Streptomyces bacteria. Coloured scanning electron micrograph (SEM) of Streptomyces sp. bacteria. These soil bacteria are responsible for the musty odour of soil. Streptomyces is the largest antibiotic producing genus, producing antibacterials (such as Neomycin), antifungals and immunosuppressants. They are infrequently pathogenic to humans, though some infections (for example mycetoma) may be caused by certain species. Magnification: x3500 when printed 10 centimetres wide.