Some Observations on the Filtrability of Bacterium Tularense Lee Foshay and W. Hesselbrock1'2

Throughout our morphologic study of Bacterium tularense in liquid medium cultures we noted constantly the presence of very minute forms (Hesselbrock and Foshay, 1945). These appeared to grade downward in size from the most prevalent coccoid or ring forms of about 0.5 ,u in diameter to bodies so small that neither size nor shape was discernible. Such particles were never seen in uninoculated media, either before incubation or for 96 hours thereafter. Some particles were discrete and freely suspended; others were attached by delicate short filaments to larger morphologic units. Many additional ones were seen lying in, and apparently arising within, delicate filaments of variable length. The number of discrete particles was never large at any time in any of the many cultures that were examined. Single particles in suspension, as well as those filamentously attached.or interconnected, stained a brick-red color when supravirtally stained with 2 per cent aqueous malachite green, whereas all large morphologic units stained green. None of six other common pathogenic bacteria, cultivable in the same medium, produced particles of this size. Prolonged intermittent observations of cultures, although inadequate to establish the fact, suggested strongly that these particles were capable of reproducing all other formns of this pleomorphic organism and, apparently by means of larger units, of reproducing themselves. Our provisional interpretation was that they represented true morphologic units of Bacterium tularense, possibly to some degree analogous in size and function to the minute elementary bodies formed by organisms of the pleuropneumonia group, appropriately called minimal reproductive units, M.R.U. (Sabin, 1941). It has long been known that virulent strains of the organism sometimes, in about one half the recorded trials, pass Berkefeld, Chamberland, and Seitz filters. We cannot find reports on the morphology of the units that passed the filters, but our observations suggested that the provisionally designated M.R.U. might have accounted for the infectivity of the filtrates. To test our views we used gradocol membranes for ultrafiltration of virulent cultures.