The Production and Selection of a Family of Strains in Penicillium Chrysogenum

SUMMARY

A ten-year study on spontaneous and induced variation in penicillin-producing molds has yielded, in the writers' laboratories, an assemblage of superior variants which have become well known in industrial circles and which are referred to as the “Wisconsin Family” of P. chrysogenum strains. The origin, inter-relationships, and characteristics of key strains in this “Family” are here described.

The methods employed in obtaining, testing, and maintaining the improved stocks are considered. Strains embodying various advantageous changes have been secured through treatment of spores with ultraviolet radiation and with a nitrogen mustard. In addition, spontaneous changes have provided some improvements in the stock. Of the two wave lengths of ultraviolet employed in mutagenic treatments, neither has appeared to have any clear advantage over the other; and desirable changes were secured at high, moderate, and low UV dosage levels.

The Wisconsin Family begins with strain Wis. Q176. This strain, obtained in 1945, gave yields approximately double those obtainable from the best strain previously known. From this outstanding variant all other members of the Wisconsin Family trace their descent. The “Pigmentless Series,” consisting of strains which secrete no yellow pigment, was inaugurated in 1947. In the “nitrogen mustard branch” of this series have emerged the best strains obtained in this selection program to date. Yields as high as 3000 O.U/ml have been secured from certain variants in this group under fermentation conditions employed in this laboratory. This is more than ten times the amount of penicillin produced under the same conditions by the improved strain from which the ‘Wisconsin Family” arose and about forty times the amount produced by the wild-type ancestor.

A more or less progressive decline in the vegetative and reproductive vigor of the strains has accompanied the increase in capacity to produce the antibiotic, not only in the Wisconsin Family itself but also in the succession of improved stocks which were ancestral to the Wisconsin Family. Feeble sporulation and slow growth of the mycelium on agar are characteristic of all of the top-ranking strains. It is emphasized, however, that reduced vigor is not an infallible criterion for the selection of superior penicillin-producing variants. It is further emphasized that there is no close correlation between rate of colony growth on agar and rate of development of mycelium in aerated liquid cultures. In both small and large scale fermentations the best lines regularly produce their high yields about as quickly as maximum yields are obtained from wild-type or inferior strains.