Effect of Sublethal Doses of Monochromatic Ultraviolet Radiation on Bacteria in Liquid Suspensions

Abstract

In the course of a quantitative investigation 1 of the lethal effect of monochromatic ultraviolet radiation below 3,000 A on stirred saline suspensions of bacteria (Escherichia coli) by a method similar to one described in a previous publication, 2 we observed the following phenomena:

If the bacteria are plated out in nutrient agar on Petri dishes subsequent to radiation, the colonies formed by the bacteria which survived the irradiation appear later than the colonies formed by the bacteria from the control suspension treated in the identical manner but protected from the ultraviolet radiation.

A careful study of the growth of the irradiated and control suspensions in a relatively poor medium (3 gm. NaCl, 0.2 gm. KCl, 0.2 gm. CaCl₂, 0.04 gm. Difco beef broth powder, 1,000 cc. distilled water, ca 10,000 bacteria/cc.) was made by determining the number of viable, colony-forming, organisms at proper intervals and several interesting observations were made:

1. The retarded growth (lag) phase of the irradiated bacteria was extended considerably over the control. This extension apparently depends on the energy applied to the suspension. When the bacteria had completed their growth, the suspension with the control and that with the irradiated bacteria contained the same number of organisms.

2. A careful determination of the lag phase revealed that where the control culture changed little in number of bacteria during this phase, the culture which had survived irradiation increased in number quite rapidly in the earlier part of the lag phase and then slowed down more or less for a certain time before it came into the log phase, thus producing a modified extension of the lag phase. The growth curve of the control rising in the beginning of the lag phase actually crossed the curve of the exposed culture and in the end an extension of the lag phase was induced by the radiation. The total number of viable bacteria of the exposed culture had increased during the lag phase. These increases were so pronounced that in the time during which the control culture showed a change in number of not more than 10 to 15%, the exposed culture had increased up to, or more than, 100%.