Living Agents of DiseasesCHAPTER 1

In the nature both non-living and living agents do exist. The relationship of the living agents with the environment has been named as “Ecology” (Gr. oikos—home, native land; logos—science). The living agents are distributed everywhere in the enviornment as well as inside and on the surface of human body. Those living agents which produce the disease in man are called pathogenic microorganisms or Patho-gens. Though the soil is an unfavourable habitat, most of the pathogens may survive for three days to three weeks; whereas the saprophytes (non-pathogens) live for longer period in the soil. The human body is the most suitable environment for the multiplication and production of disease by pathogenic microorganisms. Therefore, in medical microbiology, only the living agents of disease (Pathogens) are to be studied in detail as they are variable in their shape, size, structure and pathogenicity. Until the eighteenth century the classification of living organisms placed all organisms into one of two kingdoms: Plant or animal kingdom. But in medical microbiology, we study some organisms that are predominantly plant-like, others that are animal-like and that are variable in size—macroscopic to electron microscopic as shown in Table 1.1.

Some organisms that share characteristics common to both plants and animals are placed into a third kingdom, called Protista to include those unicellular organisms that are typically neither plant and animal. The protists encompass bacteria, fungi and protozoa.

Electron microscopic study discovered two groups of protists:

The microscope is the most important routinely used instrument in the microbiology laboratory. Its magnification enables us to see the microorganisms and their structures invisible to the naked eyes. It ranges from X100, X400 to X1000. Different kinds of microscope are available.(Fig. 1.1).

Microscopes are of two categories:

Light (or optical) and electron, depending upon the principles on which the magnification is based. Light microscope in which the magnification is obtained by a system of optical lenses using light waves includes: 1. Bright field; 2. Dark field; 3. Fluorescence; 4. Phase contrast microscope. The electron microscope uses a beam of electrons in place of light waves to produce the image. Specimens can be examined by either transmission, or scanning electron 4microscope. In the microbiology course students perform most of their examinations with a bright field microscope. This is most widely used instrument for routine microscopic work. The other types of microscopes are used for special purpose or research investigation.

It provides tremendous useful magnification because of the much higher resolution obtainable with the extremely short wavelength of the electron beam used to magnify the specimen. Electron microscope uses the electron beam and magnetic fields to produce the image, whereas the light microscope uses light waves and glass lenses.

It is a method of measuring the size of microorganisms.

Intestinal largest round worm. Female 20–35 cm in length by 3–6 mm in diameter (Fig. 1.2, left).

Tapeworm: 5–10 meters in length (Fig. 1.2, right).

W. bancrofti is a microscopic blood parasite which causes filariasis in man (Fig. 1.3).

Malarial parasite: Its trophozoite is 1.5 μ in diameter, present within the red blood corpuscles causes malaria (Fig. 1.4).

1 μ in diameter, arranged in cluster causes abscess, acne, boils, etc. (Fig. 1.5).

1 μ in diameter arranged in chain causes tonsillitis or septic sore throat (Fig. 1.6).

7–18 μ long and 0.2–0.6 μ wide, causes acute ulcerative gingivitis (Fig. 1.7).

Comma-shaped, 1.5–2.4 μ × 0.2–0.4 μ, causes cholera in man (Fig. 1.8).