Lesson 2: Public Health Microbiology

Section II: Bacteria

2-9

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2-9. IDENTIFICATION OF BACTERIA

Since there are several thousand species of bacteria, it would be impossible to identify them on the basis of appearance alone. Therefore, the bacteriologist employs a wide variety of techniques, based upon known characteristics of specific bacteria, to arrive at the identity of a given specimen. The following characteristics, which are used frequently as terms of reference, assist the microbiologist in the positive identification of bacteria as well as in eliminating them from consideration.

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.

(1) Aerobes--grow in the presence of free oxygen.

 

(2) Anaerobes--grow only in the absence of free oxygen.

 

(3) Facultative aerobes (anaerobes)--able to adjust to an aerobic (anaerobic) environment.

 

(4) Microaerophiles--require small amount of free oxygen for growth.

c. Types of Hemolysis.

(1) Beta-hemolytic--can cause complete hemolysis (rupture) of red blood cells.

 

(2) Alpha hemolytic--causes a chemical change of hemoglobin in red blood cells.

 

(3) 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 counter stain.

(1) Gram-positive--bacteria which, when stained by the gram stain

method, retain the gram stain color (dark violet or purple).

 

(2) Gram-negative--bacteria which, when stained by the gram stain method, do not retain the gram stain color (dark violet or purple), but retain the color of the counter stain (red or pink).

 

(3) Acid-fast--bacteria that when stained with certain dyes and then treated with an acid, followed by a counter stain, retain the color of the dye.

 

(4) Nonacid-fast--bacteria that when treated as in (c), above, retain the counter stain rather than the dye.

e. Microscopic Examination.

(1) Shape (refer back to para 2-7).

 

(2) Arrangement (refer back to para 2-7).

 

(3) Size.

 

(4) Spore formation--spore formers or non-spore formers. (para 2-8b(3)).

 

(5) Motility--motile or nonmotile. (determined from an instained wet preparation).

f. Pathogenicity.

(1) Enzymes. Certain bacteria produce specific enzymes that enable the bacterial infection to invade the tissues of the host. Some of these enzymes break down cell membranes or chew up the chemicals that hold tissue together. The effect of this enzymatic action is to enlarge the site of infection.

 

(2) Production of toxins.

(a) Exotoxins--extremely potent proteins that are produced in bacterial cells and which diffuse freely into the cells of host tissues, causing severe systemic poisoning.

 

(b) Endotoxins--lipopolysaccharide chemicals, less potent than exotoxins, that are part of gram-negative bacterial cells and that affect the host cells only after the bacterial cell disintegrates.

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