Bacteriophages’ abundance and high specificity to infecting only bacteria hosts within their narrow host range have made them an ideal candidate for utilization in bacteria diagnostics and bacterial infection targeting therapeutics. Genetic and chemical modifications have been performed on bacteriophages to functionalize them with a variety of molecules including fluorophores, antigens, nanoparticles, drugs, and polymers to tailor their features for specific applications. In this work, the genetic engineering tool CRISPR-Cas9 was employed to facilitate modification of the large and complex genome of bacteriophage T4. This bacteriophage is well-characterized and infects Escherichia coli hosts making it an ideal candidate for modeling bacteriophage modifications. Throughout this work, T4 bacteriophages were genetically modified to functionalize them with luciferases for quantifiable signal production, affinity peptides for immobilization and conjugation, and chimeric tail fibers for altering host range. A diagnostic assay capable of detecting <10 CFU of E. coli in 100mLs of water in low-resource settings was developed utilizing one of the modified T4 bacteriophages as a biosensor. The genetic and phenotypic information gathered in this work can be applied to bacteriophages that infect bacteria strains prominent in agriculture and pharmaceutical industries to advance bacteriophage biotechnologies and biomedicines.