Romanian Biotechnological Letters Vol. 14, No. 2, 2009, pp. 4211-4224 Copyright © 2008 Bucharest University Printed in Romania. All rights reserved Romanian Society of Biological Sciences ORIGINAL PAPER 4211 Evaluation and monitoring of quorum sensing soluble mediators implicated in the regulation of bacterial growth in Vibrio strains Received for publication, October 24, 2008 Accepted, March 5, 2009 ANCA-MICHAELA ISRAIL, *MARIANA- CARMEN CHIFIRIUC, CRISTINA DELCARU, CARMEN IORDACHE, *DIANA PELINESCU, *ELENA SASARMAN National Institute for Research and Development in Microbiology and Immunology Cantacuzino, Spl. Independentei 103, Bucharest, ROMANIA University of Bucharest, Faculty of Biology, MICROGEN, Ale. Portocalelor 1-3, Bucharest, ROMANIA Corresponding author address: <carmen_balotescu@yahoo.com> Abstract Quorum sensing (QS) is an ubiquitary regulation mechanism in the bacterial world implicated in intra and inter–bacterial communication and dependent upon the cellular density. In the present study, the authors have tried to elucidate the influence of soluble mediators accumulated in stationary phase cultures on the multiplication rate and growth curve of the homologous strains belonging to Vibrionaceae family. In this purpose, in the first step of the experiment, the growth curves of two bacterial strains, one non halophilic and one halophilic were comparatively established for the simple, control culture, culture treated by the homologous filtrate (containing soluble mediators) and culture treated by autoclaved filtrate (at 115 0 C and 130 0 C, respectively), incubated in small/big volumes at 4 0 C, 28 0 C and 37 0 C. Taking into account that the soluble mediators remain active in filtrate as well as in cultures treated at 115 0 C, for further experiments, there were used only simple, control cultures compared with cultures treated by simple filtrate, all of them being cultivated in small volumes and incubated at 37 0 C. The present study demonstrated that in stationary phase, bacterial cultures are accumulating soluble factors influencing the duration and aspect of the bacterial growth curve. In the most cases of the tested strains this influence consisted in the reduction of the multiplication rate and subsequently, of the culture density, the shortening of the lag phase and of the total duration of the growth curve. The synthesis of autoinducers proved to be dependent upon the bacterial strain, source of isolation (clinical case or aquatic environment), incubation temperature, volume of the culture medium, influencing the oxygenation surface. Keywords: Quorum Sensing / non-halophilic / halophilic vibrios / autoinducers / lag phase / exponential phase. Introduction Quorum sensing (QS) is an ubiquitary regulation mechanism in the bacterial world implicated in intra and inter–bacterial communication and dependent upon the cellular density. The positive/negative regulation of bacterial virulence factors under the control of QS phenomenon is of crucial importance for the host-infectious agent inter-relation as well as for the understanding of the mechanisms of bacterial pathogenesis (1). The first and the best studied QS system was that mediated by pheromones of homo-serin-lactone (HL) structure, described in the Gram-negative bacteria (2, 14) and namely in bioluminiscent V. fischeri and V. harveyi (Photobacterium) with natural aquatic marine habitat as well as other Vibrionaceae (V. cholerae, V. parahaemolyticus, V. alginolyticus, V. anguillarum, Aeromonas hydrophila),