With respect to temperature preference, which of the following statements correctly describes most pathogenic bacteria that infect humans?

Introduction / Context:
Bacteria can be classified according to the temperature ranges at which they grow best. These categories include psychrophiles (cold-loving), mesophiles (moderate temperature-loving), and thermophiles (heat-loving). Pathogenic bacteria, which cause disease in humans, must be able to grow in the human body to cause infection. This question asks you to identify which temperature category correctly describes most human pathogens, helping you connect microbiology with basic physiology.

Given Data / Assumptions:

• Normal human body temperature is around 37 °C.
• Pathogenic bacteria must be able to survive and multiply at or near this temperature.
• The options mention thermophiles, mesophiles, psychrophiles, and bacteria that grow equally well at all temperatures.
• We assume standard microbiology definitions for temperature categories.

Concept / Approach:
Mesophiles are bacteria that grow best at moderate temperatures, typically between about 20 °C and 45 °C, with an optimum often close to 37 °C. This range includes the temperature of the human body, making mesophiles ideal candidates for human pathogens. Thermophiles prefer much higher temperatures, often above 55–60 °C, such as those found in hot springs or industrial processes, which are far hotter than human tissue can tolerate. Psychrophiles are adapted to cold environments, with optimum growth near or below 15 °C, and do not generally thrive at body temperature. No bacteria can truly grow equally well from 0 °C to 100 °C because protein stability and membrane function break down at extremes. Therefore, most pathogenic bacteria infecting humans are mesophiles.

Step-by-Step Solution:
Step 1: Consider that human pathogens must grow at approximately 37 °C, our normal body temperature. Step 2: Recall that mesophiles have an optimum growth range around moderate temperatures, often near 37 °C. Step 3: Recognise that thermophiles have optimum growth at much higher temperatures, often above 60 °C, which would damage human tissues. Step 4: Note that psychrophiles prefer cold conditions near 0–15 °C and generally do not grow well at body temperature. Step 5: Understand that no bacteria can maintain normal growth across the entire 0–100 °C span; extreme temperatures denature enzymes and disrupt membranes. Step 6: Conclude that most pathogenic bacteria must be mesophiles, not thermophiles or psychrophiles.

Verification / Alternative check:
Microbiology textbooks commonly state that the majority of human pathogens are mesophiles, with optimum growth temperatures around 37 °C. Examples include Escherichia coli, Staphylococcus aureus, and Streptococcus species. Thermophiles are typically found in environments like geothermal vents or industrial heat tanks and are not usually associated with human disease. Psychrophiles are found in cold environments such as polar regions and deep oceans and do not cause typical human infections. This consistent classification across sources confirms that mesophiles are the correct group for most human pathogens.

Why Other Options Are Wrong:
Most pathogenic bacteria are thermophiles that grow best at very high temperatures above 60 °C is incorrect because human tissues cannot withstand such high temperatures, and thermophilic bacteria rarely infect humans.
Most pathogenic bacteria are psychrophiles adapted to near-freezing temperatures is wrong because psychrophiles prefer cold environments and typically cannot grow at 37 °C.
Most pathogenic bacteria can grow equally well at any temperature from 0 °C to 100 °C is biologically impossible; extreme cold and heat denature proteins and disrupt membranes, limiting bacterial survival and growth.

Common Pitfalls:
Some students see the word “pathogenic” and assume that pathogens must be extremely tough and therefore thermophilic, but toughness does not mean preferring high temperatures. Others may overlook the key clue of human body temperature and focus only on the category names. A reliable strategy is to always match the optimum growth temperature of the microbe with the environment where it causes disease. For human pathogens, that environment is the human body at approximately 37 °C, pointing directly to mesophiles.

Final Answer:
Most pathogenic bacteria that infect humans are Most pathogenic bacteria are mesophiles that grow best at moderate temperatures near human body temperature.