Nutrient Agar & Legionnaires' Disease

Legionnaires' disease is caused by the bacterium Legionella. It was first recognized during a 1976 outbreak at the American Legion Convention in Philadelphia. It causes pneumonia and is often found in areas with stagnant, sitting water. The best test for diagnosing Legionnaires' is to culture the microbe on selective agar media.

Agar

• Agar is derived from algae. It is a powder that can be dissolved in liquid solvents. Upon heating and cooling an agar mixture will form a gelatinous substance on which bacteria can grow. To identify special types of bacteria, different compounds and nutrients are added to the hot mixture before it solidifies. A bacterium can then be identified based on its interaction with the compounds and nutrients.

Growing Legionella

• When a physician looks for Legionella he obtains urine and respiratory secretions from his patient to create a test culture for Legionella. A sample from a normal, uninfected person will not grow any Legionella. It can take up to seven days for the Legionella to grow.

Buffered Charcoal Yeast Agar

• The best agar on which to grow Legionella is called buffered charcoal yeast agar. This agar contains antibiotics that will kill other bacteria. It also contains several dyes that react with disease causing Legionella and turn it apple-green. Other similar organisms will turn blue.