The Streptococcus

Extracellular Products:
Pathogenic Streptococci, especially S. pyogenes produce a wide array of extracellular products, many of which are considered virulence factors. Pyrogenic exotoxins (Spes) are produced by most S. pyogenes strains. The most potent of which, SpeA, is responsible for the rash in cases of scarlet fever and its gene is carried by a prophage. Nearly 10 different Spes have been reported. In all cases, Spes are superantigens, or mitogenic proteins with the capacity to crosslink the Vb domain of the T-cell receptor and the major histocompatibility complex of class II molecules on the surface of an antigen-presenting cell. This T-cell stimulation results in high systemic levels of proinflammatory cytokines and T-cell mediators, causing hypotension, fever, and shock. As a result, Spes have been suggested to be the prime mediators in streptococcal toxic shock syndrome. Interestingly, except for SpeB, Spes are not part of the bacterial genome but are carried on prophage widely found in streptococci.

Streptolysin O (SLO) is an oxygen-labile protein with the ability to lyse red blood cells under anaerobic conditions. SLO antibodies produced during infection could be quantified in relation to their capacity to neutralize the hemolytic activity of SLO. It has been shown that the titer of antibodies to SLO (ASO) is related to a recent S. pyogenes infection. Streptolysin S (SLS) on the other hand is stable in air and is the molecule responsible for the hemolytic zone around streptococcal colonies on blood plates. SLS, is a non-antigenic 2.8 kD peptide that is tightly associated with the bacterial cell surface with potent toxic effects on experimental animals in vivo and on leukocytes in vitro.

Streptokinase is a secreted molecule with no enzymatic activity, however when bound to plasminogen it initiates its conversion to plasmin, the active protease with fibrinolytic activity. Because of this activity, streptokinase has had some use in human medicine as an agent to lyse fibrln clots in coronary arterial thrombosis, acute pulmonary embolism, and deep venous thrombosis. However, because of its antigenicity, it has appreciated limited use.

Hyaluronidase, is a secreted enzyme that acts on hyaluronic acid, the base substance in connective tissue. Because of this activity, hyaluronidase was called the “spreading factor” for these streptococci. Surprisingly, the identical hyaluronic acid is also the composition of the capsule that is found on many strains of S. pyogenes, a capsule that enables the organism to resist phagocytic attack by human leukocytes.

S. pyogenes also produce a set of four enzymes that are able to cleave nucleic acids. These DNases, or sometimes referred to as streptodornase A through D all possess deoxyribonuclease activity while streptodornases B and D possess ribonuclease activity as well. It has been suggested that by digesting the DNA released from dead mammalian cells at an infected site, the enzyme reduces the local viscosity allowing the organism greater motility.