In this study, we isolated a bacteriophage T7-resistant mutant strain of Escherichia coli (named S3) and then proceeded to characterize it. The mutant bacterial colonies appeared to be mucoid. Microarray analysis revealed that genes related to
colanic acid production were upregulated in the mutant. Increases in
colanic acid production by the mutant bacteria were observed when l-
fucose was measured biochemically, and protective
capsule formation was observed under an electron microscope. We found a point mutation in the lon gene promoter in S3, the mutant bacterium. Overproduction of
colanic acid was observed in some phage-resistant mutant bacteria after
infection with other bacteriophages, T4 and lambda.
Colanic acid overproduction was also observed in clinical isolates of E. coli upon phage
infection. The overproduction of
colanic acid resulted in the inhibition of bacteriophage adsorption to the host. Biofilm formation initially decreased shortly after
infection but eventually increased after 48 h of incubation due to the emergence of the mutant bacteria. Bacteriophage PBECO4 was shown to infect the
colanic acid-overproducing mutant strains of E. coli. We confirmed that the gene product of open reading frame 547 (ORF547) of PBECO4 harbored
colanic acid-degrading enzymatic (CAE) activity. Treatment of the T7-resistant bacteria with both T7 and PBECO4 or its purified
enzyme (CAE) led to successful T7
infection. Biofilm formation decreased with the
mixed infection, too. This procedure, using a phage cocktail different from those exploiting solely receptor differences, represents a novel strategy for overcoming phage resistance in mutant bacteria.