K antigen and serum sensitivity of rough Escherichia coli

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Abstract
Bacterial hybrids were prepared which express both K1 and K27 antigens and the relative contributions of these capsules to rabbit serum resistance were examined. E. coli Hfr strain F639 (rough, K27+, serum sensitive) was conjugated with E. coli recipient strain E412 (rough, K1+, His-Trp-Strr, serum resistant). Transconjugants which inherited both the his and trp linked genes for K27 antigen synthesis were analyzed. These hybrids retained and expressed the K1 antigen since the K1 locus is nonallelic with K27 gene loci. Hybrid strains which express both K1 and K27 antigens exhibit serum resistance, but not at the level of the K1+ parental strain. An isogenic K1 derivative of a hybrid which expressed only K27 antigen was serum sensitive (> 99% kill, 60 min). Thus the presence of the K1 capsular antigen evidently can protect some rough strains of E. coli from serum bactericidal activity; K27 and perhaps other K antigens fail to provide such a protective effect.