Loss of capsule confers a growth advantage to Acinetobacter spp. under microaerophilic conditions

Our lab observed that the majority of wells of an Acinetobacter baumannii AB5075 ordered transposon library contained a mixture of VIR-O, AV-T and capsule negative cells. We hypothesized that capsule negative mutants may compete for limited oxygen under non-aerated growth conditions better than its capsulated parent. When growing A. baumannii AB5075 VIR-O cells in flat well 96 well microtiter plates at 37 degrees C under static (non-aerated) conditions, we observed the rise a significant increase in capsule negative mutants. Whole genome sequencing revealed these mutants had disrupted capsule loci. Additionally, these mutants outcompete the wildtype capsulated strain under microaerophilic conditions, but not in aerated, shaking culture. These observations have several implications, including how to grow the A. baumannii cells to maintain "pure" populations, carefully interpret results of non-aerated assays, and the use of the AB5075 ordered transposon library.