Klebsiella pneumoniae capsule expression is necessary for colonization of large intestines of streptomycin-treated mice

The role of the Klebsiella pneumoniae capsular polysaccharide (K antigen) d uring colonization of the mouse large intestine was assessed with mild-type K. pneumoniae LM21 and its isogenic capsule-defective mutant. When bacteri al strains were fed alone to mice, the capsulated bacteria persisted in the intestinal tract at levels of 10(8) CFU/g of feces while the capsule-defec tive strain colonized at low levels, 10(4) CFU/g of feces. In mixed-infecti on experiments, the mutant was rapidly outcompeted by the wild type. In sit u hybridization on colonic sections revealed that bacterial cells of both s trains were evenly distributed in the mucus layer at day 1 after infection, while at day 20 the wild type remained dispersed and the capsule-defective strain was seen in clusters in the mucus layer. These results suggest that capsular polysaccharide plays an important role in the gut colonization ab ility of K. pneumoniae.