A pH indicator incorporated into a nutrient-rich liquid medium containing a specific carbohydrate allows for the observation of changes in acidity and gas production by bacterial cultures. These changes manifest visually as color shifts within the broth and potential displacement of a small inverted tube submerged within the medium. For example, fermentation of the carbohydrate with acid production lowers the pH, causing the indicator to turn yellow, while gas production is revealed by a bubble or pocket of air within the inverted tube. Absence of fermentation may result in no color change or, if the organism utilizes peptones instead of the carbohydrate, alkalization of the broth and a color shift to pink.
Differentiating bacteria based on their metabolic capabilities is essential for identification and classification. This method provides a simple yet powerful tool for this purpose, particularly in clinical, food safety, and environmental microbiology. Its historical roots lie in the early days of bacteriology, playing a crucial role in understanding microbial physiology and paving the way for more advanced biochemical testing methods. This fundamental assay remains a cornerstone of microbiological instruction and practice.
