Bacillus subtilis. Coloured scanning electron micrograph (SEM) of Bacillus subtilis bacteria. B. subtilis, or hay bacillus, is an aerobic, gram-positive, bacilli (rod-shaped) bacteria. It is sometimes pathogenic in humans, causing severe eye infections such as anterior uveitis (inflammation of the middle layer of the eye), or iridocyclitis (inflammation of the iris). Magnification: x1800 when printed 10 centimetres wide.