245 the untreated control level of SCE was obtained at 8 × 10 -s M. Exposure for the final 24 h of culture produced a similar response but raised the maximum tolerated concentration to 2 × 10 -4 M. Exposure for 120 min in the presence of $9 mix commencing at 48 h resulted in the induction of significantly fewer SCE and less sensitivity compared with a similar non-activated 120-min exposure. Without activation doubling of the untreated control level of SCE occurred between 10 -4 and 2 X 10 -4 M while the trebling dose of 4 × 10 -4 M was also the m a x i m u m tolerated concentration. In the presence of $9 mix the doubling dose was greater than 4 X 10 -4 M and the untreated control level of SCE was more than trebled by the m a x i m u m tolerated concentration of 10 -3 M.
152 D.J.V. Paes and D.J. Tweats, Glaxo Group Research Ltd., Harefield, Middlesex (United Kingdom) Bacterial mutagenicity tests of ~-lactam antibiotics Mutagenicity screening of antibacterial compounds is at the m o m e n t effectively restricted to eukaryotic test systems. As eukaryotic microorganisms such as yeast are insensitive to m a n y mutagens due to impermeability and tests involving mammalian cells are n o t yet refined to a stage where routine screening of large numbers of compounds is practicable, it is desirable to devise effective bacterial tests for monitoring mutagenicity of realistic concentrations of these compounds. This communication describes two methods using bacterial cells for mutagenicity tests of/3-1actam antibiotics. The first m e t h o d exploits the nature of these compounds, in that t h e y axe only effective in killing growing cells. The tester strains are exposed to the test/3-1actam in buffer or unsupplemented minimal media, After exposure, the organisms are filtered, washed twice and resuspended in media suitable for bacterial fluctuation tests as described by Gatehouse (1978). The second m e t h o d , which has been devised to allow the detection of mutagenic contamination of batches of non-mutagenic antibiotics, involves the destruction of the antibacterial activity of ~-lactams by K1 ~-lactamase, isolated from Klebsiella 108ZE (K1), before testing by conventional methods. Results will be presented showing the response of model fl-lactam antibiotics and reference mutagens to both test systems.
Reference Gatehouse, D,G. (1978) Mutation Res., 5 3 , 2 8 9 - - 2 9 6 . Acknowledgement: Dr. M. Boulton, Glaxo, for supplying K1.