C. R. Aead. Sei. Paris, t. 316, Série III, p. 307-313, 1993 307 Agronomie/ZYXWVUTSRQPONMLKJIHGFEDCBA Agronomy Altérations morphologiques des cellules de Bacillus brevis par le cadmium Ali BOULARBAH, Jean-Louis MOREL et Gérard LEFEBVRE Résumé - L'effet de Cd sur la morphologie de Baeil/us brevis a été étudié par microscopie optique et microscopie électronique à transmission. En présence d'une concentration sub-Iétale en Cd, les cellules de Bacil/us brevis apparaissent sous forme sphérique et sont rassemblées en amas. On observe un épaississement et des invaginations de la paroi. Ces altérations morphologiques sont réversibles et supprimées par la présence de Mn2+. Les changements morphologiques et les teneurs élevées en Cd piégé par la paroi (88BA % de la teneur de la cellule) indiquent que celle-ci est la principale cible de Cd et pourrait constituer, chez Bacil/us brevis, une protection contre l'excès de Cd dans le milieu. Cd induced rnorphological alterations in Bacillus brevis cells Abstract - ln presence of sub-Iethal Cd concentration Bacillus brevis appeared as elusters of spherical cel/s instead of rads. MET observations showed the thickening and invagination of the cel/ wal/. Morphological changes were proved reversible when eel/s were plaeed in a Cd-free medium, and were alleoiated by addition of Mn2+. The alteration of the cel/ wal/ and ils high Cd content (88%) indicated that this eompartment was the main target for Cd and played a significant raie in the resistance of the strain ta excess Cd. Abridged English Version - INTRODUCTION.- Excess of cadmium may cause vanous metabolic inhibitions in microorganisms ([ 1] to [4]). Other metallic stresses (Pb) may also result in morphological changes in algae and bacteria ([2], [5], [6], [7]). However, no morpho- logical alteration due to Cd has been reported so far. This work was undertaken to determine whether Cd present at a sub-Iethal concentration in the growth medium has significant influence on the morphology of soil bacteria. Material and methods. - A Bacillus brevis was isolated from a soil contaminated by metallic dredged sediments [8]. This strain was shown to tolerate high concentrations of Cd (lC so = 72 mg 1-1) [8]. Cells grown on LB medium [9] were incubated for 18 hrs. at 20°C in diluted LB medium supplemented with 20 and 50 mg Cd 1-1 (Cd (N0 3 h 7 HzO). After centrifugation and washing in diluted LB, cells were disintegrated and the cell wall fragments were treated with DNase and RNase at 3rC for 30 min. [10]. Microbial material was dried for 48 hrs. at 80°C. Total Cd content in intact cells and cell walls was determined by flame AAS after digestion in 1: 1HzOz and HCl0 4 mixture. Cells of Bacillus -bretns were grown in presence and absence 50 mg Cd 1-1, and observed with a transmission electron microscope (Philips CM 120 at 80 kV) after fixation, dehydration and inclusion in Epon resin and slicing with an ultramicrotome. Cells were also observed with phase contrast microscopy in presence of Cd z + and Mn z + at similar concentrations. Viable cells were counted on LB agar. The viability was confirmed after incubation of cells with 2 (p-iodophenyl) 3-(p-nitrophenyl )-5-phenyl tetrazolium chloride (INT), and the pre- sence of INT -formazan crystals was determined under microscope [11]. Results showed that Cd accumulation in Bacillus brevis cells was correlated ta the Cd content in the medium (table 1). The cell wall contained 88% of the total cellular Cd. Observation in presence of the sub-Iethal Cd concentration (50 mg Cd 1- 1 ) showed clusters of spherical cells instead of rods (fig. 1). The reversion to normal cells (rods) was observed Note présentée par Henri DURANTON. 0764-4469/93/03160307 $ 2.00 Iêl Académie des Sciences