Immunity to Salmonella gallinarum during ontogeny of the chicken. I. Onset of resistance to infection; the minor role of opsonins.

The onset of resistance to infection with virulent and avirulent strains of has been studied in chickens during the first 5 weeks of development. Resistance and susceptibility were measured by the decline or increase, respectively, of bacterial populations in the liver and spleen, as well as by the LD. There was a dramatic increase in resistance to the avirulent strain from 1 to 5 days after hatching, but the virulent strain remained more lethal until 5 weeks of age. Virulence was associated with bacterial growth. In susceptible hosts, the incremental bacterial growth during the first 4 days after injection was inversely proportional to the LD. Natural opsonins did not appear to contribute, in more than a minor way, to resistance to infection. There was no difference in rates of clearance of either strain from the circulation during the first week, yet resistance to the avirulent strain became fully established during this period. Conversely, 5-week-old chickens, which are highly resistant to both strains, cleared the avirulent strain much more rapidly than the virulent strain. Possible reasons for different rates of clearance of various forms of bacteria are discussed. No opsonizing antibody could be detected 7 days after injection of into one or 5-week-old chickens.