Peripheral rods: a specialized developmental cell type in Myxococcus xanthus.

In response to nutrient deprivation, the ubiquitous Gram-negative soil bacterium Myxococcus xanthus undergoes a well-characterized developmental response, resulting in the formation of a multicellular fruiting body. The center of the fruiting body consists of myxospores; surrounding this structure are rod-shaped peripheral cells. Unlike spores, the peripheral rods are a metabolically active cell type that inhabits nutrient-deprived environments. The survival characteristics exhibited by peripheral rods, protection from oxidative stress and heat shock, are common survival characteristics exhibited by cells in stationary phase including modifications to morphology and metabolism. Vegetative M. xanthus cells undergo a number of physiological changes during the transition into stationary phase similar to other proteobacteria. In M. xanthus, stationary-phase cells are not considered a component of the developmental response and occur when cells are grown on nutrient-rich plates or in dispersed aqueous media. However, this cell type is not routinely studied and little of its physiology is known. Similarities between these two stress-induced cell types led to the question of whether peripheral rods are actually a distinct developmental cell type or simply cells in stationary phase. In this study, we examine the transcriptome of peripheral rods and its relationship to development. This work demonstrates that peripheral rods are in fact a distinct developmentally differentiated cell type. Although peripheral rods and stationary phase cells display similar characteristics, each transcriptomic pattern is unique and quite different from that of any other M. xanthus cell type.

Growth of Myxococcus xanthus in continuous-flow-cell bioreactors as a method for studying development.

Nutrient sensors and developmental timers are two classes of genes vital to the establishment of early development in the social soil bacterium Myxococcus xanthus. The products of these genes trigger and regulate the earliest events that drive the colony from a vegetative state to aggregates, which ultimately leads to the formation of fruiting bodies and the cellular differentiation of the individual cells. In order to more accurately identify the genes and pathways involved in the initiation of this multicellular developmental program in M. xanthus, we adapted a method of growing vegetative populations within a constant controllable environment by using flow cell bioreactors, or flow cells. By establishing an M. xanthus community within a flow cell, we are able to test developmental responses to changes in the environment with fewer concerns for effects due to nutrient depletion or bacterial waste production. This approach allows for greater sensitivity in investigating communal environmental responses, such as nutrient sensing. To demonstrate the versatility of our growth environment, we carried out time-lapse confocal laser scanning microscopy to visualize M. xanthus biofilm growth and fruiting body development, as well as fluorescence staining of exopolysaccharides deposited by biofilms. We also employed the flow cells in a nutrient titration to determine the minimum concentration required to sustain vegetative growth. Our data show that by using a flow cell, M. xanthus can be held in a vegetative growth state at low nutrient concentrations for long periods, and then, by slightly decreasing the nutrient concentration, cells can be allowed to initiate the developmental program.