Bacillus subtilis is a spore-forming bacterium that can persist for years in a resistant state. When faced with starvation, bacteria instead enter the complex developmental pathway of spore formation. González-Pastor et al. (2003) and other researchers (Engelberg-Kulka and Hazan, 2003) have found that bacteria en route to sporulation produce a toxin (sporulation-killing factor) similar to peptide antibiotics that lyses sibling cells not committed to sporulation. The killing operon also produces its own export pump and confers resistance to the killing peptide. The nutrient boost from the lysed cells allows the surviving cells to postpone sporulation, to escape its energetic costs, and to continue replication. A signaling factor (sporulationdelaying protein) mediates this escape from sporulation via a transcription factor that stimulates lipid oxidation and adenosine 5′-triphosphate production to restore energy reserves. The above studies of mixed culture require an easy, effective, and fast methodology to quantify the metabolic active cell count of different types of microorganisms. Both the survival strategy and the microbial interactions including stress response in pure and mixed culture are discussed in this chapter.