Food microbiology methods: For standard plate counts of bacteria in foods, the plating medium generally consists of which basic composition?

Introduction / Context:
Plate count methods estimate viable bacterial loads in foods. The medium used must be non-selective and nutritive enough for a wide range of heterotrophs while avoiding unnecessary inhibitors. A classic choice is Plate Count Agar (PCA), also called Standard Methods Agar, widely used in dairy, meat, and ready-to-eat food testing.

Given Data / Assumptions:

• We need the typical base composition for plate count media used in foods.
• Core components are a digest (peptone/tryptone), a vitamin source (yeast extract), a fermentable carbohydrate (glucose/dextrose), a solidifying agent (agar), and water.
• Added sodium chloride is not essential in the standard formula.

Concept / Approach:
Standard PCA contains peptone (or tryptone) for amino nitrogen, yeast extract for growth factors, and glucose as an energy source, with agar for solidification. Sodium chloride is not a required ingredient and may alter osmotic balance or selectivity unnecessarily for routine total counts. Therefore, the correct composition is the formula that includes peptone, yeast extract, glucose, agar, and water only.

Step-by-Step Solution:
Identify essential nutrients: peptone + yeast extract + glucose. Include agar to make plates; distilled water is the solvent. Exclude nonessential NaCl for standard PCA to avoid unintended selectivity. Choose the option listing peptone, yeast extract, glucose, agar, and water only.

Verification / Alternative check:
Common laboratory manuals list PCA as tryptone/peptone + yeast extract + glucose + agar + water; NaCl appears in many selective media but is not a staple of PCA.

Why Other Options Are Wrong:

• Options a–d: Either add NaCl unnecessarily, omit a key nutrient (peptone or yeast extract), or remove agar, which is essential for plating.

Common Pitfalls:
Confusing PCA with Nutrient Agar variants or selective media that do include NaCl; for broad plate counts in foods, the simpler PCA base is preferred.

Final Answer:
Peptone, yeast extract, glucose (dextrose), agar, and distilled water (no added sodium chloride).