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Our previous studies (Honjo et al., 1964, 1967, 1968; Ogawa et al., 1964, 1967; Takasaka et al., 1967) have shown that clinically pronounced dysentery was developed in about 60 per cent of cynomolgus monkeys orally administered with 109-1010 of a freshly isolated Shigella flexneri 2a. However, it seems unlikely that such a large amount as 109-1010 bacilli is ingested at one time to produce clinical manifestation of naturally occurring shigellosis in monkeys. Actually, the oral administration with 106 bacilli could not evoke clinically detectable signs of shigellosis (Takasaka et al., 1967). On the other hand, relatively small doses such as 105 bacilli directly injected into the cecal lumen could produce dysentery (Takasaka et al., 1968). Recently, Honjo et al. (1969) have demonstrated that the ascorbic acid deficient monkey has, more or less, a lowered resistance, showing the development of dysentery with a relatively small doses of S. flexneri 2a in an oral experimental infection system. To ascertain more relevantly the mode of infection in naturally occurring bacillary dysentery, we have conducted the present oral infection experiment under such a condition that the experimentally administered material may have some characteristics more similar to the naturally infecting agent than the cultivated Shigella used in the previous experiments. Cynomolgus monkeys (Macaca irus) from the Philippines and Cambodia weighing from 1.8 to 3.5 kg were used. They were conditioned at our animal room for at least 5 weeks or more prior to use. During the conditioning period, stool specimens of each monkey were tested for the presence of Shigella bacilli 2 to 8 times with negative results, except that S. dysenteriae 2 and S. sonnei were isolated from 3 monkeys 2 days before or on the day of the administration. Virulent S. flexneri 2a strain 5503 (Ogawa et al., 1966) was used in this experiment. As the 1st step of this experiment, 109-1010 bacilli grown on nutrient agar plates at 37 C for 18 hr were administered into the stomach of 12 starved monkeys by the same method as described previously (Honjo et al., 1964), resulting in the development of pronounced clinical dysentery in 8 of the 12 infected monkeys within 2 days after the challenge. Then, 5 of 8 dysenteric animals were sacrificed 2 days after the challenge when they excreted typical dysenteric diarrhea and the contents of the colon were collected and homogenized with sterile sand glass in a sterile mortar. Various amounts of the homogenized dysenteric contents were suspended in sterile saline or distilled water and the volume of suspension was adjusted to 10 ml. As the 2nd step, these suspensions were immediately administered by the

DYSENTERY PRODUCED BY ORAL ADMINISTRATION OF THE COLONIC CONTENTS OF CYNOMOLGUS MONKEYS INFECTED WITH SHIGELLA FLEXNERI 2a