Four selected smooth strains of were inoculated orally and intravenously into calves, aged 6–24 hr, whose blood, judged by the zinc sulphate flocculation test, did or did not contain immune globulins (IG+ or IG—), and the numbers of the bacteria in the tissues were estimated when the animals were killed after different intervals. In bactericidal tests one of the strains, which belonged to the serotype O78: K80, most commonly found in calves suffering from generalised infections, survived and multiplied in all samples of IG— serum, but not in most samples of IG+ serum. Two strains, O20: K?: H9 and O20: K?: H−, grew in most IG− and IG+ sera, and the fourth strain, O101: K?, which had been isolated from a calf suffering from the localised intestinal form of infection characterised by severe diarrhoea, did not grow in any sample of either IG— or IG+ serum.

After oral inoculation into IG— calves, O78: K80 bacteria rapidly invaded the blood and tissues, and their ultimate concentration in the blood was often ten times greater than that in tissues rich in reticulo-endothelial cells. The calves became ill after about 15 hr and soon collapsed and died. Similar results were obtained when O78: K80 bacteria were inoculated intravenously. The blood counts of IG+ calves into which O78: K80 bacteria had been inoculated intravenously were either negative or very low, the highest concentration of the bacteria being found in tissues rich in reticulo-endothelial cells; some of these calves, into which large doses of the O78: K80 bacteria had been inoculated, developed lesions in the kidneys, central nervous system and joints which closely resembled those seen in some natural cases of generalised infection. The blood and tissues were not invaded after oral inoculation of O101: K? bacteria in IG— calves or of O78: K80 bacteria in IG+ calves; these animals remained well. The distribution of bacteria in IG+ calves into which strains O101: K?, O20: K?: H9 and O20: K?: H— had been inoculated intravenously was similar to that obtained with the 078: K80 strain.

The rate of disappearance of bacteria of strain O78: K80 from IG+ blood was compared with its rate of disappearance from the same blood The bacteria were removed much too rapidly for the removal to be due wholly or mainly to the bactericidal action of the blood. Similar results were obtained with the O101: K? strain and IG— blood. The O20: K?: H9 and the O20: K?: H— strains disappeared rapidly from the blood of IG+ calves yet multiplied in the blood of the same calves The O78: K80 strain multiplied in IG— blood both and the rate of multiplication being faster

It is concluded that the reticulo-endothelial system constitutes the principal defence mechanism against bacteriaemia, and that in calves whose blood contains immune globulins as a result of their having been fed with colostrum this protection is of a comprehensive nature.


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