What ingredient is NOT necessarily added to modified medium?

Modified culture media are designed to support the growth of specific cells or tissues by adjusting various components according to the needs of the culture. Antibiotics are often included in media to inhibit bacterial growth and maintain sterility during cell culture; however, their inclusion is not mandatory in all modified media.

Serum is commonly added to provide essential growth factors, hormones, and nutrients that cells require for optimal growth. Glucose serves as a primary energy source for cultured cells, making it a critical component in most media formulations. Amino acids are necessary for protein synthesis and are usually included in modified media to ensure that cells have enough building blocks for growth and function.

Antibiotics, while useful in certain contexts, might not be added if the culture is conducted under conditions where sterility is maintained through other means, or if the specific cell type being cultured is sensitive to antibiotics. Therefore, antibiotics are not necessarily a required ingredient in modified media, making this choice the correct answer for the question.