Bacterial development

podcast OLD Audio recording (.mp3 format)
OLD Audio recording (.wma format)
 

Log phase cells are very different from stationary-phase cells: log cells are adapted for maximal growth rates in nutrient-saturated conditions, where growth rate is often limited only by the organisms physical ability to import nutrients, process them into cell material, and replicate. They are usually large cells with ability to replicate DNA & make RNA and proteins rapidly. Stationary cells are usually smaller, and are adapted for maximum competitiveness. In many species, the morphology of the cells types are strikingly different, such as Arthrobacter. Keep in mind, as well, that microbes generally spend most of their time in stationary phase in the environment.

The shift to stationary phase (& later back to log phase) is a complex developmental process, controlled by sigma-factor cascades (like phage infection or sporulation in Bacillus). In log-phase Bacteria, the “vegetative” sigma subunit (σ70) is the predominant sigma factor directly RNA polymerase promoter recognition. In late log phase, expression of the stationary-phase sigma (σS) is turned on. Because of its higher affinity for the core RNA polymerase, σS progressively replaces σ70. Genes needed during log phase growth require σ70-containing RNA polymerase for expression, and so these genes are progressively turned off as the concentration of σ70 declines. Genes for stationary phase growth (including the gene encoding σS) are expressed by σS, and so the expression of these genes increases until they reach normal levels for stationary phase. In the case of more complex developmental pathways, such as sporulation, heterocyst formation, or phage infection, many sigma factor transitions occur sequentially, driven by the fact that each sigma factor initiated expression of the sigma factor to follow. Each sigma factor directs expression of the genes required at that stage of the developmental pathway. These sigma factor directed developmental pathways are in many ways analogous to the homeo-box directed developmental pathways of animals.

Secondary metabolites

Many bacteria produce antibiotics and other antimicrobials, but it is very common in Streptomyces & Bacillus. Antibiotics are not produced during rapid growth, but in stationary phase, i.e. they are secondary metabolites. Secondary metabolites are compounds produced (and typically secreted into the environment) only during stationary phase, and so are not required for growth. Other secondary metabolites include iron-binding compounds (siderophores) and other compounds that help the organism compete for limited resources.

Chromobacterium
Chromobacterium violaceum (from MB 451) producing the purple pigment violacein, an antibiotic, in stationary phase (the centers of colonies)
James W. Brown

There are two ways an organism can increase its competitive fitness in a tight environment: increasing its own fitness (self-improvement), or decreasing the fitness of its competitors. Siderophores and high-affinity uptake mechanisms are examples of secondary metabolites that directly increase an organisms supply of nutrients; this is self-improvement. Antibiotics and bacteriocins are examples of secondary metabolites that provide a competitive advantage to an organism by crippling the competition, allowing the producing organism access to the resources that otherwise would have gone to the competition, or even providing the producer with the nutrients released by the extinguished competitor.