a. Food Requirements.
(1) Natural media. In their natural habitat, bacteria that cause human
disease are usually one of two types: saprophytes grow on dead organic matter and
parasites grow on living tissue.
(2) Artificial media. In the laboratory, a few bacteria will not grow on any
artificial culture medium. However, most bacteria will grow on culture medium that
provides sources of carbon, nitrogen, minerals, water, and energy. Some bacteria grow
only on special culture media that provides certain amino acids, vitamins, serum, or
other special ingredients. One of the most common media used in a clinical laboratory
contains the ingredients listed plus sheep blood. Various types of growth patterns and
hemolysis can be observed from this growth.
b. Oxygen Requirements.
Aerobes--grow in the presence of free oxygen.
Anaerobes--grow only in the absence of free oxygen.
(3) Facultative aerobes (anaerobes)--able to adjust to an aerobic
Microaerophiles--require small amount of free oxygen for growth.
c. Types of Hemolysis.
Beta-hemolytic--can cause complete hemolysis (rupture) of red blood
Alpha hemolytic--causes a chemical change of hemoglobin in red blood
Gamma hemolytic--do not cause hemolysis.
d. Staining Characteristics. Specimens are normally stained prior to
microscopic examination. Various species react differently to the stains. In the gram
stain method, a specimen is stained with crystal or gention violet, followed by iodine. It
is then washed with alcohol or acetone-ether and stained with safranin, a red or brown
(1) Gram-positive--bacteria which, when stained by the gram stain
method, retain the gram stain color (dark violet or purple).