Diversity of chromium-resistant and -reducing bacteria in a chromium-contaminated activated sludge R. Francisco, M.C. Alpoim 1 and P.V. Morais Instituto do Ambiente e Vida and 1 Departamento de Bioquı  mica, Faculdade de Cie ˆ ncias e Tecnologia, Universidade de Coimbra, Coimbra, Portugal 2001/13: received 12 March 2001, revised 18 July 2001 and accepted 13 November 2001 R. FRANCISCO, M.C. ALPOIM AND P.V. MORAIS. 2002. Aims: This study attempts to establish a relationship between the Cr(VI) resistance of the culturable microbial community and the Cr(VI) resistance and Cr(VI)-reducing ability of representative strains of each population, in order to assess whether these are exclusive characteristics of one microbial group or abilities shared among many groups. Methods and Results: A group of 48 Cr(VI)-resistant isolates, with different colony types, was isolated from chromium-contaminated activated sludge. Sodium dodecyl sulphate- polyacrylamide gel electrophoresis protein patterns and fatty acid methyl ester analysis identified six populations, representing 54% of the isolated bacteria, as belonging to the genera Acinetobacter and Ochrobactrum. The remaining populations included strains identified as species of the b-Proteobacteria and high G + C Gram-positive bacteria. The Cr(VI) resistance and reduction ability of the strains were tested. All but two isolates grew in the presence of 1 mmol l )1 Cr(VI). During enrichment, all isolates were able to survive to 2 mmol l )1 Cr(VI) and complete Cr(VI) reduction was achieved. Representative strains of each population were able to partially reduce (5®4–39®1%) the Cr(VI) present in the growth medium. Conclusions: Most of the identified isolates have never been reported to be Cr(VI)-resistant and/or Cr(VI)-reducing strains. The mechanisms of Cr(VI) resistance and reduction may differ from group to group; therefore, it is evident that both Cr(VI) resistance and reduction are shared abilities and not an exclusive characteristic of a single group, possibly reflecting horizontal genetic transfer resulting from selective pressure in this contaminated environment. Significance and Impact of the Study: To our knowledge, this is the first study of a microbial community under chronic chromate stress and, as the success of microbial-based metal remediation technologies requires a better understanding of the microbial community and the population response to metal stress, it may contribute to the implementation of a strategy of bioremediation of chromate-contaminated environments. INTRODUCTION The widespread use of chromium compounds by industries has led to large quantities of this element being released to the environment. Only hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are ecologically important because they are the more stable oxidation states. Being mutagenic, carcinogenic and teratogenic, Cr(VI) is approx. 100-fold more toxic than Cr(III) (Shen and Wang 1995). However, Cr(III) is considered to be relatively innocuous because it is less soluble and does not permeate eukaryotic and prokaryotic membranes. Serious concerns about the toxicity of Cr(VI) compounds necessitate the recovery and reuse of chromium from industrial wastes and it is essential to convert the unrecov- ered Cr(VI) to a less toxic form. Several bacteria have been described as being able to reduce Cr(VI) to Cr(III) under aerobic and anaerobic conditions and the biological trans- formation of the very toxic Cr(VI) to the less toxic and less Correspondence to: P.V. Morais, Instituto do Ambiente e Vida, A/C Departamento de Bioquı´mica, Faculdade de Cieˆncias e Tecnologia, Universidade de Coimbra, Apartado 3126, 3001-401 Coimbra, Portugal (e-mail: pvmorais@ iav.uc.pt). ª 2002 The Society for Applied Microbiology Journal of Applied Microbiology 2002, 92, 837–843