Long-Circulating Bacteriophage as Antibacterial Agents

The increased prevalence of multidrugresistant bacterial pathogens motivated us to attempt to enhance the therapeutic efficacy of bacteriophages. The therapeutic application of phages as antibacterial agents was impeded by several factors: (i) the failure to recognize the relatively narrow host range of phages; (ii) the presence of toxins in crude phage lysates; and (iii) a lack of appreciation for the capacity of mammalian host defense systems, particularly the organs of the reticuloendothelial system, to remove phage particles from the circulatory system. In our studies involving bacteremic mice, the problem of the narrow host range of phage was dealt with by using selected bacterial strains and virulent phage specific for them. Toxin levels were diminished by purifying phage preparations. To reduce phage elimination by the host defense system, we developed a serialpassage technique in mice to select for phage mutants able to remain in the circulatory system for longer periods of time. By this approach we isolated long-circulating mutants of Escherichia coli phage λ and of Salmonella typhimurium phage P22. We demonstrated that the long-circulating λ mutants also have greater capability as antibacterial agents than the corresponding parental strain in animals infected with lethal doses of bacteria. Comparison of the parental and mutant λ capsid proteins revealed that the relevant mutation altered the major phage head protein E. The use of toxin-free, bacteriaspecific phage strains, combined with the serial-passage technique, may provide insights for developing phage into therapeutically effective antibacterial agents.