Which of the following statements is incorrect for antibiotics?

We need to identify the incorrect statement about antibiotics.

Option A: "An antibiotic must be a product of metabolism."

This is correct. Antibiotics are naturally produced by microorganisms as metabolic products.

Option B: "An antibiotic is a synthetic substance produced as a structural analogue of naturally occurring antibiotic."

This describes semi-synthetic antibiotics and is an acceptable characterization. While the original definition of antibiotics refers to natural products, modern usage includes synthetic analogues.

Option C: "An antibiotic should promote the growth or survival of microorganisms."

This is incorrect. By definition, antibiotics inhibit the growth of or destroy microorganisms. They do not promote growth — that would be the opposite of their function.

Option D: "An antibiotic should be effective in low concentrations."

This is correct. A key characteristic of antibiotics is that they are effective even at very low concentrations.

The incorrect statement is Option C.