Abstract
The study of bacterial colonies has in recent years been stimulated by investigations of microbic dissociation. The existence of variant strains, variously developed, which differ in pathogenicity, in biochemical reactions, in antigenic structure, and in colonial characteristics can no longer be doubted. Furthermore, almost no attempts have been made to explain the mechanism whereby bacterial cells pile up to produce colonies varying widely in gross morphology. The relation of pleomorphic bacteria, and of organisms having abnormal staining characteristics, to the development of cells into masses containing tremendous numbers and representing a considerable number of generations is largely unknown. Technical difficulties are responsible for many of these omissions. The ordinary smear is crude. The impression colony is clumsy, and it is limited to certain types of very small colonies. Legroux and Magrou 1 described a method for making paraffin sections of old colonies of V. cholerae, but they found that their method failed with surface colonies because they were washed off by the solutions used. The method to be described overcomes these difficulties, and may be applied to any type of bacterial or mold colony.
Colonies on or in agar medium are used. For surface colonies the medium is warmed to 45°C, and over it is gently flowed 1% agar, melted and cooled to 60°C. Thus a protective layer is formed over the colonies without distortion, the upper layer adhering to the lower. Mold colonies should first be wet with 80% alcohol to prevent the formation of bubbles. When the agar hardens the desired section is cut out and dropped into the fixative. Deep colonies need no preliminary handling. Both 10% formalin and Zenker's solution have been used, with 12 to 24 hours' exposure, with equally good results.
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