What is the mode of nutrition in bacteria?

The correct option is autotrophic and heterotrophic.

Bacteria are exceptionally diverse micro-organisms that exhibit a wide variety of metabolic pathways. Unlike many other kingdoms of life that are restricted to a single mode of nutrition, bacteria display both autotrophic and heterotrophic modes of nutrition.

1. Autotrophic Nutrition:
Some bacteria can synthesize their own food from simple inorganic substances. Autotrophic bacteria are classified into two main types based on their energy source:
- Photoautotrophs: These bacteria use sunlight as an energy source to synthesize organic compounds (similar to green plants). Examples include cyanobacteria (blue-green algae).
- Chemoautotrophs: These bacteria obtain energy by oxidizing inorganic substances such as ammonia, nitrates, hydrogen sulfide, or iron. Examples include nitrifying bacteria like Nitrosomonas and Nitrobacter.

2. Heterotrophic Nutrition:
The vast majority of bacteria cannot synthesize their own food and must depend on other organic sources for nutrition. Heterotrophic bacteria are classified based on how they obtain their organic nutrients:
- Saprophytic bacteria: They feed on dead and decaying organic matter, playing a vital role as decomposers in ecosystems.
- Parasitic bacteria: They live inside or on other living host organisms and obtain nutrients from them, often causing diseases (e.g., Vibrio cholerae, Mycobacterium tuberculosis).
- Symbiotic bacteria: They live in mutually beneficial relationships with other organisms, such as Rhizobium bacteria found in the root nodules of leguminous plants, which help in nitrogen fixation while receiving food and shelter.

Therefore, because bacteria can sustain themselves through both self-synthesis (autotrophy) and by consuming external organic matter (heterotrophy), their mode of nutrition is both autotrophic and heterotrophic.