Ultramicrocells form by reductive division in macroscopic Pseudomonas aerial structures

Bacterial aerial growth with reductive cellular division and morphological development has not been reported from single-cell bacteria. Here we show that within 1 month of incubation in vaporized p-cresol, Pseudomonas sp. KL28 form shiny, highly branched specialized aerial structures of millimetre-scale diameter. The developmental process displayed spatially and temporally distinct stages; an initial sphere stage was followed by ramification, which led to highly branched tip formation. In this morphogenesis process, the extracellular matrix (ECM) played an important role for maintaining the integrity of sectional populations and the boundaries between adjacent sectors. In addition, cellular division, lysis and migration within the aerial structures were also accompanied. During prolonged incubation, clusters of short-rod cells covered by an outer layer of thick ECM underwent reductive transformation and then replicative reductive division to form oval ultramicrocells of < 0.4 microm in diameter. In addition, the aerial structures protected these rather fragile cells from desiccation and served as a selection pressure for wrinkly, spreading cell variants. The formation of aerial structures is affected positively and negatively by a GacA regulator and RpoS, respectively, and is linked to other phenotypes. Our results demonstrate that Pseudomonas has an ecological adaptation to form mushroom-like aerial structures, which can be a tool for studying cell-cell interactions and bacterial development.