Phenotypic Characterization of pilA, pilB, and pilD Mutants of Acinetobacter baumannii 5075: Impacts on Growth, Biofilm Formation, and Tazobactam Response.

BACKGROUND/OBJECTIVES: The Type IV pilus assembly system in Acinetobacter baumannii is a major determinant of its pathogenicity, playing a role in surface-associated functions via the biogenesis of Type IV pili (T4P). Tazobactam (TAZ) is a well-characterized β-lactamase inhibitor, primarily used in combination with β-lactam antibiotics such as piperacillin (PIP) to counteract bacterial resistance mechanisms. While A. baumannii resistance to β-lactam antibiotics has been well studied, the influence of T4P on its susceptibility to TAZ remains largely unexplored. For this reason, we investigated how multidrug-resistant A. baumannii 5075 (AB5075) responds to TAZ by assessing the roles of pilA, pilB, and pilD in bacterial growth and biofilm formation under direct TAZ exposure, with a focus on phenotypic characterization rather than molecular mechanisms. METHODS: Bacterial growth kinetics were quantified by measuring the optical densities of cell suspensions and the colony forming units per volume (CFUs/mL) at different time intervals. Time-kill assays and microtiter dish biofilm formation assays were used to evaluate how effectively TAZ can inhibit growth and biofilm formation, respectively. RESULTS: Time-kill assays confirmed that 32 µg/mL of TAZ inhibited growth in both wild-type (WT) and mutant strains, with the pilD mutant showing initial resistance before eventual inhibition. Biofilm assays showed that the pilA mutant had the highest biofilm formation at 8 h, surpassing the WT strain. A prolonged 32 µg/mL of TAZ exposure (24-36 h) significantly reduced biofilm production across all strains, with inhibition rates reaching 89% for the WT, 82% for the pilA mutant, 91% for the pilB mutant, and 86% for the pilD mutant. CONCLUSION: These findings deepen our understanding of the strain-specific roles of T4P components in growth and biofilm regulation in AB5075, and highlight the potential of TAZ as a therapeutic strategy against biofilm-associated infections.