The Gram-negative bacterium Holospora obtusa is a macronucleus-specific symbiont of the ciliate Paramecium caudatum. The infectious form of this bacterium infects the host macronucleus through digestive vacuoles and differentiates into the reproductive form two days after the
infection in the nucleus. The
monoclonal antibodies IF-3-1 and IF-3-2 reacted with 39 and 15 kDa
periplasmic proteins, respectively, that were specific for the infectious form of H. obtusa. Because the
antigens were not detected in the reproductive form of the bacterium, it appears that expression of the
proteins decreases during or soon after the
infection. Using these
antibodies, quantitative changes in the
antigens in the early
infection process were examined by immunoblotting and immunogold electron microscopy. Immunoblotting showed that the amounts of both
antigens were reduced within 1 h after the bacteria were engulfed into the digestive vacuoles of the paramecia, but that the amounts of IF-3-2
antigens declined earlier than the IF-3-1
antigen. Immunogold labeling showed that the level of IF-3-2
antigens became very low in the bacteria in the host digestive vacuoles, whereas there was no similar decrease in amount of IF-3-1
antigens. Possible functions of the
antigens are discussed. The IF-3-1
antigens decrease in concentration in parallel with the decrease in the periplasmic region.