Currently, urban and industrial wastewater treatment plants (WWTP), are focused mainly on the reduction of biological oxygen demand and sometimes on the removal of nutrients such as nitrogen and phosphorus without paying too much attention neither the microorganisms that perform the treatment nor the microorganisms that interfere in the process.
      
      In these environments there is a large diversity of microorganisms that haven’t been studied in detail and that could provide real and practical solutions to the problems that could be caused by the wastewater treatment process. These microorganisms could also provide important biotechnological applications such as the elimination of recalcitrant or toxic compounds in different contaminated environments. That is why studies of bacterial biodiversity in these environments would substantially improve the efficiency of water treatment process and therefore the environment, promoting sustainable development that would have a positive impact on the society’s welfare.
      
      On the basis of the above, this work propounded a study of the biodiversity of actinomycetes isolated from different wastewater treatment plants (WWTP) and the identification of the species involved in the processes of foam formation and degradation of toxic compounds.
      
      Twenty eight WWTP from different localities were studied, most of them with frequent episodes of biological foaming. First of all, plate isolation in three different culture media was carried out in order to obtain the maximum possible biodiversity. A total of 152 isolates were obtained with typical morphology of nocardioform actinomycete. These isolates were characterized, first, by morphological observation, Gram staining and mycolic acids detection to verify they belong to the suborder Corynebacterineae. Then, detection of diaminopimelic acid isomer and of the predominant sugar in cell wall was performed. All isolates, except five, contain mycolic acids, meso-diaminopimelic and arabinose and galactose as predominant sugars, therefore these isolates belong to the suborder Corynebacterineae.
      
      To identify the isolates to species level, sequence analysis of 16S rDNA was performed. 16S gene was amplified with universal primers, sequenced and phylogenetic trees were constructed with the corresponding nucleotide similarity matrix. The results show that 66% of the identified isolates belong to the genus Gordonia, of which only 22% belong to the species G. amarae. 11% of the isolates belong to the genus Mycobacterium, 10% to the genus Tsukamurella, 6% to the genus Rhodococcus and the remaining 6.5% distributed among the genera Corynebacterium, Dietzia, Mycobacterium, Pseudonocardia and Williamsia. To complete the identification of the isolates by the means of polyphasic taxonomy, a series of phenotypic tests was conducted to corroborate the phylogenetic identification. In some species, the identification is not conclusive, so may be considered they are new species.
      
      Once the isolates were identified, studies of biodegradation of two toxic compounds from petroleum, such as phenol and naphthalene, were carried out. For this purpose, strains were grown in three mineral culture media, using as the sole carbon source phenol and naphthalene, because these substances cause the worst problems in industrial water treatment plants. Finally, PCR technique adjustment was conducted for the detection of catechol 1,2-dioxygenase gene and so to determine the genetic potential of biodegradation of identified microorganisms.
      
      The result indicates that 50% of strains degrade at least one of the toxic compounds. The catechol 1.2-dioxygenase gene was detected in 20% of the isolates.
      
      Polyphasic taxonomy was found to be the most reliable method for species level identification of isolates from WWTP. It was also noted that there is a great diversity of species that are not often found in other treatment plants and that a high percentage of the species had the ability to degrade toxic compounds. In this way, besides the fact that the biological process can be optimized depending on the filamentous bacteria found and the effluent quality can be improved, these degrading bacteria can be used in contaminated environments to improve the environment in general.