polarization value (least fluid membrane) of 0.446, followed by values of 0.392, 0.371, 0.344 and 0.293, respectively, for B. cereus, Arthrobacter sp., P. fluorescens and P. putida. At an assay temperature of 30jC, the polarization values ranged from 0.357 to 0.288 for cells grown in the absence of tetracycline, regardless of the species. B. cereus grown in the presence of 1 Ag/ml tetracycline had lower polarization values than when grown in the absence of this antibiotic at all assay temperatures. Regardless of the absence or presence of 1 Ag/ml tetracycline in the growth medium, all bacterial species generally exhibited a more fluid membrane as the assay temperature increased from 10 to 50jC. To our knowledge, these are some of the first cytoplasmic membrane polarization values reported for these Gram-negative and Gram-positive bacteria over a broad temperature range and also for cells grown in the presence of tetracycline. D 2004 Elsevier B.V. All rights reserved