UV-C inactivation in Escherichia coli is affected by growth conditions preceding irradiation, in particular by the specific growth rate.
OBJECTIVE
The objective was to analyse the impact of growth conditions, in particular of the specific growth rate, on the resistance of Escherichia coli towards UV-C irradiation.


RESULTS
Escherichia coli K12 wild-type bacteria (and in some experiments also a mutant not expressing RpoS, the global regulator of the general stress response; rpoS(-) mutant) were cultivated either in batch culture until stationary phase was reached or in continuous culture at different specific growth rates (μ) and then irradiated with UV-C light. Inactivation was determined by plating. The specific growth rate had a profound effect on UV-C resistance. Stationary phase or very slowly growing cells (0≤μ<0·1 h(-1)) as well as fast-growing cells exhibited a high resistance compared to bacteria growing at an intermediate rate (between 0·2 and 0·4 h(-1) ). The rpoS(-) mutant was more susceptible to UV irradiation than the wild-type when obtained from stationary phase, while mutant cells from continuous culture (μ=0·2 h(-1)) revealed a UV-C resistance similar to the wild-type grown under the same conditions.


CONCLUSIONS
Antecedent growth conditions determine the physiological state of bacteria including the resistance towards UV-C irradiation. In particular, the specific growth rate was shown to markedly affect UV-C resistance of E. coli. The observed pattern of UV-C resistance exhibiting a minimum at intermediate specific growth rates must be explained by two or several counteracting mechanisms. For lower specific growth rates, the regulator of the global stress response, RpoS, is at least partly involved in the physiological processes responsible for UV-C resistance.


CONCLUSIONS
The observed impact of antecedent growth conditions on UV-C resistance of E. coli stresses the necessity to use clearly defined cultivation conditions and to report them to gather meaningful and comparable data on the UV-C resistance of micro-organisms.
Bucheli-Witschel
M
M
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Environmental Microbiology, Dübendorf, Switzerland.
Bassin
C
C
Egli
T
T
D001426:Bacterial Proteins / Q000235:genetics; D046508:Culture Techniques; D048168:Escherichia coli K12 / Q000254:growth & development / Q000528:radiation effects*; D009154:Mutation; D011836:Radiation Tolerance / Q000235:genetics / Q000502:physiology*; D012808:Sigma Factor / Q000235:genetics; D014466:Ultraviolet Rays
D001426:Bacterial Proteins; D012808:Sigma Factor; C069567:sigma factor KatF protein, Bacteria