When we talk about microbial growth, we are really referring to the number of cells, not the size of the cells. Microbial populations can become very large in a very short time and unchecked microbial growth can cause serious disease or food spoilage. By understanding microbial growth, we can determine how to control the growth. Bacterial growth refers to an increase in bacterial cell number and it normally happens by binary fission. During active bacterial growth, the size of the microbial population is continuously doubling. The normal growth curve of bacteria has four phases: lag phase, exponential growth phase, stationary phase, and death or logarithmic decline phase. Most bacteria grow best in a narrow range of pH, neutrality, pH 6.5-7.5. Very few bacteria grow at an acid pH and are called acidophiles. Some bacteria have adapted so well to an environment of high salt concentrations that they actually require salt concentrations for growth.