How to Identify Bacteria

Bacteria are microscopic organisms with one cell. Some use photosynthesis to make their own food, while others are parasites that feed off the people, animals or plants they live in. Some bacteria live in human or animal intestines and break down food so it can be digested. Others live in soil or water where they break down dead matter and recycle chemicals like carbon and nitrogen. But the bacteria that cause disease need to be identified so the right medicine can be prescribed. It's not easy to identify bacteria. After years of study, scientists still have not identified and classified some bacteria. But there are a few ways to identify bacteria.

Difficulty:

Moderately Challenging

Instructions

Things You'll Need

• Violet dye
• Light red dye
• Iodine
• Microscope
• Slides
• Cultures

How to Identify Bacteria

1. Use a gram stain for young and growing bacteria. Stain the bacteria with violet dye and apply iodine to fix the stain inside the bacteria's cell. Wash the bacteria with alcohol and stain it with a light red dye. Gram positive bacteria with thicker cell walls will stay violet, and gram negative bacteria, which has thinner cell walls, will turn pink. Many gram negative bacteria can cause disease because of a layer of toxins in the cell wall. The outer cell membrane protects gram negative bacteria from some antibiotics including penicillin. Ampicillin and other drugs were developed to destroy the membrane. Only six gram positive bacteria can cause disease in humans and penicillin can kill them.

2. Try taking cultures of bacteria and exposing them to antibodies. If the bacteria clumps together, it can be identified as the bacteria that is killed by that antibody.

3. Find the DNA and RNA characteristics of bacteria. Using DNA-based tools is faster and more specific than culture-based methods of identifying bacteria. Determine the shape of the bacteria. Is it rod shaped, spiral or sphere shaped?

4. Compare how bacteria react to viruses. Some viruses can kill or affect certain types of bacteria. If you expose bacteria to a virus and it reacts as expected, you can identify the bacteria.

5. Look in reference books. Bergey's Manual of Systematic Bacteriology and Bergey's Manual of Determinative Bacteriology are used by many experts, as is Cowan and Steel's Manual for the Identification of Medical Bacteria, 2nd edition.

Resources

• The Prokaryotes-updated information on identification of bacteria