Mode of action — The best-described action of Pluronic F-68 (non-ionic surfactant) in a mammalian cell-culture medium is that it primarily...

Introduction:
Poloxamers such as Pluronic F-68 are amphiphilic block copolymers. Their amphiphilicity makes them surface-active, altering interfacial phenomena that are central to shear-related injury in mammalian cell cultures. The question seeks the primary mechanistic category for F-68's protective effect.

Given Data / Assumptions:

• Aerated, agitated suspension of shear-sensitive cells.
• F-68 at standard concentrations.
• No nutrient limitation in the base medium.

Concept / Approach:
By adsorbing at gas–liquid and cell–liquid interfaces, F-68 changes surface tension and interfacial rheology. This reduces the probability and severity of damaging events during bubble formation and collapse and decreases cell adhesion to interfaces. The mode is interfacial modification, not nutrition, coagulation, or antimicrobial action.

Step-by-Step Solution:
Identify the interface-centric damage pathway in aerated cultures.Map F-68's amphiphilic adsorption to reduced interfacial stress transmission.Conclude that interfacial property modification is the dominant protective mechanism.

Verification / Alternative check:
Lower LDH release and higher viability in sparged reactors with F-68 vs controls support a protective interfacial mechanism. Foam behavior changes also indicate surface-activity as the operative mode.

Why Other Options Are Wrong:

• (b) F-68 is not metabolized as a nutrient source.
• (c) and (d) would hinder suspension health and mass transfer.
• (e) It has no antibiotic function.

Common Pitfalls:
Equating F-68 with classic antifoams or antibiotics; ignoring that too high a concentration may affect membrane interactions undesirably.

Final Answer:
acts on the surface properties of the cells and the culture medium, modifying interfaces to reduce shear damage.