Loss of capsule confers a growth advantage to <i>Acinetobacter</i> spp. under microaerophilic conditions

<i>Acinetobacter baumannii</i> is a strict aerobe and unable to carry out fermentative metabolism. When strain AB5075 was grown under static, non-aerated conditions, capsule-deficient mutants predominated in the culture after 24 h. In contrast, capsule mutants were not observed when cultures were aerated. In media containing thioglycolate to reduce free oxygen, capsule mutants grew to a higher density than isogenic wild-type cells, and in competition experiments, capsule mutants strongly outcompeted wild-type cells in static, low-oxygen growth conditions. This phenomenon was observed in another <i>A. baumannii</i> strain AB0057 but not with AB307-0294. However, the latter strain exhibited robust growth under static growth conditions when compared with AB5075. When <i>Acinetobacter baylyi</i> ADP1 was grown under static conditions, capsule mutants also predominated in the culture and exhibited a strong competitive advantage over wild-type cells under static growth conditions. Overall, these findings have implications when working with the comprehensive <i>A. baumannii</i> transposon insertion library, as the majority of individual T26 insertions are highly mixed with capsule-deficient cells.IMPORTANCE<i>Acinetobacter baumannii</i> is a pathogen of increasing frequency and responsible for a variety of infections. In this study, we demonstrate an unexpected role for capsule as a factor that limits the ability of <i>Acinetobacter</i> spp. to grow under low oxygen conditions. The ability to modulate capsule expression may serve as a mechanism to cope with low-oxygen levels at human infection sites, while still maintaining virulence.