Phase-Variable Control of Multiple Phenotypes in Acinetobacter baumannii Strain AB5075.

Area of Science:

• Microbiology
• Bacterial genetics
• Pathogenesis

Background:

• Acinetobacter baumannii strain AB5075 displays two colony opacity phenotypes: opaque and translucent.
• These phenotypes are unstable and interconvert frequently, suggesting a phase-variable mechanism.
• The frequency of this phase variation increases with cell density, mediated by an extracellular factor.

Purpose of the Study:

• To identify and characterize a novel phase-variable mechanism in Acinetobacter baumannii.
• To investigate the regulation of this phase variation by cell density and extracellular signals.
• To determine the impact of this phase variation on bacterial phenotypes, including virulence and antibiotic resistance.

Main Methods:

• Phenotypic analysis of opaque and translucent colony variants.
• Investigation of phase variation frequency in relation to cell density.
• Identification of the extracellular factor mediating phase variation.
• Assessment of phenotypic differences including motility, biofilm formation, antibiotic resistance, and virulence in a Galleria mellonella model.

Main Results:

• A novel, high-frequency phase-variable mechanism controls colony opacity in A. baumannii.
• Phase variation is regulated by cell density and a novel extracellular signaling molecule.
• Opaque and translucent variants exhibit significant differences in cell morphology, motility, biofilm formation, antibiotic resistance, and virulence.
• Similar phase-variable control of colony opacity was observed in additional clinical isolates.

Conclusions:

• This study reports the first example of phase variation in Acinetobacter baumannii.
• Phase variation is controlled by quorum sensing, a phenomenon not previously described in bacteria.
• The findings highlight the importance of recognizing colony opacity as an indicator of underlying phenotypic variation that affects key bacterial traits.