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Nalidixic acid (nal) susceptibility of clinical isolates of Serratia marcescens from urinary tract infections, wounds, and blood was determined by the standard disk method, and results were compared with results obtained by a standardized tube dilution procedure using five different broth media. An attempt was made to approximate in vivo conditions by using urine as a test medium and correlating the activity of nal in urine with its activity in nutrient, Trypticase soy, brain-heart infusion, and Mueller-Hinton broths. The isolates were consistently susceptible to lower concentrations of nal in nutrient broth than in the other media. The results showed that Mueller-Hinton broth most closely reflected nal activity in urine. A standardized procedure for nal susceptibility testing of urinary tract isolates of S. marcescens is proposed.

SUMMARY: Auxotrophic, phage-resistant and streptomycin-resistant mutants of Caulobacter were used to demonstrate genetic recombination in this genus. Forty of a possible 170 mating pairs gave recombinant frequencies of from 100 to 1000 times greater than reversion frequencies for the markers used. Stability of the recombinants was demonstrated by successive cloning on both complete and minimal medium. Cell contact is apparently required for recombination since both cell types of a mating pair must be present for prototroph formation.