Aerotolerant anaerobes are incapable of aerobic respiration, but can nevertheless grow in the presence of air. Many lactic acid bacteria fall into this category. They can only generate energy by fermentation and lack both catalase and superoxide dismutase, but are able to grow in the presence of oxygen because they have a mechanism for destroying superoxide. Microorganisms affect the Eh of their environment during growth. This is true especially of aerobes, which can lower the Eh of their environment while anaerobes cannot. As aerobes grow, oxygen in the medium is depleted, resulting in the lowering of Eh. Growth is not slowed, however, due to the ability of cells to make use of oxygen donating or hydrogen-accepting substances in the medium. The result of this is that the medium becomes poorer in oxidizing and richer in reducing substances. Microorganisms canreduce the Eh of a medium by their production of certain metabolic by-products such as hydrogen sulphide, which has the capacity to lower Eh to -300 mv.