Microbial nutrition classification: Organisms such as Lactobacillus spp. that require complex, specific nutrients (e.g., vitamins, amino acids, and other growth-promoting substances) are generally referred to as what type of heterotrophs?

Introduction / Context:
In clinical and food microbiology, some bacteria grow only when the medium provides preformed growth factors such as vitamins, amino acids, nucleotides, or heme. These microbes are described as “fastidious,” and recognizing this term guides media selection (e.g., enriched or selective-enriched media) and interpretation of growth failures.

Given Data / Assumptions:

• Lactobacillus and related genera often need complex nutrients and growth factors.
• Question asks for the correct descriptor for such nutrient-demanding organisms.
• We contrast this with terms describing energy sources or carbon sources (chemo-, litho-, photo-).

Concept / Approach:
“Fastidious” refers to organisms with complex or specific nutritional requirements beyond basic mineral salts and a carbon source. They commonly require enriched media (e.g., blood, serum, yeast extract). The other listed categories describe how organisms obtain energy or electrons, not the complexity of nutritional needs.

Step-by-Step Solution:
Identify that the stem emphasizes “elaborate requirements” for vitamins and growth factors. Recall the standard term: fastidious organisms (here, fastidious heterotrophs). Differentiate from chemo-/photo- terms, which relate to energy metabolism rather than nutritional complexity. Select the option that directly matches the definition.

Verification / Alternative check:
Classic examples include Neisseria, Haemophilus, and Lactobacillus requiring factors like NAD, hemin, or complex peptides; they grow best on enriched media (e.g., chocolate agar, tomato juice agar for lactobacilli).

Why Other Options Are Wrong:

• Chemo-lithotrophs: Use inorganic electron donors (e.g., NH3, Fe2+); says nothing about fastidiousness.
• Chemotrophs: Very broad; includes fastidious and non-fastidious organisms alike.
• Photo-organotrophs: Light-driven energy with organic carbon; again, not about complex requirements.
• Oligotrophs: Thrive at very low nutrient concentrations, not necessarily complex supplements.

Common Pitfalls:
Confusing energy/electron-source terminology with nutritional complexity; these describe different dimensions of microbial ecology.

Final Answer:
Fastidious heterotrophs.