Chemical Changes in Submerged Penicillin Fermentations

Abstract

The chemical changes induced by Penicillium notatum and P. chrysogenum during submerged penicillin fermentations follow a predictable pattern. The fermentation can be divided into 3 phases. On a medium containing 2% corn steep solids and 2% lactose, the metabolic changes proceed in the following manner: The lactic acid and nitrogenous compounds of corn steep, being more readily available than lactose as carbon source, are rapidly attacked during fermentation phase 1. As the soluble organic N compounds of corn steep are utilized, the N they contain is partially liberated in the form of NH3 and partially used in building mycelium. When the bulk of the readily available C is exhausted, phase 2 begins. Lactose is now rapidly oxidized by the mycelium which has been formed in phase 1, and oxidation of this sugar is usually completed by the end of phase 2. Coincident with the rapid oxidation of lactose, NH3 is utilized more rapidly than it is formed and its conc. falls. The nonammonia Kjeldahl N furnished by the corn steep decreases rapidly during phase 1 but remains without great change through phase 2. In phase 3 marked liberation of soluble N from the mycelium is apparent; NH3 is often freed and nonammonia Kjeldahl N always is freed. The nitrate N in the medium is utilized slowly; if large amts. of nitrate are added, or if the level of corn steep is low, more nitrate is utilized. The initial rise in pH during phase 1 can be accounted for largely by an accumulation of NH3, by liberation of Na ions from the utilization of nitrates, and by a rapid breakdown of lactic acid. Phase 2 is characterized by a pH plateau, or some depression in the pH resulting from NH3 utilization. A plateau near a neutral reaction is more favorable for penicillin accumulation. Penicillin concs. decrease during the period of the final pH rise that accompanies the liberation of NH3 and basic N compounds from the mycelium. The effect of the composition of the medium on metabolic changes has been illustrated by specific examples, including fermentations carried out on a lactose, glucose, corn steep medium, on a medium, without nitrate, on low corn steep media, on high corn steep media, and on a glucose, corn steep medium. The different strains investigated have shown but slight differences in their metabolism. Data concerning the oxygen requirements of various strains on different media have been obtained and related to O2 supply. The influence of aeration has been illustrated by data on fermentations carried out in aerated bottles, tanks, and flasks agitated in a rotary shaker. There has been no indication that the pO2 of the flask atmosphere influences penicillin yields in the shake flask fermentations employing a lactose, corn steep medium; the PCO2, however, seems to affect penicillin production. Among fermentations set up under similar conditions a correlation exists between ammonia levels and penicillin yields; this interrelationship has been discussed.