Light micrograph of a pus smear from a wound infected with Clostridium perfringens (formerly C. welchii), the bacteria causing gas gangrene, appearing as blue-black rods. The blue-black spheres are the bacteria Staphylococcus aureus, the commonest cause of pyogenic (pus-forming) infections in man. Also visible (more feintly) are the smaller red rods of the bacteria Escherischia coli; the blobs stained red are leucocytes (white blood cells). C. perfringens is present in soil and the intestines of man. Infection occurs from soil in accidental wounds and from the gut via the skin during surgery. Magnification: x500 at 35mm size.