Clostridium difficile drug-resistant bacteria, coloured transmission electron micrograph (TEM). This bacterium, also known as C. diff, is a normal member of the intestinal flora in humans. It is resistant to many antibiotics, however, and if the rest of the gut flora is killed by a course of antibiotics, C. diff can flourish. In such cases it can cause severe gut infections, including the potentially fatal pseudomembranous colitis. The bacteria can form spores (pink ovals), which are resistant to heat and many chemicals. C. diff is one of the most common hospital-acquired infections. Magnification: x4300 when printed 10cm wide.