Nutritional classification of human-associated bacteria Considering metabolic requirements, all bacteria that regularly inhabit the human body are best described as:

Introduction / Context:
Microbial ecology categorizes organisms by energy and carbon sources. Understanding the trophic types of human-associated bacteria clarifies why certain nutrients and environments support or inhibit their growth and informs strategies for probiotics, infection control, and culture media formulation.

Given Data / Assumptions:

• Human-associated bacteria colonize nutrient-rich, organic environments (skin, gut, mucosa).
• Host tissues and secretions provide organic carbon and energy sources.
• Sunlight is not directly harnessed by human commensals or pathogens for photosynthesis.

Concept / Approach:
Heterotrophs use organic compounds as carbon sources and usually as energy sources. This describes essentially all bacteria that inhabit humans, regardless of oxygen preference. Chemolithotrophs oxidize inorganic compounds (for example, ammonia, hydrogen sulfide) and are characteristic of environmental niches such as soil or hydrothermal vents. Autotrophs fix carbon dioxide; phototrophs use light as an energy source—neither category represents typical human-associated microbiota.

Step-by-Step Solution:
Identify the host environment: abundant organic substrates (amino acids, sugars, fatty acids).Match to trophic category: heterotrophy.Exclude chemolithotrophy (inorganic donors), autotrophy (CO2 fixation), and phototrophy (light as energy source).Select “Heterotrophs.”

Verification / Alternative check:
Standard culture media for human pathogens are formulated with organic nutrients (peptones, extracts, blood), reflecting heterotrophic growth needs.

Why Other Options Are Wrong:

• B: Chemolithotrophs are rare in the human body and not typical colonizers.
• C: Autotrophs are not a feature of human microbiota.
• D: Phototrophs require light and photosynthetic machinery absent in human-associated bacteria.
• E: Methanotrophy is a specialized metabolism not characteristic of the human microbiome.

Common Pitfalls:
Confusing oxygen requirement (aerobe/anaerobe) with nutritional class (heterotroph/autotroph).

Final Answer:
Heterotrophs