Multiple choice: In your laboratory tests, you find that your shredded cheese that was made with a protective culture containing Streptomyces natalensis is less likely to have visible mold growth. What is the most likely explanation for this observation?

Question

BlackTom AI · Accepted Answer

Natamycin is an antifungal polyene produced by Streptomyces natalensis, so S. natalensis produces natamycin.

1) Relevant knowledge points and concepts
- Streptomyces species are well known producers of secondary metabolites, including antibiotics and antifungals.  
- Natamycin (also called pimaricin, food additive E235) is a polyene antifungal produced by S. natalensis and related Streptomyces.  
- Natamycin binds to ergosterol in fungal cell membranes, disrupting membrane function and inhibiting fungal growth, especially on food surfaces.  
- Nisin is a bacteriocin active mainly against Gram-positive bacteria, not molds. Nitrate production or bacterial lysogeny do not directly prevent fungal growth.

2) Step-by-step reasoning
- Observation: shredded cheese made with a protective culture containing S. natalensis shows less visible mold.  
- Known fact: S. natalensis synthesizes natamycin, a potent surface-active antifungal used commercially to prevent mold on cheeses and cured meats.  
- Mechanism: natamycin binds fungal ergosterol, impairing membrane-dependent processes necessary for growth and spore germination. Because natamycin is poorly soluble, it remains mainly on the cheese surface where molds initiate growth, providing effective surface protection.  
- Therefore, the decreased visible mold is most directly explained by production of natamycin by S. natalensis.

3) Calculations
- No quantitative calculation is required for this conceptual question.

4) Why the correct answer is correct
- The protective effect against molds specifically implicates an antifungal compound; S. natalensis’s known product is natamycin, an established antifungal food preservative used to prevent visible mold on cheeses.

5) Why other options are incorrect
- “Produces nitrate”: nitrate production does not prevent fungal growth on cheese surfaces.  
- “Produces nisin”: nisin is antibacterial (bacteriocin) effective against Gram-positive bacteria, not molds/fungi.  
- “Encodes lysogenic phage”: lysogeny affects bacterial genomes, not fungal growth; it would not produce a surface antifungal.

Conclusion: The most likely explanation is that S. natalensis produces natamycin, matching the provided correct answer: S. natalensis produces natamycin.

Multiple choice: In your laboratory tests, you find that your shredded cheese that was made with a protective culture containing Streptomyces natalensis is less likely to have visible mold growth. What is the most likely explanation for this observation?单项选择题

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