MutantsofSalmonella typhimurium LT2:Development and Characterization 1

A series ofrepeated exposures to gamma irradiation withintervening outgrowth ofsurvivors was usedtodevelop radioresistant cultures ofSalmonella typhimuium LT2.Stepwise increases inresistance to bothionizing and ultraviolet irradiation were obtained independently ofthepresenceorabsence ofintegrated P22prophage. Single clonal isolates, representing parentand radioresistant populations, retained thegeneral characteristics oftheLT2 parent, including serological properties, phagetyping, antibiotic sensitivities, mouse virulence, andmostbiochemical testreactions. Resistant cells were generally larger andcontained 1.8to2.1times more ribonucleic acidandprotein thanparentcells, butdeoxyribonucleic acid(DNA)contents were similar. Heterogeneity inthepopulations withrespect torelease ofH2S,utilization of carbon sources,andgrowth on minimal mediumisconsidered tobeancillary, rather thancausally related, toincreased radioresistance. Theresistant isolates displayed an increased ability toreactivate gamma-irradiated P22phage. DNA polymerase Iandpolynucleotide-joining enzyme activities were elevated in extracts ofradioresistant cells relative toparentcells. Itissuggested thatthe observed increases inradioresistance result fromaselection ofmutations leading toan increased capacity torepair DNA. Theexposure ofmicroorganisms tocontinued environmental stress frequently results inthe selection ofstrains resistant totheimposed conditions. Whenthestress agent isalso mutagenic, asinthecaseofionizing irradiation, an increase ingeneral mutation rates, combined withtheselective process, mayaccelerate the emergence ofmoreradioresistant strains (44). Several investigators havereported thedevelopment ofincreased radioresistance within bacterial populations whenthesurvivors were repeatedly grownandreexposed toirradiation (12,31,45).Generally, fora givencycle of irradiation andgrowth, amoderate increase in radioresistance toa plateau level hasbeen obtained (45). A modified growth-irradiation

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