Genetic determinants of capsular serotype K1 of Klebsiella pneumoniae causing primary pyogenic liver abscess.

BACKGROUND Primary pyogenic liver abscess (PLA) caused by Klebsiella pneumoniae is an emerging infectious disease. Capsular serotype K1 and the magA gene have been reported to be associated with this disease. METHODS The prevalence of magA was determined by polymerase chain reaction (PCR). The sequences of the magA flanking region were completed by inverse PCR and direct sequencing. Serotyping was performed by double immunodiffusion. Insertion mutations and trans-complementation were used to define the K1 genetic determination region. RESULTS Thirty-five of 42 strains from patients with PLA were magA positive, whereas only 1 of 32 non-PLA strains was magA positive. All 36 magA-positive strains were serotype K1, and the 38 magA-negative strains were not (36/36 vs. 0/38; P<.0001). Sequencing of the magA flanking region revealed a putative capsular polysaccharide synthesis (cps) region; this region was 25 kb in length and contained 20 open reading frames (ORFs); of these ORFs, 9 were cotranscribed as part of an operon and differed from both MGH78578 and the Chedid strain. Mutation of 4 genes in this region turned the mutant strains anti-K1 negative. Trans-complementation restored the K1 phenotype. CONCLUSIONS The operon containing magA is responsible for capsular serotype K1 of K. pneumoniae. Several loci in the operon are unique determinants of K1 strains.