The ability of some bacteria to ferment citrate can be useful for identifying bacteria. Bacteria with the enzyme citrate lyase can break down citrate to form pyruvate. Which can be reduced is used to determine use. When bacteria use citrate and ammonium, the medium is alkalized because of ammonium (NH3) produced from ammonium (NH4+). The indicator bromthymol blue changes to blue when the medium is alkalized, indicating a positive citrate utilization test