Infection at the Subcellular Level I. Localization of Intravenously Injected Brucellae in the Vacuolar Apparatus of Cells of Guinea Pig Liver

The distribution of Brucella melitensis in various tissues and in subcellular fractions obtained from liver was investigated to evaluate the initial phases of brucellosis in the guinea pig. Fifty minutes after intravenous infection, brucellae were found principally in the blood and liver, with a substantial number recovered from spleen. Fractionation of liver established that most bacteria were found in the mitochondrial plus lysosomal (M + L) fraction; a significant number, however, sedimented in the nuclear (N) fraction. With time, there was a progressive shift of bacteria from the M + L to the N fraction, accompanied by a similar shift in acid phosphatase activities. Isopycnic centrifugation of mixtures of M + L fractions and brucellae permitted complete separation of acid phosphatase-bearing particles from bacteria. Similar experiments with fractions from infected animals showed that viable bacteria were found in both the acid phosphatase and free brucellae regions of the gradient. At 10 min postinfection, 52% of the recovered organisms were in the acid phosphatase region; at 30 min, 65%; at 60 min, 85%; and at 315 min, 79%. Detergent plus sonic treatment of an M + L fraction from the liver of an animal killed 50 min after infection caused most of the bacteria in the acid phosphatase region to shift to the region where free bacteria were found. These data suggested that brucellae sequestered in the liver were located primarily in the vacuolar apparatus of the cells which phagocytized them.