Filterable Forms of Bacteria

Numerous authors have reported the occurrence of filterable elements in cul-tures of bacteria. These elements which were, of course, viable, passed through filters which held back the normal forms. Most of the records on the filterable forms were based on filtration experiments with emulsified cultures or pathogenic material. The filtrates were tested either in laboratory media or by injection into animals, and the results were called positive, if a culture grew or a reaction in the animal was observed. The reports were at first received with the widest interest, but on repetition many workers were unable to obtain positive results. Those which had first proclaimed the existence of a filterable phase could not answer the challenge of the newcomers by devising well defined conditions under which positive results occurred with reasonable frequency. Therefore the interest in the problem gradually subsided. It is significant that newer bacteriological books such as Dubos ' book on the bacterial cell (38) do not mention the word filterability in connection with bacteria. The object of this article is to give an account of recent studies dealing with the