The long-term effects of phage concentration on the inhibition of planktonic bacterial cultures

Since the early 1920s there has been an interest in using bacteriophages (phages) for the control of bacterial pathogens. While there are many factors that have limited the success of phage bio-control, one particular problem is the variability of outcomes between phages and bacteria. Specifically, there is a significant need for a better understanding of how initial phage concentrations affect long-term bacterial inhibition. In work reported herein three phages were isolated for Escherichia coli K12, Pseudomonas aeruginosa PAO1, as well as Bacillus cereus and bio-control experiments were performed with phage concentrations ranging from 105 to 108 plaque forming units per mL over the course of 72 h. For four of the nine phages isolated there was a linear relationship between inhibition and phage concentration, suggesting the effect of phage concentration is important at longer time scales. For three of the isolated phages, phage concentrations had no effect on bacterial inhibition suggesting that even at the lowest concentration the method of action was saturated and lower concentrations might still be effective. Additionally, a cocktail was created and was compared to the previously isolated phages. There was no statistical difference between the cocktail and the best performing phage highlighting the importance of selecting the appropriate phages for treatment. These results suggest that, for certain phages, there is a strong relationship between phage concentration and long-term bacterial growth inhibition and the initial phage concentration is an important indicator of the long-term outcome.