The inclusion of bioregenerative life support elements (i.e., plant growth systems and bioreactors) will significantly increase the total abundance of microorganisms in extraterrestrial facilities. If the microbial communities associated with these systems (e.g., biofilms attached to plant roots or hardware surfaces) serve as reservoirs for potentially pathogenic human-associated bacteria, then bioregenerative systems may represent a human health risk. Research at the Kennedy Space Center during the past several years has attempted to quantify this risk by assessing the capacity of different human-associated bacteria to survive in prototype ALS systems. Preliminary, short-term studies indicated that many potentially pathogenic human-associated bacterial species identified from past space missions (Pseudomonas aerugi-nosa, Pseudomonas cepacia, Escherichia coli, Staphylococcus aureus, and Streptococcus pyogenes) have the capacity to grow on the roots of plants, one of the largest potential sites of microbial activity in bioregen-erative life support systems. However, only P. aeruginosa could persist at detectable levels when competition from typical root-associated bacteria was present. Subsequent long-term plant growth experiments have confirmed the greater capacity of P. aeruginosa to persist in plant growth systems, although no human-associated bacteria tested to date have proliferated in the systems. Rather, relative success is measured by the rate at which bacterial numbers decrease following introduction. Recent and current studies have focused on the influence of community richness (i.e., the number of microbial species) on the ability of introduced human-associated bacteria to persist within prototype systems. Richness may be manipulated in a bioregenerative system; a stringent decontamination approach could lead to very low richness, but specific inoculation with either defined bacterial isolates or undefined mixtures of microbial communities would increase richness.