Cell envelope defects of different capsule-null mutants in K1 hypervirulent Klebsiella pneumoniae can affect bacterial pathogenesis.

Hypervirulent Klebsiella pneumoniae (hvKP) causes Klebsiella-induced liver abscess. Capsule is important for the pathogenesis of Klebsiella in systemic infection, but its role in gut colonisation is not well understood. By generating Delta wcaJ, Delta wza and Delta wzy capsule-null mutants in a prototypical K1 hypervirulent isolate, we show that inactivation of wza (capsule exportase) and wzy (capsule polymerase) confer cell envelope defects in addition to capsule loss, making them susceptible to bile salts and detergent stress. Bile salt resistance is restored when the initial glycosyltransferase wcaJ was inactivated together with wzy, indicating that build-up of capsule intermediates contribute to cell envelope defects. Mouse gut colonisation competition assays show that the capsule and its regulator RmpA were not required for hvKP to persist in the gut, although initial colonisation was decreased in the mutants. Both Delta rmpA and Delta wcaJ mutants gradually outcompeted the wild type in the gut, whereas Delta wza and Delta wzy mutants were less fit than wild type. Together, our results advise caution in using the right capsule-null mutant for determination of capsule's role in bacterial pathogenesis. With the use of Delta wcaJ mutant, we found that although the capsule is important for bacterial survival outside the gut environment, it imposes a fitness cost in the gut.