All growth-based methods (metabolic methods) rely on an electrical or an optical signal change as a result of microbial metabolism. Most of the work with growth-based methods has been applied to bacteria, yeast, and mold. These microorganisms become metabolically active and grow in a broth medium, consequently changing the chemical composition of the medium. The two main growth-based methods operate on the principles of impedance and optical signal changes. The impedance method relies on the generation of small molecules or ions from metabolism of the larger molecules in the medium. These changes in the composition of the medium increase conductivity, thereby decreasing the impedance signal. The optical method relies on optical changes resulting from pH change (using pH indicators such as bromocresol purple or phenol red) or cleavage of unique molecules.