Bacteriophage titering by optical density means: KOTE assays

Bacteriophages, or phages, are the viruses of bacteria. Since at least the later 1940s, researchers have studied phages using spectrophotometrically determined measures of bacterial-culture turbidity (optical density). Recently, two groups have proposed the use of kinetic visualizations of phage-induced bacterial lysis to estimate phage titers ('KOTE' assays). Provided here is an overview of the two new publications and comparison of different approaches to interpreting resulting turbidimetric data. The latter includes especially peak culture turbidities (OD_max) versus the timings of 'Deviation' of the turbidity of phage-containing curves from those of phage-free controls. Also addressed is the possible impact of the phage lysis inhibition phenotype. Overall, KOTE assays seem to provide somewhat consistent titer-estimating power, though not necessarily always with precision as high as that of plaque-based titering. This ability comes, though, at a cost of a high preliminary workload that is required to establish calibration curves. An important possibility emerging from these efforts is that OD_max or its timing might serve as superior indicators of phage antibacterial virulence than area under the curve measures. Additional approaches to phage titering are also reviewed, along with exploration of the long, nearly 100-year history of the KOTE technique.