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ABSTRACTThe capsular polysaccharides in different serotypes of Klebsiella pneumoniae(KP) coded by the (CPS) gene cluster are characterized by a conserved and a hyper-variable region. We performed a virulence study by switching genes in the highly conserved region of the CPS cluster between strains. Six genes in the CPS conserved region in serotype K20, including galF, acidPPc, wzi, wza, wzband wzc, were knocked out and replaced by the homologous genes from serotype K1. Compared to the parental K20 strain, the mutants showed a decline in lethality (LD50) in mice from 10-fold to > 105-fold and were categorized in terms of the effect on virulence as low (L) for galFand acidPPC, moderate (M) for wzi, and high (H) for wza, wzband wzc. Although substituting the acidPPCgene from K1 for acidPPCin the K20 strain fully restored virulence, substitution with the wzi, wza, wzbor wzchomologs from K1 did not. The restoration with wzifrom K1 led to a partial restoration of virulence, with the LD50in mice changing from 104to 103CFU.For the wza, wzband wzcgenes, Complementation of K20 wza, wzband wzcfrom K1 resulted in varied degrees of lethality in mice. Variable improvement in serum killing and phagocytosis was observed when the knockout mutants were compared with the gene-switched strains. In conclusion, homologous genes for capsule synthesis failed to exhibit the same functionality when switched between serotypes and virulence was decreased in different degree in according to the genes' homology.