Complex pattern formation by Pseudomonas strain KC in response to nitrate and nitrite

Pattern formation by enteric bacteria growing on semi-solid agar plates has recently been described, and in this paper a similar phenomenon is reported in an environmental isolate, Pseudomonas strain KC. This organism reproducibly formed complex patterns on 3-mm-thick, semi-solid agar (0·25%, w/v) mineral medium motility plates containing either 5 mM 2-oxoglutarate or 5 mM glycerol, and 2 mM NO- 2 or 3 mM NO- 3. When the plates were inoculated at the centre and incubated in air at 30 °, a growth zone formed that migrated slowly (<0·5 mm h-1) and uniformly outward. Within 24 h, a dense outer growth ring formed which then coalesced into discrete aggregates (approx. 0·5 mm in diameter) with uniform spacing; these aggregates moved radially at approximately 0·7 mm h-1and maintained their density and spacing, to form a spoke-like pattern. Microscopic observations revealed that cells within an intact aggregate were very motile, but did not appear to leave the aggregate. Pattern formation did not occur if the NO-2or NO- 3 concentration was altered, if the agar layer was thicker or thinner, if the plates were incubated under strictly denitrifying conditions, or if carbon sources that were chemoattractants, e.g. acetate, were used. Five other species of pseudomonads were tested under identical conditions, and none formed patterns. Oxygen microelectrode studies indicated that there was little or no oxygen within the aggregates. The growth of strain KC was progressively inhibited in imposed diffusion gradients of NO- 2 or NO- 3 of which NO- 2 was the more potent inhibitor. These results suggest that pattern formation by strain KC may reflect an adaptive response to adverse environmental conditions.