Discovering the Molecular Determinants of Phaeobacter inhibens susceptibility to Phaeobacter phage MD18

Bacteriophage technologies have immense potential as antibiotic therapies and in genetic engineering. Understanding the mechanisms that bacteriophages implement to infect their hosts will allow researchers to manipulate these systems and adapt them to specific bacterial targets. Here, we isolated a bacteriophage capable of infecting the marine alphaproteobacterium Phaeobacter inhibens and dissected its mechanism of infection. Phaeobacter phage MD18, a novel species of bacteriophage isolated in Woods Hole, MA, exhibits potent lytic ability against P. inhibens and appears to be of the Siphoviridae morphotype. Consistent with this finding, the sequence of the MD18 revealed significant similarity to another siphophage, the recently discovered Roseobacter phage DSS3P8. We incubated MD18 with a library of barcoded P. inhibens transposon insertion mutants and identified 22 genes that appear to be required for phage predation of this host. Network analysis of these genes using genomic position, GO term enrichment, and protein associations reveals that these genes are enriched for roles in assembly of a type IV pilus (T4P) and regulators of cellular morphology. Our results suggest that T4P serve as receptors for a novel marine virus that targets P. inhibens. ImportanceBacteriophages are useful non-antibiotic therapeutics for bacterial infections as well as threats to industries utilizing bacterial agents. This study identifies Phaeobacter phage MD18, the first documented phage of Phaeobacter inhibens, a bacterium with promising use as a probiotic for aquatic farming industries. Genomic analysis suggests that the P haeobacter phage MD18 has evolved to enhance its replication in P. inhibens by adopting favorable tRNA genes as well as through genomic sequence adaptation to resemble host codon usage. Lastly, a high-throughput analysis of P. inhibens transposon insertion mutants identifies genes that modulate host susceptibility to phage MD18 and implicates the type IV pilus as the likely receptor recognized for adsorption. This study marks the first characterization of the relationship between P. inhibens and an environmentally sampled phage, which informs our understanding of natural threats to the bacterium and may promote the development of novel phage technologies for genetic manipulation of this host.