Comparative analysis of phage cocktail activity against uropathogenic E. coli strains using spot overlay assay

Spot overlay assays are a common method for measuring phage activity against target microorganisms and determining % host range activity. Dilutions of phage demonstrate activity by creating a zone of clearing indicative of bacterial killing; however, this yields only semi-quantitative results for each target strain tested. In addition, zones can be completely clear or incomplete cloudy zones of varying intensity. These factors make it difficult to compare the overall strength of activity between monophages or phage cocktails when numerous bacterial strains are involved. Here we demonstrate a method to analyze spot overlay results to provide insight beyond just host range activity; three phage cocktails with host range activity of 92-96 % had different overall activities. While the spot overlay is a qualitative assay, consolidation of activity results can derive semi-quantitative information. This method converts activity of phage dilutions from a spot overlay assay into an overall activity of cocktails for comparison. Our method:•Uses standard laboratory protocol for analysis•Expands information beyond host range activity•Consolidates results of all strains tested to assess overall cocktail strength of activity for comparison.