Acinetobacter baumannii is one of the most clinically important nosocomial pathogens. The World Health Organisation refers it to its «critical priority» category to develop new strategies for effective therapy. This microorganism is capable of producing structurally diverse capsular polysaccharides (CPSs), which serve as primary receptors for A. baumannii bacteriophages carrying polysaccharide-depolymerasing enzymes. In this study, eight novel bacterial viruses that specifically infect A. baumannii strains belonging to K2/K93, K32, K37, K44, K48, K87, K89 and K116 capsular types were isolated and characterized. The overall genomic architecture demonstrated that these viruses are representatives of the Friunavirus genus of the family Autographiviridae. The linear double-stranded DNA phage genomes of 41,105–42,402 bp share high nucleotide sequence identity, except for genes encoding structural depolymerases or tailspikes which determine the host specificity. Deletion mutants lacking N-terminal domains of tailspike proteins were cloned, expressed and purified. The structurally defined CPSs of the phage bacterial hosts were cleaved with the specific recombinant depolymerases, and the resultant oligosaccharides that corresponded to monomers or/and dimers of the CPS repeats (K-units) were isolated. Structures of the derived oligosaccharides were established by nuclear magnetic resonance spectroscopy and high-resolution electrospray ionization mass spectrometry. The data obtained showed that all depolymerases studied were glycosidases that cleave specifically the A. baumannii CPSs by the hydrolytic mechanism, in most cases, by the linkage between the K-units. IMPORTANCE Acinetobacter baumannii, a nonfermentative, Gram-negative, aerobic bacterium, is one of the most significant nosocomial pathogens. The pathogenicity of A. baumannii is based on the cooperative action of many factors, one of them being the production of capsular polysaccharides (CPSs) that surround bacterial cells with a thick protective layer. Polymorphism of the chromosomal capsule loci is responsible for the observed high structural diversity of the CPSs. In this study, we describe eight novel lytic phages which have different tailspike depolymerases (TSDs) determining the interaction of the viruses with corresponding A. baumannii capsular types (K-types). Moreover, we elucidate the structures of oligosaccharide products obtained by cleavage of the CPSs by the recombinant depolymerases. We believe that as the TSDs determine phage specificity, the diversity of their structures should be taken into consideration as selection criteria for inclusion of certain phage candidate to the cocktail designed to control A. baumannii with different K-types.

Acinetobacter baumannii plays a significant role in infecting patients admitted to hospitals. Many A. baumannii infections, including ventilation-associated pneumonia, wound, and bloodstream infections, are common for intensive care and burn units. The ability of the microorganism to acquire resistance to many antibiotics, disinfectants, and dehydration assures its long-term survival in hospital settings. The application of bacteriophages is a potential tool to control A. baumannii infections. Bacteriophage AP22 lytic for A. baumannii was isolated from clinical materials and classified as a member of the Myoviridae family. The phage had an icosahedral head of 64 nm in diameter and a contractile tail of 85-90 nm in length. According to restriction analysis, AP22 had 46-kb double-stranded DNA genome. The phage AP22 exhibited rapid adsorption (> 99% adsorbed in 5 min), a large burst size (240 PFU per cell), and stability to the wide range of pH. The bacteriophage was shown to specifically infect and lyse 68% (89 of 130) genotype-varying multidrug-resistant clinical A. baumannii strains by forming clear zones. Thus, it could be used as a candidate for making up phage cocktails to control A. baumannii-associated nosocomial infections.

Acinetobacter baumannii is a gram-negative, non-fermenting aerobic bacterium which is often associated with hospital-acquired infections and known for its ability to develop resistance to antibiotics, form biofilms, and survive for long periods in hospital environments. In this study, we present two novel viruses, vB_AbaP_AS11 and vB_AbaP_AS12, specifically infecting and lysing distinct multidrug-resistant clinical A. baumannii strains with K19 and K27 capsular polysaccharide structures, respectively. Both phages demonstrate rapid adsorption, short latent periods, and high burst sizes in one-step growth experiments. The AS11 and AS12 linear double-stranded DNA genomes of 41,642 base pairs (bp) and 41,402 bp share 86% nucleotide sequence identity with the most variable regions falling in host receptor–recognition genes. These genes encode tail spikes possessing depolymerizing activities towards corresponding capsular polysaccharides which are the primary bacterial receptors. We described AS11 and AS12 genome organization and discuss the possible regulation of transcription. The overall genomic architecture and gene homology analyses showed that the phages are new representatives of the recently designated Fri1virus genus of the Autographivirinae subfamily within the Podoviridae family.