Some microbes, most notably Proteobacteria, modify specific PLFA biomarkers during periods of slow growth or in response to environmental stress. For example, cis monounsaturated fatty acids may be modified to cyclopropyl fatty acids during periods of slowed growth or modified to trans monounsaturated fatty acids to decrease membrane permeability in response to environmental stress. The ratio of product to substrate fatty acid thus provides an index of their health and metabolic activity.