The Filtration of Bacteria

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Abstract
A Rawlin strain of Bacillus typhosus growing in K medium at 37[degree] C was filtered through a Berkefeld "N" filter. The filtrate was inoculated in varying amounts into nutrient broth and K medium. The resulting growth in K medium was filtered and the filtrate inoculated as before. The same procedure was carried out a 3rd time. There occurs a progressive diminution of growth in broth, and a progressive increase in K medium as indicated by turbidities, resulting from serial passage through K media. Growth in the K culture, after the 3rd filtration, failed to develop in nutrient broth. This transformation is associated with development of mucoid colonies on agar, delayed fer-mentation, and agglutination reactions. K cultures finally failed to agglutinate. Culturing in K medium therefore results in production of filterable forms of B. typhosus. The organism changes from a "peptophilic" to a "proteo-philic" type, as shown by its failure to grow in nutrient broth. These changes are ascribed to a peptone-poor, protein-rich medium. Sources of error, morphological changes and relation of filterable state to infection are discussed.