Fundamentals of Microbiology

2.3.4 Endospore Staining

Endospore formation is a distinguishing feature of the family Bacillaceae, which includes members of the aerobic genus, Bacillus and the anaerobic genus, Clostridium. Endospore resists adverse environmental conditions such as dryness, heat and poor nutrient supply. The endospore is a highly retractile body formed within the vegetative bacterial cell at a certain stage of growth. The size, shape, and position of the spore are relatively constant characteristics of a given species and are thereof, of some value in distinguishing the kind of bacillus from another. The position of the spore in the cell may be central, sub terminal or terminal. It may be the same diameter as the cell, smaller, or larger causing a swelling of the cell.

Endospores strongly resist application of simple dyes, but once stained are quiet resistant to decolorization. This character suggests one way to make the structure visible. If simple stains are used, the body of the bacillus is deeply colored, whereas the spore is unstained and appears as a clear area in the organism. By vigorous staining procedures the dye can be introduced into the substance of the spore. When thus stained, the spore tends to retain the dye after treatment with decolorizing agents.

To make the distinction clear between the spore and the vegetative portion of the cell, a contrasting counter stain is usually applied in the ordinary fashion and the resulting picture shows the initial stain taken up by the spore and the second stain appear in the cytoplasm. Thus, it makes for a very simple method of distinguishing the endospore from the vegetative cell.