Biochemical aspects of the growth of feline pneumonitis virus in the chick embryo yolk sac.

The many unusual properties of the psittacowith elementary bodies. About 48 hours after sis-lymphogranuloma group of infectious agents the host cell is infected, the vesicle bursts, have made their exact taxonomic position a ruptures the cell and releases a swarm of new matter of doubt (Rake, 1953; Meyer, 1953). elementary bodies to infect fresh cells (Weiss, However, whether these intracellular parasites 1949). are really viruses or rickettsiae or neither, there When feline pneumonitis virus is inoculated is no doubt that they are a large and important into the yolk sac of a 6 day chick embryo, the group of organisms lying close to the nebulous demonstrable virus titer falls sharply during the line which separates virus from not-virus. first few hours after infection and does not rise For several years, my students and I have for about 24 hours. The work of other investibeen studying some of the biochemical aspects gators (Sigel et al., 1951) suggests the presence of the growth of feline pneumonitis virus in of noninfectious virus in this interval. After 24 chick embryo yolk sac. Feline pneumonitis virus hours, new infectious units rapidly appear, and is a typical member of the psittacosis-lymphothe virus titer rises at an increasing rate until granuloma group and was first described by the embryo dies between the fourth and fifth Baker (1942). Like other members of its group, day of infection. Then the embryo dies, virus this agent grows to a high titer in the yolk sac, multiplication ceases, and the titer in the yolk and the very thin and easily separated yolk sac sac slowly declines (Moulder and Weiss, 1951b; may be conveniently used both for investigating Moulder et al., 1954). Just as the embryo dies, the relation of virus multiplication to host cell the virus reaches its maximum concentration, metabolism and for preparing purified virus and almost all of the entodermal cells of the yolk suspensions. sac are infected. One yolk sac contains approxiIn beginning, I should like to recall to you the mately 108 LDso's of virus, and each LD50 is cycle of development in host cells exhibited by equivalent to about 25 elementary bodies feline pneumonitis virus and the other agents of (Moulder et al., 1954). From data obtained in the psittacosis group (Weiss, 1949). The basic purification experiments, we have estimated that infectious unit is the elementary body (figure 1). this amount of virus represents about one per In electron micrographs of air-dried metalcent of the fat-free drv weight of the yolk sac shadowed preparations, the elementary body is (Zahler and Moulder, 1953). about 0.4 micron in diameter (Hamre, Rake and One of the most obvious ways to investigate Rake, 1947; Moulder and Weiss, 1951a) and the biochemical events associated with the reexists in two different forms: one shows distinct production of an intracellular parasite is to comcentral and peripheral components, whereas the pare the biochemical properties of normal and other appears homogeneous. Both are probably infected tissue at different times during growth infectious forms of the virus (Heinmets and of the parasite. From the time chick embryos Golub, 1948; Crocker, 1954). Virus growth is are infected with feline pneumonitis virus until started when an elementary body enlarges into they succumb to the infection, there is no differan initial body. The initial body then develops ence in either the oxygen consumption (Moulder into a cluster of initial bodies, apparently by a and Weiss, ] 951b) or the concentration of organic series of binary fissions (Rake and Jones, 1942). phosphorus compounds (Zahler, 1953) in normal The clusters finally form large vesicles packed and infected yolk sac. Since feline pneumonitis ' The Eli Lilly and Company Award Address in virus never makes up more than one per cent of Bacteriology and Immunology, delivered at the the active cell mass, it is not surprising that its 54th annual meeting of the Society of American multiplication causes no gross disruption in the Bacteriologists in Pittsburgh, Pa., May 6, 1954. metabolic pattern of the yolk sac.