Bacterial Swarming:  A Biochemical Time-Resolved FTIR−ATR Study of Proteus mirabilis Swarm-Cell Differentiation

Fourier transform infrared spectroscopy was applied to the study of the differentiation process undergone by Proteus mirabilis. This bacterium exhibits a remarkable dimorphism, allowing the cells to migrate on a solid substratum in a concerted manner yielding characteristic ring patterns. We performed an in situ noninvasive analysis of biochemical events occurring as vegetative cells differentiate into elongated, multinucleate, nonseptate, and hyperflagellated swarm cells. The major findings arising from this study are (i) the real-time monitoring of flagellar filament assembly, (ii) the evidence for de novo synthesis of qualitatively different lipopolysaccharides (LPS) and/or exopolysaccharides (EPS) constituting the slime into which bacteria swarm, and (iii) the alteration in the membrane fatty acid composition with a concomitant 10 °C decrease in the gel/liquid crystal phase transition resulting in an elevated membrane fluidity in swarm cells at the growth temperature. The time course of events shows that the EPS−LPS syntheses are synchronous with membrane fatty acid alterations and occur about 1 h before massive flagellar filament assembly is detected. This study not only provided a time sketch of biochemical events involved in the differentiation process but also led to the identification of the major spectral markers of both vegetative and swarm cells. This identification will allow to resolve the time−space structure of P. mirabilis colonies by using infrared microscopy.