Many Enterococcus faecalis strains display tolerance or resistance to many antibiotics, but genes that contribute to the resistance cannot be specified. The multiresistant E. faecalis V583, for which the complete genome sequence is available, survives and grows in media containing relatively high levels of chloramphenicol. No specific genes coding for chloramphenicol resistance has been recognized in V583. We used microarrays to identify genes and mechanisms behind the tolerance to chloramphenicol in V583, by comparison of cells treated with subinhibitory concentrations of chloramphenicol and untreated V583 cells. During a time course experiment, more than 600 genes were significantly differentially transcribed. Since chloramphenicol affects protein synthesis in bacteria, many genes involved in protein synthesis, for example, genes for ribosomal proteins, were induced. Genes involved in amino acid biosynthesis, for example, genes for tRNA synthetases and energy metabolism were downregulated, mainly. Among the upregulated genes were EF1732 and EF1733, which code for potential chloramphenicol transporters. Efflux of drug out of the cells may be one mechanism used by V583 to overcome the effect of chloramphenicol. 1. Introduction Chloramphenicol (Cm) has been used as an broad-spectrum antibiotic in human and veterinary medicine since the 1950s, but the use of chloramphenicol in humans is now rather limited [1]. In animals, chloramphenicol use is limited to pets and non-food-producing animals. The structure of chloramphenicol is relatively simple, and it was the first chemically synthesized antibiotic on the market [1].Chloramphenicol inhibits translation in bacteria, by inhibition of the peptidyl transferase reaction of the large subunit of the ribosome. The inhibition of the peptidyl transferase activity is mediated by binding to several proteins in the 50S ribosomal subunit [2]. A number of resistance mechanisms to chloramphenicol in bacteria has been described, of which the most common is enzymatic inactivation by acetylation of chloramphenicol via chloramphenicol transferases (CATs) [1]; Chloramphenicol acetyltransferases (CATs) have been described in both Gram-positive and Gram-negative bacteria. CATs catalyze the hydroxylation of chloramphenicol, thereby leaving the antibiotic inactive [1]. The inactivation of chloramphenicol can also be performed by xenobiotic acetyltransferases [1, 3]. A third mechanism of chloramphenicol inactivation is performed by chloramphenicol phosphotransferases [1]. Several examples of chloramphenicol resistance or lowered
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