Streptococci

1. Classification of Streptococci
2. Virulence Factors
3. Diseases caused by S. pyogenes infections
4. Quiz

• Streptococci are a group of Gram-positive cocci usually grouped in chains
• They are classified based upon their haemolytic properties (alpha haemolysis, beta haemolysis and non-haemolytic gamma), serological specificity and biochemical characteristics.
• Streptococcus pyogenes contains multiple virulence factors which facilitate bacterial replication in the host.
• S. pyogenes can cause a variety of serious conditions due to its virulence factors machinery that allows it to withstand adverse conditions from the host immune system.
• Conditions caused by S.pyogenes include: streptococcal pharyngitis, impetigo, cellulitis, necrotising fasciitis and toxic shock syndrome among others.

Classification of Streptococci

Streptococci are a genus of Gram-positive cocci that usually assemble linearly forming chains.

Streptococci are classified based on their haemolytic properties:

• (Alpha) α haemolysis - these species have the ability to oxidise iron in haemoglobin molecules (partial haemolysis), hence the characteristic greenish colour that emanates around them when cultured on a blood agar plate. Some examples belonging to this group include: S. pneumoniae and the group of ‘viridans’ (literally green) streptococci.
• (Beta) β haemolysis - these species cause complete breakdown of red blood cells, hence resulting in the characteristic transparent ‘halo’ (full haemolysis) surrounding the colonies when cultured on a blood agar plate. An important example being S. pyogenes.
• (Gamma) γ haemolytic species - these species cause no haemolysis and simply appear as normal colonies on a blood agar plate, not creating any discolouration or ‘halo’ around them. One example is Streptococcus bovis.

Figure: Different haemolytic patterns

Creative commons source by Y tambe, edited by Dr. Elena Perez [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)]

There are a few other ways to describe streptococci. One of them is known as Lancefield grouping and is based on cell wall antigens: it describes the specific carbohydrates found on the bacterial cell wall (group A to V)(group B strep which causes neonatal sepsis is based on this grouping). The other way of classifying Streptococci, known as Sherman’s grouping, is based on their physiological properties and describes whether the streptococcus species is pyogenic, viridans, enterococcal or lactic. So, for example, S. pyogenes is described as beta-haemolytic, pyogenic (both Sherman) and group A (Lancefield).

Virulence Factors

Many streptococci are part of our normal flora. However, there are a number of virulence factors (molecules and mechanisms that increase a bacterium’s effectiveness and ability to multiply) that render pathogenic species challenging for the body to eliminate on its own.
Let’s look at Streptococcus pyogenes as an example: some of its virulence factors proper to group A streptococci include:

• M protein which promotes resistance by inhibiting activation of the alternative complement pathway; it also promotes initial attachment
• A large hyaluronic acid capsule that prevents recognition and phagocytosis by macrophages and neutrophils (as it mimics human connective tissue hyaluronate);
• Pyrogenic exotoxins that act as superantigens, triggering widespread T cell activation and cytokine release
• Streptokinase which dissolves clots by converting plasminogen into plasmin;
• Streptolysins O and S that cause lysis of the host’s erythrocytes, neutrophils and platelets;
• DNAses A, B, C and D that cause degradation of DNA.

N.B. Not all S. pyogenes are equipped with all these virulence factors as there is variation between strains.

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