Step 1: Understanding the Problem:
The question asks to identify the optimal temperature range for the growth of thermophilic bacteria.
Step 2: Key Concepts and Approach:
Bacteria are classified into groups based on their preferred growth temperatures:
1. Psychrophiles: Grow best at cold temperatures (typically $< 15^\circ\text{C}$).
2. Mesophiles: Grow best at moderate temperatures (typically $20\text{--}45^\circ\text{C}$).
3. Thermophiles: Grow best at high temperatures.
4. Hyperthermophiles: Grow best at extremely high temperatures ($> 80^\circ\text{C}$).
Step 3: Detailed Explanation:
• Thermophiles: The term "thermophile" is derived from Greek words meaning "heat-loving."
These organisms possess specialized enzymes and membrane lipids that remain stable and functional at elevated temperatures.
• Growth Range: Thermophilic bacteria generally have an optimum growth temperature between $45^\circ\text{C}$ and $70^\circ\text{C}$.
They are commonly found in hot springs, compost heaps, and are of concern in dairy pasteurization and canning processes if cooling is inadequate.
• Other Ranges: Temperatures below $20^\circ\text{C}$ favor psychrophiles and psychrotrophs, while $100^\circ\text{C}$ is lethal to almost all vegetative bacterial cells, though some hyperthermophiles can survive near boiling points in deep-sea vents.
Step 4: Final Answer:
Therefore, thermophilic bacteria grow best at $45\text{--}70^\circ\text{C}$, corresponding to Option (C).