Final answer:
Germ-free mice lack immune function due to absence of microbiota, while colonized mice have a microbiota that helps to protect against infections and contributes to normal immune system development. Studies have shown that the microbiota influences growth and disease susceptibility, challenging the belief that all individuals are equally susceptible to diseases.
Step-by-step explanation:
When comparing germ-free mice (GF) to colonized mice, it's important to understand the role of the microbiota in the immune system. Germ-free mice are raised in a sterile environment and lack microbiota, which means they often have underdeveloped immune systems and therefore do not have 'normal' immune function. Contrarily, colonized mice, which include wild-type (WT) and those with specific bacterial colonization like Lactobacillus (L1/L2), have a microbiota that can protect against infections by outcompeting potential pathogens, producing substances to lower intestinal pH, and consuming nutrients required by pathogens. Scientific studies, including those by Blanton et al. and Schwarzer et al. published in Science, have observed the impact of the microbiota on growth, development, and immune response in mice. These studies demonstrate that mice with a healthy human microbiota transplanted into them exhibit better growth and health outcomes than those with microbiota from undernourished children.
Koch's postulates, which had stated that all healthy test subjects are equally susceptible to disease, are now considered flawed because of our understanding of the unique microbiotas and varying states of the immune system in each individual. The absence of key microbiota is associated with immune system dysfunctions and an increased risk of allergies, autoimmune disorders, and other diseases. The microbiota's role extends to the influence it has on the severity and symptoms of an infection, illustrating the critical partnership between our bodies and the microbes that inhabit us.
Thus, the correct answer to the student's question should be: a) Germ-free mice lack immune function, and d) Colonized mice have a microbiota which helps in immunity and protecting against infections.