Questions: Which of the following statements is true regarding the human microbiome? The human microbiome must be controlled to avoid disease. Most bacteria do not cause disease in humans. The human microbiome is established before birth. Human cells vastly outnumber microbial cells in the body.

The answer is the second one: Most bacteria do not cause disease in humans.

Explanation for each option:

1. The human microbiome must be controlled to avoid disease.

   • This statement is not entirely true. While it is important to maintain a balanced microbiome to prevent dysbiosis (an imbalance that can lead to disease), the microbiome itself does not need to be "controlled" in a strict sense. Many microorganisms in the microbiome are beneficial and essential for health.

2. Most bacteria do not cause disease in humans.

   • This statement is true. The majority of bacteria in the human microbiome are either harmless or beneficial. They play crucial roles in processes such as digestion, immune function, and protection against pathogenic bacteria.

3. The human microbiome is established before birth.

   • This statement is not accurate. The human microbiome begins to establish at birth, particularly during the passage through the birth canal and through early interactions with the environment. There is some evidence of microbial presence in the womb, but the significant establishment of the microbiome occurs postnatally.

4. Human cells vastly outnumber microbial cells in the body.

   • This statement is incorrect. In fact, microbial cells in the human body are estimated to be roughly equal to or even outnumber human cells. The commonly cited ratio is approximately 1:1 or slightly more microbial cells than human cells.

Summary: The correct statement regarding the human microbiome is that most bacteria do not cause disease in humans.