The six phages that displayed the widest bactericide host range in the spot assays were selected for a more thorough assessment of productive infection as defined by the efficiency of plating (EOP). Each phage was tested three times for each of four different dilutions against all the bacterial strains that it had been shown to be able to lyse in the spot assays. This was done under the same conditions as in the spot assays, i.e. using stationary phase bacteria. Thus, all bacterial strains to be tested were grown overnight (18 hours) at 30°C and 200 μl of each of those cultures was used in double layer plaque assays together with 100 μl of diluted phage lysate. The four phage lysates were 106–109 times dilutions from the phage stock. This means that EOP assay replicates for a particular phage were done in parallel on all bacterial strains tested, and also at concentrations comparable to what was used in the spot tests. The plates were incubated overnight at 30°C and the number of plaque forming units (PFU) was counted for each combination. When the 106 dilution did not result in any plaques, a lower dilution was tried afterwards to verify that the EOP was lower than 0.001. Finally, the EOP was calculated (average PFU on target bacteria / average PFU on host bacteria) along with the standard deviation for the three measurements (S1 Table ).
The average EOP value for a particular phage—bacterium combination was classified as “High production” when the ratio was 0.5 or more, i.e. when the productive infection on the target bacterium resulted in at least 50% of the PFU found for the primary host. An EOP of 0.1 or better, but below 0.5, was considered to be of “Medium production” efficiency, and between 0.001 and 0.1 as “Low production” efficiency. An EOP equal to or under 0.001 was classified as inefficient [34 (link)].
The average EOP value for a particular phage—bacterium combination was classified as “High production” when the ratio was 0.5 or more, i.e. when the productive infection on the target bacterium resulted in at least 50% of the PFU found for the primary host. An EOP of 0.1 or better, but below 0.5, was considered to be of “Medium production” efficiency, and between 0.001 and 0.1 as “Low production” efficiency. An EOP equal to or under 0.001 was classified as inefficient [34 (link)].
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