ИСТИНА

Acinetobacter baumannii is gram-negative conditionally pathogenic bacteria, which cause nosocomial infections, such as pneumonia, wound and catheter-related urinary tract infections, peritonitis, meningitis, endocarditis, and bloodstream infections. Treatment of the infections is complicated by the ability of the bacteria to acquire and accumulate various mechanisms of antibiotic resistance, which poses a serious public health problem. An alternative method of combating A. baumannii may become phage therapy, which is a highly specific approach with minimum medical contraindications. One of the virulence factors of A. baumannii is a capsular polysaccharide (CPS) composed of many oligosaccharide repeats (K units), which forms a thick protective layer around the bacterial cell. Due to polymorphism of the capsule gene locus (K locus, KL) CPS structures are highly diverse (more than 120 KL types have now been identified). A prerequisite of infection and lysis of A. baumannii cells by a bacteriophage is specific recognition followed by cleavage of the CPS by phage tail-spike receptor proteins that possess a CPS depolymerizing activity. The aim of this work was the establishment of the biochemical basis for phage therapy of the infections caused by A. baumannii, including elucidation of the mechanisms of cleavage of the CPSs with recombinant bacteriophage depolymerases or depolymerases derived from prophages found in the same or a different A. baumannii strain. The СPSs were isolated by phenol-water extraction from cells of clinical isolates of A. baumannii belonging to different KL types and purified by Sephadex G-50 gel-permeation chromatography. Their structures were either identified by comparison with published data or de novo established by sugar analysis along with 1H and 13C one- and two-dimensional NMR spectroscopy, including 1H,1H ROESY and 1H,13C HMBC experiments. New CPS structures of three strains (RCH52, B11911, NL4) containing derivatives of either legionaminic or pseudaminic acid were determined. The CPSs from seven strains (NIPH70, NIPH146, NIPH601, LUH5549, B05, AbWRMAC3340, D4) were cleaved with recombinant bacteriophage depolymerases and those from three strains (AYE, AB5256, B8300) with prophage-derived depolymerases. The oligosaccharide products were fractionated by Fractogel TSK HW-40 (S) gel-permeation chromatography and studied by negative or positive ion mode high-resolution electrospray ionization mass spectrometry and NMR spectroscopy. It was found that all depolymerases studied possess a glycosidase activity and cleave specifically the CPSs of A. baumannii by the hydrolytic mechanism to give a monomer or/and an oligomer (a dimer and in one case a trimer) of the K units. This work was supported by the Russian Foundation for Basic Research (project No. 17-04-01254).