A primary concern in creating a water reclamation system for long-duration manned space flight is the control of microbial contamination which can jeopardize water quality, compromise human health, and degrade constituent materials of the system. The microbial ecology facility in the Analytical and Physical Chemistry Branch of the Materials and Processes Laboratory at NASA's Marshall Space Flight Center (MSFC) is addressing this concern by means of experiments investigating the interaction of bacterial species in the development of a biofilm and their response to the introduction of additional species or to disinfection. Both static and recycling water systems are used.In static experiments, varied sequence and timing of species introduction in binary bacterial biofilms on 316L stainless steel elucidate the mechanisms involved in biofilm formation. In a second type of static experiment, biofilms constructed of one to seven species are challenged by the introduction of a new organism or by disinfectant in order to examine the relationship between community structure and biofilm response to these challenges.In bench-scale, closed-loop recycling water systems, the response of biofilms to iodine disinfection is investigated. Studies of NASA's environmental control and life support system water recovery test (ECLSS-WRT) have shown 1 to 2 mg/L I2 to be ineffective against some planktonic bacteria and mixed population microbial biofilms. Much higher iodine concentrations (15 to 20 mg/L I2), tested in the bench-scale system, were necessary to completely disinfect a stable biofilm. When Staphylococcus aureus and Escherichia coli were introduced to this system which contained natural mixed culture biofilms, it was found that their colonization in the biofilms increased their survival time from 3 to 5 days as unattached cells to over 60 days when protected in the biofilms. Iodine concentrations of 0.5 to 1.0 mg/L eliminated these organisms from the bulk water while over 4.0 mg/L was necessary to completely eliminate them from the biofilm.The factors of biofilm age, species composition, and sequence of introduction of bacterial types to a biofilm are shown to be significant in terms of the response of the biofilm to disinfection and the addition of new organisms.