Swarmer cell differentiation of Proteus mirabilis in fluid media

After 3-4 h in a rich fluid medium such as brain--heart infusion broth, motile nonseptate filaments developed from normal short rods and formed about 80% of the cell mass of Proteus mirabilis PM23. This developmental pattern was not observed in any of the other nine representatives of the species. These filaments were considered to be equivalent to swarmer cells formed on agar media because these cells ceased tumbling (i.e., chemotaxis was repressed), they developed large numbers of flagella (i.e., flagella synthesis and insertion was derepressed), and the distribution of nuclei in the filaments indicated that there was normal segregation. The population of cells grown in a minimal medium supplemented with amino acids and nicotinic acid consisted only of short cells with tumbling motility, despite the production of long cells and swarming on the same medium solidified with ordinary agar (refined agar was not effective). These short cells differentiated in 1-1.5 h in brain--heart infusion broth at 37 degrees C after an initial division. The requirements for initiation of differentiation were good basal nutrition, suitable cations (probably Ca2+ and Na+, or K+), and unknown heat-stable organic factors (molecular weight less than 10 000) present in crude agar and yeast extract. Other components of media promoted swarmer differentiation if it was initiated and these included organic acids (lactate), amino acids (proline or serine), phosphate, and an appropriate ionic environment. Comparison of the observed sequence of length classes in brain--heart infusion broth culture with computer generated growth models suggested that, at the outset of growth, 50% of the products of each short cell division ceased septation but grew in length for about five doubling periods and then divided cells from each end at a faster rate (3-5 times per hour) for return to the short cell pool.