Brown and Balish (1) reported that there were no marked differences in the gastrointestinal flora between BALBjc athymic nude (nujnu) mice and their heterozygous (nuj +) littermates. Investigating the intestinal flora of BALBjc nujnu mice and their nuj + littermates, however, we found that although there were no significant differences in the indigenous flora of the intestine between them, nujnu mice showed a remarkably high incidence of spontaneous colonization by Pseudomonas aeruginosa in their small intestines (91 % of 11 animals tested) and feces (100% of 13 animals) as compared with nuj+ mice (0% of 14 and 47% of 15, respectively) .The high incidence of P. aeruginosa colonization in the nujnu mice we observed might be due to a lack of a thymus-dependent immune system (6). Therefore, to confirm this finding nujnu mice were compared with their nuj + littermates with respect to intestinal colonization by P. aeruginosa administered orally.BALBjc nujnu mice and their nuj + littermates (6 to 10 weeks old) were used.They were produced in our laboratory by mating nude (nujnu) males and heterozygous (nuj +) females. The nujnu and nuj + mice were separately housed in metal cages with filter caps (Sanki Kagaku Kogei Co.) and kept on a rack with a laminar air filter unit (Sanki Kagaku Kogei Co.). They were given a 3-Mradirradiated diet (MB-3, Funabashi Nojyo) and given fresh tap water each day with the exception of the time of the administration of a P. aeruginosa culture. Cages and bedding were sterilized at 160 C for an hour.For administration of P. aeruginosa, animals were given drinking water containing, per ml, approximately 1 X 10 6 viable cells of P. aeruginosa grown overnight in brain heart infusion broth (Difco Laboratories). The drinking water bottles were autoclaved after two days of feeding. Feces and intestines with their contents, as bacteriological samples, were aseptically collected, weighed, homogenized, and diluted serially 10-fold in phosphate-buffered solution (pH 7.2). NAC agar (Eiken Chemical Co.) was used for enumeration of P. aeruginosa. P. aeruginosa strains were serologically identified with antisera obtained from Denka Seiken Co. Bacteria other than P. aeruginosa were not counted, because there would be no significant difference in their counts between the nujnu and nuj + mice as we found on preliminary examination.

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