Clostridium difficile bacteria, illustration. This bacterium, also known as C. diff, is a normal member of the intestinal flora in humans. Some strains produce toxins and can be resistant to many antibiotics. If the rest of the gut flora is killed by a course of antibiotics, pathogenic C. diff can flourish and cause severe gut infections. The bacteria can form spores (seen here within the cells) which are resistant to heat and many chemicals. C. diff is one of the most common hospital-acquired infections. The illustration could also represent anthrax bacteria, Bacillus anthracis, or other bacteria which form spores.