Microbiology Bacteria Identification

Since the discovery of microorganisms a few centuries ago, scientists have been looking for better ways to identify bacteria. Some methods are old and time tested, while some methods are new and genetically based. Either way, a very specific work flow is required to identify bacteria.

Microscopic Examination

• Microscopic examination is usually done by way of the gram stain procedure. This staining method will differentiate gram-positive (purple) bacteria from gram-negative (pink) bacteria. This is based on the composition of the cell wall of the organism. Common gram-positive bactera are staphylococcus and streptococcus. Gram-negative bacteria include E. coli and Klebsiella.
  In addition, microscopic morphology that is useful in identification can be noted, for example staphylococci are gram-positive cocci in clusters. So the gram stain appears microscopically as purple spheres in a formation resembling a group of grapes.

Media Selection

• Based on the specimen source (urine, blood, etc.) and the likely organisms that would be present, the appropriate bacteriological media are selected to grow the organism for further work-up. In addition, selecting the appropriate temperature and atmospheric conditions (aerobic versus anaerobic) will allow bacterial growth.

Colony Morphology

• How the bacterial colony appears on bacteriological growth media offers key characteristics about the bacteria. Characteristics like size, color, odor and the ability to lyse (break apart) blood cells are all important identification criteria for determining what the unknown bacteria is.

Biochemical and Enzymatic Tests

• The ability of a bacterial species to use a biochemical, a sugar, amino acid or a enzymatic substrate is very useful in identifying bacteria. These tests can be used individually (coagulase for Staphylococcus aureus) to identify a organism, or in a battery of tests to identify gram-negative bacilli. Many commercially prepared tests kits are available to identify bacteria using biochemicals and enzymes.

Serological Methods

• Serologic methods for identifying bacteria are frequently used in the laboratory. This typically involves testing an unknown antibody against a known antigen bound to a latex particle or similar structure. After mixing the two together and rotating, a visible agglutination (clumping) will appear if positive for the organism tested. Other tests look for antibody neutralization, inhibition and other end products.

Molecular Methods

• The latest and most specific methods available in the laboratory are molecular methods. These tests are based on the ability to detect, identify and characterize microorganisms based on their DNA or RNA. Polymerase chain reaction (PCR) is a common molecular method used to identify bacteria.