Streptococcus pyogenes bacteria, light micrograph. These are Gram-positive non-motile cocci (round) that are often found in chains or pairs. S. pyogenes is one of the most frequent pathogens of humans. Each cell is covered by a capsule consisting of a mucopolysaccharide found in human connective tissue. Therefore, the bacteria can infect tissues largely unnoticed by the body's immune system. The most common disease caused by the bacteria is tonsillitis and pharyngitis (Strep throat), it also causes scarlet fever. Severe invasive infections can lead to necrotising fasciitis (flesh-eating disease) and Streptococcal toxic shock syndrome. Magnification: x500 when printed at 10 centimetres wide.