Which organism is well known to continue active metabolism at very high glucose concentrations by switching to overflow pathways (e.g., ethanol), thereby tolerating high substrate levels?

Introduction / Context:
High substrate concentrations can inhibit many microbes via substrate inhibition or by causing redox imbalances. Some organisms, however, maintain metabolism by redirecting flux to overflow pathways. Recognizing such organisms informs fed-batch feeding strategies.

Given Data / Assumptions:

• Focus on glucose as the substrate.
• “Continue metabolism” refers to sustaining high flux even when respiration saturates.
• Comparison among common biotech organisms.

Concept / Approach:
Saccharomyces cerevisiae is Crabtree-positive: at high glucose (even with oxygen), it favors fermentative metabolism (ethanol production) to sustain glycolytic flux. This allows continued metabolism at substrate levels that might otherwise slow strictly respiratory organisms. In contrast, many bacteria reduce growth when facing substrate inhibition or rely on respiratory capacity limits without strong overflow to ethanol.

Step-by-Step Solution:

Verification / Alternative check:
Industrial high-gravity brewing and bioethanol processes exploit yeast’s ability to metabolize at very high sugar concentrations, confirming practical tolerance and overflow.

Why Other Options Are Wrong:

• Pseudomonas aeruginosa: largely respiratory; less associated with ethanol overflow.
• Bacillus subtilis: can overflow acetate, but classical high-sugar ethanol overflow is a yeast trait.
• Escherichia coli: shows acetate overflow but is more sensitive to substrate inhibition than yeast at very high sugars.
• Nitrosomonas: chemolithoautotroph; not relevant to high-glucose regimes.

Common Pitfalls:
Equating any overflow (e.g., acetate in E. coli) with robust high-sugar tolerance; yeast remains the canonical example for high-glucose ethanol overflow.

Final Answer:
Saccharomyces cerevisiae