How are auxotrophic markers used in industrial strain development?

Auxotrophic markers are mutations preventing synthesis of essential nutrients, used for genetic selection without antibiotics. Host strains lack specific biosynthetic genes (e.g., LEU2, URA3 in yeast; proA in E. coli). Plasmids carrying corresponding wild-type genes are maintained when cells are grown in minimal medium lacking that nutrient. Advantages over antibiotic markers: regulatory acceptance (especially for food/pharma products), no antibiotic costs, no resistance gene concerns, and stable selection pressure. Disadvantages: requires defined media, may reduce growth rate, and adds metabolic burden. Complementation systems can combine multiple markers for multi-plasmid systems. For CHO cells, DHFR and GS systems provide both selection and amplification. Auxotrophic markers are preferred for industrial production strains.