Formation of specialized aerial architectures by Rhodococcus during utilization of vaporized p-cresol

When grown with vaporized alkylphenols such as p-cresol as the sole carbon and energy source, several isolated Rhodococcus strains formed growth structures like miniature mushrooms, termed here specialized aerial architectures (SAA), that reached sizes of up to 0.8 mm in height. Microscopic examination allowed us to view the distinct developmental stages during the formation of SAA from a selected strain, Rhodococcus sp. KL96. Initially, mounds consisting of long rod cells arose from a lawn of cells, and then highly branched structures were formed from the mounds. During the secondary stage of development, branching began after long rod cells grew outward and twisted longitudinally, serving as growth points, and the cells at the base of the mound became short rods that supported upward growth. Cells in the highly fluffy structures were eventually converted, via reductive division, into structures that resembled cocci, with a diameter of approximately 0.5 mu m, that were arranged in chains. Most cells inside the SAA underwent a phase variation in order to form wrinkled colonies from cells that originally formed smooth colonies. Approximately 2 months was needed for complete development of the SAA, and viable cells were recovered from SAA that were incubated for more than a year. An extracellular polymeric matrix layer and lipid bodies appeared to play an important role in structural integrity and as a metabolic energy source, respectively. To our knowledge, similar formation of aerial structures for the purpose of substrate utilization has not been reported previously for Gram-positive bacteria.