Industrial fermentation — In producing L-glutamic acid via fermentation with Micrococcus (Corynebacterium) glutamicus, why is the medium's biotin concentration considered critically important for both growth and product formation?

Introduction / Context:
L-glutamic acid (a key flavor enhancer and amino acid feedstock) is classically produced by Micrococcus/Corynebacterium glutamicus. Process engineers tune vitamin levels—especially biotin—to control cell membrane physiology and carbon overflow. Understanding how biotin impacts growth versus secretion is essential to reliable high-titer production.

Given Data / Assumptions:

• L-glutamic acid is produced by actively growing cells that secrete glutamate when membrane permeability is altered.
• Biotin is a growth factor (cofactor for carboxylation reactions) required in small amounts.
• Extremes of biotin level can either stunt growth or suppress glutamate excretion.

Concept / Approach:
Biotin limitation modifies fatty acid synthesis and membrane composition, increasing membrane permeability and promoting glutamate efflux. However, if biotin is too scarce, cells fail to grow and product formation collapses. Conversely, excess biotin restores tight membrane control, reducing glutamate secretion even if biomass is adequate.

Step-by-Step Solution:

Verification / Alternative check:
Industrial practice deliberately employs low-biotin media or uses surfactants/biotin antagonists to trigger glutamate overflow while ensuring minimal but sufficient biotin for cell growth.

Why Other Options Are Wrong:

• Only (a): ignores the well-known suppression of glutamate secretion at high biotin.
• Only (b): ignores growth failure at too-low biotin.
• None: contradicts established process control strategies.

Common Pitfalls:
Confusing “more vitamin equals better production.” For glutamate processes, controlled limitation—not abundance—drives secretion, provided growth is not starved.

Final Answer:
both (a) and (b)