Measurement of bacterial resistance
The resistance of bacteria to phages was measured for the coevolution
lines (BP, BP+IP, and BP+IBP) from transfer 8 and 20. Bacterial
resistance to within-microcosm phages was determined by streaking 20
independent bacterial colonies across a line of 20 µL phages
pre-streaked on agar plates. Specifically, bacterial colonies were
isolated by plating dilutions on KB agar plate and incubating for 48 h,
grown in 96-well plates at 28°C for 48 h, and then reconditioned at 10°C
for 48 h before streaking. Phage samples were extracted using chloroform
as described above. A bacterial colony was scored as resistant if there
was no inhibition of growth after incubated at 10°C for 72 h and
population-level bacterial resistance was calculated as proportions of
resistant bacterial colonies. We also measured resistance of bacterial
populations from those microcosms against phages from the source
habitats (SBP), where each source-habitat phage population was paired
with one bacterial population from each of the following evolution
regime: BP, BP+IP, BP+IBP.