Correct answer: (1) Streptomyces

Streptomyces (a genus of filamentous soil bacteria belonging to actinomycetes) is the single largest source of known microbial antibiotics, contributing the majority of clinically and agriculturally important compounds such as streptomycin, tetracyclines, chloramphenicol, erythromycin, rifamycin, and many more.​

Option analysis

(1) Streptomyces – correct

• Streptomyces species produce around two‑thirds of all known microbial antibiotics, and roughly 70–80% of bioactive secondary metabolites from actinomycetes.​

• Because of this enormous diversity, they are described as “biofactories” of antibiotics and are the primary industrial and natural source of new antibiotic molecules.​

(2) Penicillin

• Penicillin is not a microorganism but an individual antibiotic (or a group of β‑lactam antibiotics) originally produced by the fungus Penicillium notatum / P. chrysogenum.

• It is one product, not a source genus, so it cannot be the “source of maximum antibiotics”.

(3) Actinomyces

• Actinomyces is a different genus of Gram‑positive bacteria, some species of which are commensals or opportunistic pathogens in humans and animals.

• They do not match Streptomyces in breadth or number of antibiotic metabolites and are not classically cited as the main antibiotic‑producing group.

(4) Bacillus

• Bacillus species do produce several important antibiotics (e.g., bacitracin, polymyxin, gramicidin), as well as enzymes and insecticidal proteins.

• However, the quantity and diversity of antibiotics from Bacillus are far lower than from Streptomyces, so they are not the maximal source.