What factor does NOT contribute to fluctuations in normal flora?

Normal flora, or the collection of microorganisms that inhabit various sites in the human body, can be influenced by multiple factors. The correct answer indicates that genetics does not typically contribute to fluctuations in these microbial populations.

Diet changes can significantly alter the composition and abundance of microbial communities because nutrients affect microbial growth, while specific dietary components can encourage the proliferation of certain bacteria over others. Antibiotic use can drastically disrupt normal flora by killing off sensitive bacteria, leading to increased colonization by resistant strains or opportunistic pathogens. Stress levels have been shown to affect immune function and, subsequently, the balance of microbial populations, potentially leading to changes in the normal flora.

In contrast, while genetics can influence certain aspects of an individual’s microbiome, such as predispositions to certain infections or how one's immune system responds to microbes, the direct fluctuations in normal flora over time are more influenced by environmental factors, behaviors, and health-related interventions rather than inherent genetic factors.