Acinetobacter baumannii is an emerging nosocomial pathogen worldwide with increasing prevalence of multi-drug and pan-drug resistance. A. baumannii exists widely in natural environment, especially in health care settings, and has been shown difficult to be eradicated. Bacteriophages are often considered alternative agent for controlling bacterial infection and contamination. In this study, we described the isolation and characterization of one virulent bacteriophage AB1 capable of specifically infecting A. baumannii.

Our understanding of the molecular mechanisms behind bacteria-phage interactions remains limited. Here we report that a small protein, SrpA, controls core cellular processes in response to phage infection and environmental signals in Pseudomonas aeruginosa. We show that SrpA is essential for efficient genome replication of phage K5, and controls transcription by binding to a palindromic sequence upstream of the phage RNA polymerase gene. We identify potential SrpA-binding sites in 66 promoter regions across the P. aeruginosa genome, and experimentally validate direct binding of SrpA to some of these sites. Using transcriptomics and further experiments, we show that SrpA, directly or indirectly, regulates many cellular processes including cell motility, chemotaxis, biofilm formation, pyocyanin synthesis and protein secretion, as well as virulence in a Caenorhabditis elegans model of infection. Further research on SrpA and similar proteins, which are widely present in many other bacteria, is warranted.

Phage therapy requires the comprehensive understanding of the mechanisms underlying the host-phage interactions. In this work, to identify the genes related to Pseudomonas aeruginosa phage K8 receptor synthesis, 16 phage-resistant mutants were selected from a Tn5G transposon mutant library of strain PAK. The disrupted genetic loci were identified and they were related to O-specific antigen (OSA) synthesis, including gene wbpR, ssg, wbpV, wbpO, and Y880_RS05480, which encoded a putative O-antigen polymerase Wzy. The LPS profile of the Y880_RS05480 mutant was analyzed and shown to lack the O-antigen. Therefore, the data from characterization of Y880_RS05480 by TMHMM and SDS-PAGE silver staining analysis suggest that this locus might encode Wzy. The complete phage K8 genome was characterized as 93879 bp in length and contained identical 1188-bp terminal direct repeats. Comparative genomic analysis showed that phage K8 was highly homologous to members of the genus PaP1-like phages. On the basis of our genetic findings, OSA of P. aeruginosa PAK is proven to be the receptor of phage K8. The highly conserved structural proteins among the genetic closely related phages suggest that they may recognize the same receptor.

Pseudomonas aeruginosa is an opportunistic human pathogen which poses a major threat to patients with cystic fibrosis (CF). Excessive amounts of mucus present in the lungs of CF patients promotes the colonization of P. aeruginosa. The migA gene, encoding a putative glycosyltransferase, has been shown to be highly inducible by respiratory mucus derived from CF patients. In this study, it is further demonstrated by population transcript analysis that the migA gene is highly expressed in the CF lung environment. Deletion analysis of the migA promoter identified a las-box-like sequence commonly found in promoters that are responsive to quorum sensing regulation. Further analysis of migA expression in quorum-sensing-defective strains, as well as its expression in response to autoinducer molecules, demonstrated that migA is regulated by the RhlI/RhlR quorum sensing regulatory system. Functionally, as the MigA sequence homology data suggested, the migA gene indeed affects the structure of LPS in P. aeruginosa. Increased expression of the migA gene results in a loss of core-plus-one LPS, while having no obvious effect on the long-chain O-antigen-bearing LPS. Although the exact biological role of the core-plus-one LPS is not clear, these experimental results suggest that migA up-regulation in the CF lung environment is part of the adaptive response which confers on P. aeruginosa a survival advantage.