Bacterial loads in infected mice. (A) Histopathology in the liver. Low power (20X objective) views of the liver of infected IFN-γ +/+ (top) or IFN-γ −/− (bottom) mice show that numerous granulomatous lesions form in the liver of IFN-γ −/− mice, while IFN-γ +/+ mice have few granulomas. Granulomas in IFN-γ +/+ mice contained no acid-fast bacteria (right-hand panel) at the time assayed (5 weeks postinfection), while IFN-γ −/− mice contained numerous bacteria (red rods within the granuloma structure). At the stage shown, 100% of granulomas contained acid-fast bacteria (data not shown) (B) Quantitation of bacterial numbers in the spleen. Closed symbols: IFN-γ +/+ mice, open diamonds: IFN-γ −/− mice. Time after infection is shown in the abscissa. Dilutions of spleen homogenates were plated onto MH9 media and bacterial colonies measured 21 to 28 days later.