# Rapid bacteriophage quantification with a particle size analyzer combined with polarization intensity differential scattering (PIDS) detector

## TL;DR

This paper introduces PhageFOTO, a fast and label-free method for counting bacteriophages using particle size analysis and polarization technology.

## Contribution

PhageFOTO is a novel, rapid, and label-free phage quantification method that does not require bacterial host knowledge.

## Key findings

- PhageFOTO achieved 89 ± 4.3%, 98 ± 1.7%, and 94 ± 2.7% accuracy for PhiX, M13, and T4 phages/mL compared to plaque assays.
- The method's limit of detection is around 107 phages/mL.
- PhageFOTO works across a wide range of phage capsid sizes (6–90 nm).

## Abstract

The study of bacteriophages (phages) and effects on their microenvironments expanded exponentially within the last decade. While there are multiple described methods for phage quantitation, there is still a need for a rapid, label-free method. To this end, we established a procedure for rapid phage quantitation through novel use of a particle size analyzer with Polarization Intensity Differential (PIDS) technology and eliminated the need for labels or knowledge of bacterial host. We validated the procedure and analysis method, termed PhageFOTO (Fast Optical Tallying of Objects) using several physiologically different phages ranging from ~6 nm capsid width (Inoviridae) to ~90 nm capsid width (Caudoviricetes). PhageFOTO demonstrated 89 ± 4.3%, 98 ± 1.7%, and 94 ± 2.7% accuracy for quantitating PhiX, M13, and T4 phages/mL respectively as compared to the gold standard plaque assay with limit of detection for particle concentration occurring around 107 phages/mL. PhageFOTO proved to be a novel, rapid, label free method for phage counting that does not rely on knowledge of the bacterial host presenting unique capability for quantitation of phage samples.