Streptococcus - Streptococci

Streptococci

Very common in nature, streptococci represent a heterogeneous group of spherical, Gram-positive bacteria; some of these, together with other microorganisms, normally inhabit the mucous membranes of the organism (above all oral, pharyngeal, intestinal and vaginal).

However, not all streptococci are commensal bacteria: some species are in fact endowed with a remarkable pathogenic potential (eg S. pneumoniae ) and, by attacking the host, can cause damage. Other streptococcal species, under favorable conditions, may undergo a commensal to opportunistic transformation, penetrate the circulatory stream and, having reached an organ, trigger a series of even serious morbid processes (eg viridating streptococci).

Microbiological description

In microbiology, the term " streptococcus " refers to the genus to which about twenty different species belong. Streptococci are gram-positive, roundish bacteria capable of dividing along their own axis; the peculiarity of growing in chains or in pairs distinguishes streptococci from staphylococci.

NB Staphylococci, dividing along multiple axes, generate clusters of cells.

The etymological analysis of the term " streptococcus " also suggests the particular "couple" arrangement of these bacteria: the term streptos derives from the Greek "στρεπτος‚ ", which means" easily bent ", just to give the idea of a chain it twists easily.

The streptococcus has a variable diameter from about 0.5 to 1.25 µm and is an optional anaerobic, non-spore and immobile (in most species). These pathogens are gram positive, negative oxidase and negative catalsi.

Most streptococci produce capsules composed of hyaluronic acid, a kind of defense weapon of the bacterium, since it hinders phagocytosis by macrophages and neutrophils.

Hyaluronic acid is considered an element of virulence of the pathogen, precisely because it delays phagocytosis by the cells of the immune system.

The cell wall is composed of antigens characterized by the letters M (virulence factor endowed with immunogenic power), R (antigen not involved either in virulence, or in immunity) and T (epidemiological marker). In the wall there are also group-specific carbohydrates, peptidoglycans and polysaccharide C. In addition to those just listed, among the virulence factors of streptococcus we also mention the exotoxins (streptolysin O, streptolysin S and erythrogenic toxin) and exoenzymes (hyaluronidase, DNAse, streptokinase and NADase).

General classification

Streptococci can be classified according to two criteria:

behavior on blood agar soils (parameter: haemolytic capacity)
antigenic structure (in this case, the classification is performed within the group of beta hemolytic streptococci)

Streptococci were classified into three large groups, based on their behavior on blood agar media:

STREPTOCOCCHI - EMULSIFIED ALPHA: in blood agar soils, this class of streptococci is placed in green colonies, an expression of both incomplete hemolysis and hemoglobin processing (iron hemoglobin oxidation → greenish staining). In this group, streptococci exhibit pili of adhesion (consisting of protein M and covered with lipoteicoic acid) useful for anchoring to the epithelial cells of the host. A typical exponent of this category is the pneumococcus ( S. pneumoniae ).
STOLF-HEMOLYTIC BETA: cultivated in blood agar soils, these streptococci are placed in colonies surrounded by a rosacea color: the above color is the result of the complete rupture of the red blood cells (eg S. pyogenes )
STREPTOCOCCHI-EMOLITHIC RANGE (also called anemolytics): they do not generate any haemolysis (eg enterococcus). The term "haemolytic" referring to the group of these pathogens is used improperly.

The growth of streptococci in blood agar media appears to be favored by incubation in the atmosphere of 10% CO ₂, and at a temperature of 37 ° C. Only type D streptococci (analyzed below) require temperatures ranging from 15 ° to 45 ° C, even at high concentrations of Salts (6.5%).

Structural classification

Depending on the antigenic structure, gram positive streptococci can be further classified: this particular cataloging is called " Lancefield classification ", from the name of the discoverer. Streptococci are thus classified in several serological groups based on the polysaccharide antigen C of cell wall, whose characteristics are extremely heterogeneous in the different types of streptococcus.

Polysaccharides C: components of the bacterial wall consisting of a fundamental skeleton linked to peptidoglycan.

The different serum groups are identified by a letter of the alphabet, which goes from A to H and from K to V (in other words, neither group J nor I) exists. Human pathogenic streptococci are members of the serum groups A, B, C, D and G.

Streptococci of pathological interest

In the medical field, the streptococci of greatest pathological interest are certainly the beta-hemolytics of groups A and B, implicated in numerous typically infantile pathologies:

group A streptococci → pharyngitis, tonsillitis, scarlet fever, pneumonia, rheumatic fever, skin infections and impetigo
group B streptococci → meningitis, endocarditis, septic arthritis and sepsis

The table shows the most important streptococci from the pathological point of view

streptococcus	General characteristics	Diseases conveyed
Streptococcus pyogenes	Group A beta-hemolytic streptococcus, involved in the vast majority of streptococcal infections in humans	Pharyngitis, acute rheumatic fever, necrotizing fasciitis, nephritis glomerulus, impetigo, scarlet fever
Streptococcus agalactiae	Group B hemolytic streptococcus	Systemic bacteremia (rare), meningitis and sepsis in newborns and the elderly
Enterococcus faecalis	Non-hemolytic group D streptococcus	Abdominal abscess, endocarditis, bacteremia, urinary tract infection, meningitis
Streptococcus pneumoniae	Haemolytic streptococcus alpha	Septic arthritis, bacteremia, endocarditis, meningitis, osteomyelitis, pneumonia, septicemia
Streptococci viridans	Haemolytic streptococcus alpha	Abscesses, tooth decay, endocarditis