The present invention generally relates to engineered bacteriophages which express amyloid peptides for the modulation (e.g. increase or decrease) of protein aggregates and amyloid formation. In some embodiments, the engineered bacteriophages express anti-amyloid peptides for inhibiting protein aggregation and amyloid formation, which can be useful in the treatment and prevention of and bacterial infections and biofilms. In some embodiments, the engineered bacteriophages express amyloid peptides for promoting amyloid formation, which are useful for increasing amyloid formation such as promoting bacterial biofilms. Other aspects relate to methods to inhibit bacteria biofilms, and methods for the treatment of amyloid related disorders, e.g., Alzheimer's disease using an anti-amyloid peptide engineered bacteriophages. Other aspects of the invention relate to engineered bacteriophages to express the amyloid peptides on the bacteriophage surface and/or secrete the amyloid peptides, e.g., anti-amyloid peptides and pro-amyloid peptides, and uses thereof for modulation protein aggregates and amyloid formation.