Optical Density-Based Methods in Phage Biology: Titering, Lysis Timing, Host Range, and Phage-Resistance Evolution.

More than a century ago, bacteriophages (phages) were discovered as entities that could both replicate and dramatically reduce bacterial culture turbidities. By the late 1940s, phage impact on broth turbidity was being studied using electronic detectors. This review examines such turbidimetric, also known as colorimetric or optical density means of studying phage biology. The focus is especially on relatively rapid and higher throughput phenotypic phage characterization versus methods that rely instead on phage plaques, spots, or genotype determinations. Topics covered include (i) the most probable number method along with Appelmans' approach, (ii) estimation of phage growth parameters including especially that of phage lysis timing, (iii) consideration of lysis inhibition as a complicating factor, (iv) phage titering based on degrees of optical density change, (v) detection of both lysis from without and resistance to lysis from without, (vi) phage host-range determination, and (vii) study of post-lysis culture grow back, that is, of bacterial evolution of phage resistance. Based on over 30 years of experience using and studying optical density approaches to the exploration of broth-culture phage biology, the author takes a critical look at both the benefits and limitations of this increasingly common approach to phage biological characterization.