Abstract

Recent volunteer and controlled epidemiologic field studies have provided evidence which firmly associates the agent first recovered by Eaton in 1944 with lower respiratory tract illness of man.1-3 A serologic response to the Eaton agent occurs in approximately 90 per cent of pneumonia illnesses in which cold agglutinins develop during convalescence as well as in a significant but variable proportion of cold agglutinin-negative pneumonias.2 4 The development of pneumonia and other forms of respiratory disease following the administration of tissue culture-grown Eaton agent to volunteers and the demonstration that naturally acquired antibody offered protection against such illness supports the contention that the agent is a respiratory tract pathogen.5 For many years, the agent was tentatively classified as a virus. The large size of the agent (180-250 m,i) and its sensitivity to streptomycin and various tetracycline derivatives, however, posed some difficulty with such a classification.6-8 Recently, Marmion and Goodburn were able to visualize small cocco-bacillary bodies on the mucous layer covering the bronchial epithelium of the Eaton agent infected chick embryo.9 The distribution of these bodies corresponded with the localization of Eaton agent as visualized by the fluorescent antibody technique. These workers also demonstrated that the Eaton agent was inhibited by an organic gold salt. Clyde visualized extracellular colony-like structures in stained preparations of infected tissue culture; these structures corresponded with the areas of specific immunofluorescence.U? Both groups of workers suggested the possibility that the Eaton agent may be a pleuropneumonia-like (PPLO) rather than a virus. Cultivation of the organism in cell-free media, however, was not achieved. T