Substances, produced by microorganisms, that in very small amounts inhibit other microorganisms are called?

The correct answer is antibiotics. These are specific substances produced by certain microorganisms, which can inhibit or kill other microorganisms, particularly bacteria. Antibiotics are a key part of our medical toolkit, as they are used to treat bacterial infections by targeting specific processes or structures within bacterial cells, thereby disrupting their growth and survival.

The defining feature of antibiotics is their effectiveness in very small amounts, allowing for targeted treatment with minimal side effects. This property distinguishes them from other substances like antiseptics, which are used to disinfect surfaces and prevent infection but are not specifically produced by microorganisms.

While antibodies are indeed a critical component of the immune response, produced by the body in response to pathogens, they are not naturally occurring substances produced by microorganisms to inhibit other microbes. Therefore, antibiotics are the correct choice as they align with the criteria outlined in the question.