Q.35 The growth medium for mammalian cells contains serum. One of the major functions of serum is to
stimulate cell growth and attachment. However, it must be filter sterilized to
(A) remove large proteins
(B) remove collagen only
(C) remove mycoplasma and microorganisms
(D) remove foaming agents

Why Serum in Mammalian Cell Culture Medium Must Be Filter Sterilized

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Serum Filter Sterilized Mammalian Cells: The Key to Contaminant-Free Cell Culture

In mammalian cell culture, serum plays a vital role as a supplement in growth media. It provides essential growth factors, hormones, and proteins that promote cell attachment, proliferation, and survival. Fetal bovine serum (FBS) is the most common type used in labs worldwide. However, raw serum isn’t ready for direct use—it must undergo filter sterilization to ensure sterility.

This process typically involves passing serum through a 0.22 μm or 0.1 μm membrane filter, removing harmful contaminants without compromising its bioactive components. But why is this step non-negotiable? Let’s dive into a common MCQ from biotechnology exams that tests this concept.

Correct Answer: (C) Remove Mycoplasma and Microorganisms

Serum filter sterilized mammalian cells requires this step primarily to eliminate mycoplasma and microorganisms. Serum sourced from animals can harbor bacteria, fungi, viruses, and especially mycoplasma—tiny, cell wall-less bacteria that are notorious contaminants in cell culture. Mycoplasma infections can alter cell metabolism, inhibit growth, and produce false results in experiments like enzyme assays or genetic studies.

Filter sterilization uses sterile filters to physically trap these microbes (typically >0.2 μm in size), ensuring the serum is free of viable contaminants. This is crucial because heat sterilization (autoclaving) would denature serum proteins, destroying its ability to support cell growth and attachment. Studies from the American Type Culture Collection (ATCC) show mycoplasma contaminates up to 30% of cell lines, underscoring why filter sterilization serum is a standard protocol in biotech labs.

Detailed Explanation of All Options

Understanding why other options are incorrect helps solidify your grasp of cell culture techniques. Here’s a breakdown:

• (A) Remove large proteins: Incorrect. Serum is valued for its large proteins like albumin and transferrin, which aid nutrient transport and cell attachment. Filtration targets microbes, not proteins—most serum proteins are smaller than the filter pores (e.g., albumin ~66 kDa passes through 0.22 μm filters easily).

• (B) Remove collagen only: Incorrect. Collagen is a key extracellular matrix component in serum that promotes mammalian cell attachment to culture flasks. Filter sterilization doesn’t selectively remove it; the goal is broad microbial control, not targeting specific proteins like collagen.

• (C) Remove mycoplasma and microorganisms: Correct, as explained above. Mycoplasma (0.2-0.3 μm) and larger microbes are effectively trapped, preventing undetectable infections that evade routine checks.

• (D) Remove foaming agents: Incorrect. Foaming in serum arises from proteins or handling but is managed by antifoams or gentle mixing, not filtration. Sterile filters don’t target foam; they’re for biological sterility.

Best Practices for Serum in Mammalian Cell Culture

To optimize serum filter sterilized mammalian cells workflows:

• Use certified, pre-filtered FBS from reputable suppliers (e.g., Gibco, Sigma-Aldrich).

• Store at -20°C and thaw slowly to preserve activity.

• Test for mycoplasma post-preparation using PCR kits.

• Consider serum alternatives like serum-free media for defined experiments.

This MCQ highlights a cornerstone of microbiology and bioengineering: contamination control. Mastering it prevents costly experiment failures.