FIG 4.

Phage resistance caused by induction of the lgfp gene. (A) Effects of lgfp induction on T4 and T7 phage infections. Cultures of E. coli carrying pHGFP (blue) or pLGFP (red) were incubated at 37°C for 30 min with 10 mM IPTG (solid symbols). No IPTG was added to controls (open symbols). The OD₆₆₀ of each culture was adjusted to about 0.4, and phage T4 (circles) or T7 (triangles) was added. No phage was added to controls (squares). The multiplicities of infection of T4 and T7 were 0.088 and 0.0012, respectively. E. coli carrying pHGFP (±IPTG) or pLGFP (−IPTG) was grown to stationary phase at 2 h. The PFU count (upper graph) and OD₆₆₀ (lower graph) of each culture were measured periodically. Error bars show standard deviations (n = 3). (B) Effects of lgfp expression on the rates of expansion of various phages. E. coli JM2.300 carrying pHGFP or pLGFP was used as a host for phage λ, and E. coli AK4 carrying pHGFP or pLGFP was used as a host for phages f1 and MS2. Culture conditions and IPTG induction were as described for panel A. Phages were added to the cultures at a multiplicity of infection of 0.01. The rate of expansion of each phage was calculated from the phage titer after 4 h of incubation at 37°C. Error bars show standard deviations (n = 3). (C) Construction of a phage response genetic system. pPAL contains the lgfp gene downstream of the modified promoter of the psp operon, the ampicillin resistance gene, and ColE1. pPAH has hgfp instead of lgfp of pPAL. RBS, ribosome binding site. (D) GFP expression from pPAH by phage f1 infection. E. coli AK4 carrying pPAH was incubated with phage f1 (multiplicity of infection, 1) at 37°C for 2 h. No phage was added to controls. GFP expression was then measured as described in Materials and Methods. Mean fluorescent signal values (arbitrary units) are indicated in the graphs. (E) Phage resistance of E. coli AK4 carrying pPAL. Cultures of E. coli AK4 carrying pPAH (blue) or pPAL (red) were incubated with various phage concentrations at 37°C for 4 h. Growth is indicated as the ratio of the OD₆₆₀ to that of a culture of AK4 without the addition of phage f1 (circles, lower graph). All cultures without phage addition were grown to an OD₆₆₀ of 0.65 to 0.90. The titers of phage f1 in the cultures were measured as described in Materials and Methods. The reproductive rates were calculated from the titers (squares, upper graph). Error bars show standard deviations (n = 3).