Which grow best at moderate temperatures in the range 20°C to about 45°C and are often described as including many pathogens?

• A. Psychrophiles
• B. Thermophiles
• C. Mesophiles
• D. Halophiles

Answer: (C)

Growing best at moderate temperatures around room to body temperature is a hallmark of mesophiles. Their enzymes and cell membranes are optimized for activity around 20–45°C, so they thrive in environments that humans and many other animals inhabit, including the human body at about 37°C. This is why many pathogens are mesophiles: they’re well-suited to grow at the temperatures found in the human host, making infections more likely.

In contrast, psychrophiles are adapted to cold environments and grow best below about 20°C; thermophiles prefer heat and grow best at higher temperatures, often above 45°C; halophiles are characterized by their tolerance to high salt concentrations, with temperature preferences varying but not defining them as the group best suited for moderate temperatures.