The K139 capsular polysaccharide produced by Acinetobacter baumannii MAR17-1041 belongs to a group of related structures including K14, K37 and K116

Capsular polysaccharide (CPS) is a key target for bacteriophage and vaccine therapies currently being developed for treatment of infections caused by the extensively antibiotic resistant bacterial species, Acinetobacter baumannii. Identification of new CPS structures and the genetics that drive their synthesis underpins tailored treatment strategies. A novel CPS biosynthesis gene cluster, designated KL139, was identified in the whole genome sequence of a multiply antibiotic resistant clinical isolate, A. baumannii MAR-17-1041, recovered in Russia in 2017. CPS material extracted from A. baumannii MAR-17-1041 was studied by sugar analysis and Smith degradation along with one- and two-dimensional H-1 and C-13 NMR spectroscopy, and the structure was found to include a branched pentasaccharide repeating unit containing neutral carbohydrates. This structure closely resembles the topology of the A. baumannii K14 CPS but differs in the presence of D-Glcp in place of a D-Galp sugar in the repeat-unit main chain. The difference was attributed to a change in the sequence for two glycosyl-transferases. These two proteins are also encoded by the A. baumannii KL37 gene cluster, and a multiple sequence alignment of KL139 with KL14 and KL37 revealed a hybrid relationship. The global distribution of KL139 was also assessed by probing 9065 A. baumannii genomes available in the NCBI non-redundant and WGS databases for the KL139 gene cluster. KL139 was found in 16 genomes from four different countries. Eleven of these isolates belong to the multidrug resistant global lineage, ST25.