Which statement is true about pathogenic and nonpathogenic bacteria?

• A. Pathogenic bacteria are harmful to humans, whereas nonpathogenic bacteria are not.
• B. All bacteria are harmful to humans.
• C. Nonpathogenic bacteria cause disease in everyone.
• D. Pathogenic bacteria are harmless.

Answer: (A)

The key idea here is the difference in disease-causing potential between bacteria. Pathogenic bacteria have traits that let them cause illness in humans, such as toxins or ways to invade tissues. Nonpathogenic bacteria, on the other hand, do not cause disease in healthy individuals and are often harmless or even beneficial (like gut flora). So the statement that pathogenic bacteria are harmful to humans while nonpathogenic bacteria are not is the accurate reflection of this distinction.

The other options don’t fit because they contradict these definitions: not all bacteria are harmful—many are harmless or helpful; nonpathogenic bacteria do not cause disease in everyone (they’re typically non-disease-causing in healthy hosts, though some can be opportunistic in weakened individuals); and pathogenic bacteria are not harmless by definition.