By applying random genetic modification techniques, also called mutagenesis, mutations in the microbial DNA are induced through employing physical or chemical agents (mutagens). The mutations are produced randomly in the DNA chain and lead to the production of mutant microorganisms that can transfer these changes in the genome to the subsequent generations. Among the most used physical agents, ultraviolet radiation (usually with a wave length of 260 nm) is highlighted. This radiation is absorbed by the double bonds of pyrimidines composing the DNA chain. As a result, DNA photoproducts are obtained that provoke drastic changes in the nucleic acids. This process is not directed and leads to the death of most irradiated cells. However, a small number of cells survive and even acquire some desirable traits as a higher product yield. The best mutants are selected by screening procedures and are irradiated again in order to obtain mutants with better yields. Besides ultraviolet radiation, x-rays and ionizing radiations are also employed though these agents can cause the death of all the irradiated cell population. Among the most employed chemical mutagens are the nitrogenated base analogues (5-bromouracile, 2-aminopurine), deami — nating or hydroxylating agents (nitrous acid, hydroxyl amine), alkylating agents (mustard gas, ethyl-ethanesulfonate, ethyl-methanesulfonate, nitrosoguanidine), intercalant agents (acridine orange, ethidium bromide, proflavine), and pairing blocking agents (benzopyrene, aflatoxine B1; Crueger and Crueger, 1993). The selection programs of industrial microorganisms by mutagenesis are very pro­longed, tedious, and expensive, but can cause significant increases in the yield of the overall process.

In the case of fuel ethanol production, the development of mutant cells has been oriented, besides increasing ethanol yield, to the enhancement of tolerance to salts and impurities contained in the medium (e. g., for the case of yeasts culti­vated on molasses) or to the acquiring of flocculating properties. This latter trait allows the ready separation of yeast cells during schemes of continuous fermenta­tion or by repeated-batch regimes (see Chapter 7) because the cells agglomerate and settle (or float) allowing their rapid removal from the cultivation broth.