General concepts of bacterial growth may often envisage the cells as simply floating or "swimming" in water. In reality, bacteria tend to attach onto surfaces and then colonize the surface entirely to form a biofilm. The bacterial cells tend to be negatively charged and are attracted to positively charged surfaces. When this happens, the bacterial cells become anchored to the surface by extending organic polymers which make the primary attachment. Biofilms are dynamic and there is an ongoing competition among the incumbent microbial strains that may result in three possible effects. First, the range of strains that may be present can become reduced or stratified. The second possible effect relates to the incumbency density of cells within the biofilm that may fluctuate with environmental conditions. Competition among the incumbent microbial strains in a maturing biofilm in response to environmental conditions can lead to a third effect which is related to the degree of imposed stress.