Abstract
Cultures of bacteria placed on agar are as a rule easily killed by exposure to ultra-violet light from a quartz mercury arc. The amount of radiation necessary for devitalization depends upon several factors. Perhaps the most intricate of these is the variation of resistance with age. This relation between resistance and age has not been determined in detail. The following study was aimed as a contribution to our knowledge in this respect.
In the following experiments a new Victor quartz mercury arc lamp was used without any filter. The current was 4.5 amperes at 65 volts and the distance from burner to plate was 24 inches.
An old culture was one that was grown and transplanted daily for one week and then sealed and left in the museum till needed; a period of from one to 6 weeks.
A new culture was one that was grown and transplanted daily from the old culture for a period of 5 weeks or so. Coli communis was used as the culture in most of our experiments, a control being run on each plate irradiated.
Series I. A small amount of culture was removed with a wire loop 2 mm. in diameter and this was mixed with 2 cc. water. Then one loopful of this suspension was spread on one-half of an agar plate and another loopful on the other half and these were marked “control” and “irradiated”.
Series II. As much of the suspension as would adhere to the wire loop was removed from the agar after the exposure and planted in 5 cc. of beef broth. The same was done with the controls.
All cultures were irradiated for 3/4 min. Four plates were used in each set of experiments.
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