An agar or liquid medium containing haemoglobin, high density horse lipoprotein, trypsin, Tween 80, phosphate buffer, CaCl, glucose, glutamic acid and NaCl supported growth of ileal fluid dependent organism (IFDO). Glucose, glutamic acid and NaCl were not essential but enhanced growth. Trace amounts of lipoprotein were sufficient to support growth, and some human sera and serum fractions rich in low density lipoprotein could be substituted for horse lipoprotein. Addition of lipase enhanced the growth rate, and reduced the requirement for lipoprotein. No nucleic acid precursors were identified as essential for growth. However, nucleosides, especially cytidine, accelerated the growth rate. The growth rate was also increased by DNAase and RNAase. These observations indicate that the organisation of the IFDO particle is more complex than that of a crystal. They are consistent with the hypothesis that IFDO is a replicating agent that utilises specific preformed protein to assemble a proteinaceous particle, and support the postulated relationship of IFDO to transmissible spongiform encephalopathy agents.


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