Food and industrial microbiology — Among the following groups, which has the greatest tolerance to acidic conditions (i.e., can grow and spoil products at lower pH values than the others)?

Introduction / Context:
Acidity (pH) is a critical intrinsic factor that governs microbial growth in foods and fermentation systems. Understanding which microbial groups tolerate lower pH helps predict spoilage risks in acidic products (juices, pickles, jams) and choose appropriate preservation hurdles. This question compares the acid tolerance of yeasts and moulds with that of typical bacteria, including the well-known neutrophile Escherichia coli.

Given Data / Assumptions:

• Bacterial pathogens and many spoilage bacteria prefer near-neutral pH (about 6.5–7.5).
• Yeasts and filamentous fungi (moulds) are often active at lower pH ranges common in acidic foods.
• E. coli is a neutrophile and not adapted to strong acidity without special conditions.

Concept / Approach:
Microorganisms are broadly classified by pH preferences: acidophiles (low pH), neutrophiles (near-neutral pH), and alkaliphiles (high pH). Many food-related bacteria are neutrophiles and become inhibited as pH drops below about 4.5–5.5. In contrast, numerous yeasts and moulds retain metabolic activity and growth at pH values where bacteria are largely suppressed, explaining why acidic foods often suffer yeast or mould spoilage rather than bacterial proliferation.

Step-by-Step Solution:

Verification / Alternative check:
Food spoilage surveys repeatedly show yeast/mould growth in acidic products like fruit juices and jams, while bacterial growth is limited unless pH rises or protective niches form.

Why Other Options Are Wrong:

• Bacteria: generally less acid tolerant than fungi, with exceptions (e.g., some LAB) but not overall.
• E. coli: neutrophile; acid-sensitive in most foods.
• None of these: incorrect, because yeasts/moulds are the recognized acid-tolerant spoilers.

Common Pitfalls:
Assuming all bacteria are equally sensitive to acidity; lactic acid bacteria complicate the picture but fungi still dominate very low pH spoilage.

Final Answer:
Yeast and moulds