What Is a Dichotomous Key to Identify Bacteria?

Items covered in a dichotomous key include whether bacteria are Gram positive or negative, catalase positive or negative, have nitrate enzymes, and contain the enzyme coagulase. The key is a set of questions that when complete help identify the bacteria. Each question requires completion of a certain test.

The Gram-positive or Gram-negative test determines if bacteria are present in the sample. The material is stained and put under a microscope for study. A negative Gram stain usually shows up as pink and indicates no organism is found. Positive Gram stains are usually purple and typically have more structures on the slide.

The catalase test is used to identify staphylococci, a positive reading, or streptococci, a negative reading. Catalase-positive bacteria intake oxygen, while catalase-negative bacteria are usually anaerobic.

The nitrate reductase test determines whether a bacterium produces one of two enzymes, nitrate reductase or nitrite reductase. The material is grown in a culture medium containing potassium nitrate. If the growing medium turns red and bubbles appear in an attached Durham tube, nitrogen is present.

The coagulase test determines if the bacteria contain coagulase, a blood-clotting enzyme. The test is performed using clear blood serum. If the enzyme is present, the serum clouds up and thickens.