Inès Mehri 1 Yousra Turki 1 Hanène Chérif 2 Amel Khessairi 1 Abdennasser Hassen 1 Maher Gtari 2 1 Laboratoire Traitement et Recyclage des Eaux, Centre de Recherche et des Technologies des Eaux, Borj- Cédria, Tunisia 2 Laboratoire Microorganismes et Biomolécules Actives Faculté des Sciences de Tunis, Université Tunis El Mana, Tunis, Tunisia Research Article Influence of Biological Treatment and Ultraviolet Disinfection System on Pseudomonas spp. Diversity in Wastewater as Assessed by Denaturing Gradient Gel Electrophoresis Biological treatment methods use natural processes of ubiquitous living organisms to improve or upgrade the quality of a wastewater. To investigate the dominance of Pseudomonas spp. community in untreated and treated wastewater from full-scale bio- discs treatment plant, PCR–denaturing gradient gel electrophoresis coupled with sequence analysis of 16S rRNA gene fragments from dominant bands, were performed. The Pseudomonas species distribution over the total treatment process and over seasons (summer and winter) was obtained by comparing the denaturing gradient gel electrophoresis (DGGE) patterns to a normalized mix of nine reference Pseudomonas strains. Results indicated that Pseudomonas putida and Pseudomonas fluorescens strains showed dominance in wastewater, remarkable stability in the biofilm and resistance to UV irradiation. A reduction of Pseudomonas aeruginosa in treated wastewater was also observed. In addition, climatic and operational conditions results in selection of microbial community. The high temperature and intense solar radiation contribute to the total absence of the Pseudomonas syringae strain. In this work, the effect of increasing UV 254 germicidal dose on the Pseudomonas community in secondary treated wastewater effluent was also tested. At a dose of 1200 mWs/cm 2 , total disappearance of the band corresponding to P. aeruginosa, was noted. The DGGE approach can be used as an effective method to assess directly the Pseudomonas community shifts in studied wastewater treatment plant. Keywords: Biological inactivation; PCR–DGGE; Pseudomonas community; 16S ribosomal DNA; UV-C Received: October 24, 2012; revised: April 11, 2013; accepted: April 30, 2013 DOI: 10.1002/clen.201200589 1 Introduction The genus Pseudomonas is a heterogeneous group of bacteria due to the elevated metabolic versatility capable of utilizing a wide range of simple and complex organic compounds [1]. Therefore, they are found ubiquitously in water, soil, and plants. Pseudomonas “sensu stricto” group I includes, firstly, the type species of the group, Pseudomonas aeruginosa. This bacterium is an opportunistic human pathogen responsible for frequently lethal hospital infections [2, 3]. Secondly, plant deleterious pseudomonads, like P. syringae, produce toxins that affect the plant growth and their primarily foliar pathogen producing diverse types of disease symptoms [4]. Finally, P. fluorescens and P. putida considered among saprophytic bacteria can exert a toxic activity against various deleterious microorganisms [5]. Consequently, the genus Pseudomonas is not only plant growth- promoting bacteria, but is able to degrade, alter or relocate the structure of a variety of pollutants (hydrocarbons, heavy metals, dioxins, etc.) existing in soil or polluted water [6–8]. This prominent property makes these microorganisms attractive candidates for use in bioremediation and biocontrol activities [6, 9]. Untreated or insufficiently treated wastewaters can cause several problems, such as bacterial contamination, eutrophication, oxygen consumption, and toxicity, when discharged to environment [10]. The biological process is considered one of the most frequently used techniques to treat municipal wastewater. Therefore, understanding the biodiversity and the dominant species of the Pseudomonas community is of great importance in studying contaminant degradation pathways, optimizing treatment processes, and improv- ing removal efficiencies [11]. Attempt to improve the microbiological water quality, UVC light could have a germicidal effect on bacteria, bacterial spores, viruses, mold spores, yeast, and algae, but the doses needed to inactivate them vary [12]. The effectiveness of UVC light in biological inactivation arises primarily from the fact that DNA molecules absorb UV photons between 200 and 300 nm, with peak absorption at almost 260 nm. This absorption creates damage in the DNA by altering nucleotide base pairing; thereby creating new linkages between adjacent nucleotides on the same DNA strand [13]. The majority of studies on wastewater Pseudomonas communities have been relied on cultivation-dependent techniques. However, the Correspondence: Dr. I. Mehri, Laboratoire Traitement et Recyclage des Eaux, Centre de Recherche et des Technologies des Eaux, Borj-Cédria, Tunisia E-mail: mehri_ins@yahoo.fr Abbreviations: DGGE, denaturing gradient gel electrophoresis; DTI, distribution tank input 578 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com Clean – Soil, Air, Water 2014, 42 (5), 578–585