Laundry detergent contains surfactants and alcohol ethoxylates that can pollute aquatic environments. These cleaning agents are toxic to plants and animals, but little is known about their effects on microbial communities. Testing the impacts of laundry detergent in the environment can be challenging. Microcosms are used to model natural systems and facilitate manipulative experiments; however, once the system is established in the laboratory it will begin to diverge from the natural state. In this project, microcosms (~ 4 L) were established with water collected from a pond on the campus of the University of Houston – Clear Lake. The microcosms were stocked with ten representative aquatic plants and animals. Readily-culturable bacteria from a microcosm that received laundry detergent were compared to a parallel control.  Libraries of bacteria were generated, by culturing on tryptic soy agar, after one and five weeks. Bacteria with distinct phenotypes were identified with Matrix Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) mass spectrometry. Acinetobacter bayli, an opportunistic pathogen commonly found in soil and water, was numerically abundant in the library generated from the control library on week one. A month later (week 5), Rhizobium radiobacter and Bacillus megaterium were numerically abundant in the control microcosm. These bacteria are associated with soil, not aquatic, systems. At that same time point (week 5), Citrobacter freundii, an emerging pathogen that frequently causes urinary tract infections, dominated the library generated from the microcosm that was amended with laundry detergent. This suggests that laundry detergent changes the structure of the microbial communities and can select for pathogens; however, the change in structure over time in the control microcosm, suggests that microcosms may model aquatic systems well.