Diverse morphological types of dormant cells and conditions for their formation in Azospirillum brasilense

Differences in generation of dormant forms (DF) were revealed between two strains of non-sporeforming gram-negative bacteria Azospirillum brasilense, Sp7 (non-endophytic) and Sp245 (endophytic strain). In post-stationary ageing bacterial cultures grown in a synthetic medium with a fivefold decreased initial nitrogen content, strain Sp7 formed two types of cyst-like resting cells (CRC). Strain Sp245 did not form such types of DF under the same conditions. CRC of the first type were formed in strain Sp245 only under phosphorus deficiency (C > P). The endophytic strain was also shown to form structurally differentiated cells under complete starvation, i.e. at a transfer of early stationary cultures, grown in the media with C > N unbalance, to saline solution (pH 7.2). These DF had a complex structure similar to that of azotobacter cysts. The CRC, which are generated by both azospirilla strains and belong to distinct morphological types, possessed the following major features: absence of division; specific ultrastructural organization; long-term maintenance of viability (for 4 months and more); higher heat resistance (50-60°C, 10 min) as compared with vegetative cells, i.e. the important criteria for dormant prokaryotic forms. However, CRC of non-endophytic strain Sp7 had higher heat resistance (50, 55, 60°C). The viability maintenance and the portion of heat-resistant cells depended on the conditions of maturation and storage of CRC populations. Long-term storage (for 4 months and more) of azospirilla DF populations at -20°C was optimal for maintenance of their colony-forming ability (57% of the CFU number in stationary cultures), whereas the largest percentage of heat-resistant cells was in CRC suspensions incubated in a spent culture medium (but not in saline solution) at room temperature. The data on the intraspecies diversity of azospirilla DF demonstrate the relation between certain type DF formation to the type of interaction (non-endophytic or endophytic) with the plant partner and provide more insight into the adaptation mechanisms that ensure the survival of gram-negative non-spore-forming bacteria in nature.