SUMMARY: Some of the recorded properties of a filterable organism (the PAP organism) isolated by Eaton and colleagues from cases of human primary atypical pneumonia suggest that it may be related biologically to the Mycoplasma (pleuropneumonia-like) group. In further investigation of this possibility now reported it was found that the organic gold salt, sodium aurothiomalate, which is known to inhibit certain mycoplasmas, would also inhibit infection with the PAP organism in the hamster and chick embryo lung. Some accepted viruses of a similar order of size to the PAP organism —namely, Nigg's pneumonitis virus of mice (psittacosis group), three strains of influenza virus A, vaccinia and variola viruses—were not inhibited by sodium aurothiomalate. On the other hand, grey lung virus of mice, an organism with some properties resembling those of the PAP organism, was strongly inhibited. Diethyl ether and the antibiotic kanamycin sulphate both inhibited the growth of the PAP organism in chick embryo lung. An intensified Giemsa-staining method revealed small red-purple coccobacillary bodies (later called elementary bodies, EB) in the lungs of chick embryos infected with three different strains of PAP organism isolated, respectively, in 1944, 1954 and 1960 in different parts of the United States. The EB were not seen in uninfected chick embryo lungs. The EB and the antigen of the PAP organism stained by the fluorescent antibody technique occupied essentially the same position in relation to the mesobronchial epithelial cells of chick embryo lung. The formation of both was suppressed in parallel by treatment of inoculum or eggs with diethyl ether, kanamycin sulphate or sodium aurothiomalate. There was a good but not absolute correlation between the presence or absence of EB and antigen in experiments on the growth or survival of the PAP organism in chick embryos, in cell-free media, and in titrations in eggs.


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