MX80 bentonite clay has been selected as the buffer and backfill in a proposed method for long-term deep geological storage of nuclear waste. Extensive studies have been carried out on the geomechanical properties of the clay; however, it is not clear what effect microbes, specifically iron-reducing bacteria, will have on its ability to function as an affective barrier. Iron-reducing bacteria can reduce structural or external Fe(III) to Fe(II) and have been previously identified in the indigenous microbial community of MX80 bentonite. Experiments to assess bacterial survival at the high temperature and low water conditions likely to exist in the repository were carried out at different temperatures with the addition of steel to represent a nuclear waste canister. The resulting microbial enrichments were analysed, and mineralogical and geomechnical analysis was carried out on the clay. Microbial sequencing revealed that iron-reducing bacteria, and other indigenous species can survive these conditions in MX80 bentonite in either an active or dormant state. Microbial influenced mineralogical changes may lead to a loss of silica from the clay and reduction of Fe(III) to Fe(II). These changes could alter the ability of the clay to act as an effective barrier in nuclear waste disposal. Furthermore, evidence of reduced steel corrosion when microbes were present suggested that microbial activity may lead to either a protective coating on the steel or depletion of oxygen to slow corrosion. The production of such a layer would benefit nuclear waste disposal by inhibiting corrosion of a metal waste canister.