Recurrent Mutation and Selection for Increased Penicillin Titre in Aspergillus nidulans

This study demonstrates the effectiveness of recurrent mutation and selection for improving penicillin production in Aspergillus nidulans. Two parallel selection programs were conducted over six cycles using different mutagens: near-ultraviolet light with 8-methoxypsoralen (program A) and ethyl methanesulphonate (program B). Each cycle involved screening 100 mutant strains and advancing the top 5% to the next round. Both programs achieved approximately threefold increases in penicillin yield, with the best strains reaching 20.5 u/ml and 17.6 u/ml respectively, compared to parental starting titres of 6.8-5.5 u/ml. The authors employed a single-stage screening approach rather than traditional multi-stage testing, which proved efficient given high mutation frequencies (>5%) and low testing errors. Standardization against control strains and contemporaneous comparison of selected lines confirmed genuine selection responses. The results validate theoretical predictions that mass selection of the top few percent strains per cycle can achieve rapid improvement in secondary metabolite production with less labor-intensive screening procedures than previously standard approaches.

Key findings

• Both selection programs achieved approximately 300% increases in penicillin titre over six cycles of recurrent mutation and selection.
• Single-stage screening combined with 5% selection intensity (carrying forward 5 of 100 strains per cycle) proved as efficient as multi-stage testing approaches used in previous strain development programs.
• Two mutagens with different mechanisms of action—8-methoxypsoralen/near-UV and ethyl methanesulphonate—produced comparable results despite their different mutagenic pathways.
• The final improved strains (A6-9 at 20.5 u/ml and B6-27 at 17.6 u/ml) represented approximately threefold improvements over their isogenic parental strains (6.8-5.5 u/ml).