”

34 total strains, comprising 27 clinical isolates, 3 standard strains, and 4 polymyxin-resistant strains.

“A diverse panel of clinical A. baumannii isolates from Australia (n = 9), Pakistan (n = 15), and Switzerland (n = 3), and standard strains (n = 3), including polymyxin-resistant laboratory and clinical mutant strains (n = 4), was used.”

Polymyxin-resistant isolates exhibited lower MICs to the disinfectants PQ-1 and PHMB compared to susceptible isolates, indicating collateral susceptibility.

“Within this exploratory strain collection, polymyxin-resistant isolates exhibited lower MICs to PQ-1 and PHMB, indicating collateral susceptibility.”

Significant negative correlations were observed between polymyxin susceptibility and MICs for PQ-1 and PHMB, while no consistent correlation was found between polymyxins and the antimicrobial peptide LL-37.

“Correlation analysis revealed significant negative correlations between polymyxin susceptibility and disinfectant MICs (colistin–PQ-1 (r = −0.67 and p = 0.0001), colistin–PHMB (r = −0.44 and p = 0.05), and polymyxin B–PHMB (r = −0.48 and p = 0.005), whereas no consistent correlation was observed between polymyxins and AMPs, including LL-37.”

PQ-1 demonstrated the greatest antibacterial activity among the tested disinfectants, while LL-37 showed the lowest and most consistent MICs among the tested antimicrobial peptides.

“PQ-1 exhibited the greatest antibacterial activity among disinfectants, while LL-37 showed the lowest and most consistent MICs among AMPs; lactoferricin was largely inactive.”

The study had a small number of polymyxin-resistant isolates, and the isolates were selected based on phenotype rather than known genetic resistance mechanisms, making the findings hypothesis-generating.

“Given the small number of polymyxin-resistant isolates, these findings should be considered hypothesis-generating and require confirmation in larger collections of genetically characterized strains.”