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The streptococcus group includes spherical bacteria usually in chains of variable lengths of 2 to many cells. All of the species are Gram-positive and do not form spores. They are widely distributed in nature. Most species of streptococci are catalase negative, do not ferment insulin, do not reduce nitrates and are not soluble in bile salts. If a carbohydrate is fermented by streptococci, large amount of lactic acid are produced without gas. The pathogenic streptococci grow readily on most of the media in the laboratory. However, the organisms are grown on blood or serum media for optimum growth.
Rivolta (1873) described chain forming organisms in pus from a case of strangles in horses. In 1878-79, Pasteur recognized this organism as a pus-forming agent. In 1903, Hugo Schottmuller introduced blood to differentiate various types of hemolysis. In 1928, Rebecca Lancefield reported a serological method of grouping streptococci.
Streptococcal infections are associated with bovine mastitis, strangles in horses, naval infections in horses (naval ill), pharyngeal abscesses in hogs, metritis and cervicitis in cows and horses, arthritis in hogs, and miscellaneous effects from a wide variety of abscess formation to septicemia.
In humans, streptococcal infections are associated with scarlet fever, sore-throat, tonsillitis, bacterial endocarditis, rheumatic fever, and pneumonia. Streptococci may also cause secondary infections following some viral diseases.
Many species of Streptococci form capsules which are antigenically distinct and species specific. Chain formation is best seen in preparations from liquid media. Although most of the streptocci are facultative anaerobes, some are strict anaerobes. A and C strains of this organism mostly produce capsules composed of hyaluronic acid and are mucoid. Some strains may show different colony forms particularly the Group A giving rise to either matt or glossy colonies. The matt type colonies have "M" protein and tend to be virulent while the glossy type produce little "M" protein and often are non-virulent.
All streptococci may be assigned to one of three types - alpha, beta and gamma, when grown aerobically, on sheep erythrocytes.
A.Alpha-Type (Hemolytic) Streptococci-
These so-called viridans (viridis, green) species of streptococci produce a brown to green zone due to the discolored erythrocytes around their blood-agar colonies with or without an outer clear, colorless area of hemolysis. The outer clear area around the greenish zone may best be seen after the cultures are stored in the refrigerator. Such colonies are said to be of the alpha-type, and the streptococci producing them are called alpha-type hemolytic streptococci, viridans streptococci, or streptococcus viridans.
B. Beta-Type (Hemolytic) Streptocci-
In this group, the streptococci produce perfectly clear zones around their subsurface colonies on blood agar. These zones are due to hemolysin secreted by the streptococci in the colony. The clear, colorless zone or hemolysis is called beta-type hemolysis and the streptococci are known as beta-type hemolytic streptococci.
C.Double-Zone Beta-Type Streptococci-
These streptococci are of bovine origin, and are often found in dairy products. The hemolysis produced is of the beta-type, but when the plates are refrigerated, a second ring of hemolysis appears, separated from the first one by a ring of red erythrocytes. This is commonly called "hot-cold hemolysis."
D.Gamma-Type (Nonhemolytic) Streptococci-
Some streptococci produce no visible change in the blood agar around their colonies. These are called gamma-type streptococci, or nonhemolytic streptococci. These streptococci are not commonly pathogenic; some species are found in milk and sometimes are isolated from lesion
Classification on the basis of hemolytic reaction
Classification of streptococci can also be based on
- (1) action on RBC;
- (2) resistance to physical or chemical agents and
- (3) biochemical tests.
Accordingly, they are classified in the following 4 divisions:
elaborate group-specific C carbohydrates. Acid extracts contains these C carbohydrates and give precipitin reactions with specific antisera which permit classification into groups A to O. blood. The effect on blood cell depends on the species of erythrocyte, environmental conditions, (e.g., pH, temperature of incubation and moisture) and some unknown factors.
Serologic Groups:
The beta-type hemolytic streptococci can be divided into at least 13 to 15 serologic groups (Lancefield groups) on the basis of carbohydrate antigenic substances (C) contained in the cell walls of the cocci. By using precipitin tests, streptococci have been grouped ranging from groups A to O. The terms beta-type streptococci of group A and "group A streptococci" or "hemolytic strep" are virtually synonymous with Streptococcus pyogenes.
Serologic Types of Group A:
Group A streptococci are further divided into some 50 or more types, each containing a different protein called an M antigen. These M types are differentiated by precipitin tests and may be further subdivided within the group A streptococci by T-agglutination. The various antigenic substances isolated from streptococci are used for serological typing and are listed below:
C (Carbohydrate) is present in cell walls of streptococci and can be extracted with hot hydrochloric acid or by enzymatic lysis
M (Protein) is closely associated with virulence of group A streptococci. It is type specific. More than 50 types have been identified.
R-antigens have been identified in some M protein types.
T substance has no relationship to virulence. It is destroyed by heat and acid extraction.
The table needs alignment-----------------
Table 1 - Antigens of Streptococcus Pyogenes
________________________________________Chemistry
Antigen Serological Nature Properties Activity
__________________________________________________________
C Group-specific Polysaccharide Polymer 0f N- Acetylglucosamin e and rhamnose; occurs in cell wall
M Type-specific Protein Alcohol-soluble; resistance to heating in dilute acid; destroyed by proteolytic enzymes; related to virulence and antibody is protective
T Occurs in several Protein Resistant to Pro-types but may be teolytic enzymes; type-specific unstable to heat in dilute acid, but resistance in slightly alkaline solution
R Occurs in types 2, Protein Destroyed by peptic 3, 28 and 48 and but not tryptic diges-in strains of Groups tion,unstable to heat B, C, and G in dilute acid,but stable to heat in dilute alkali
__________________________________________________________
Source: Textbook of Microbiology by Burrows et al.
VII. In addition to these antigens, the group A hemolytic streptococci, have one potential surface antigen-the hyaluronate of the capsule which is not immunogenic (fig. 1).
The members of this group produce group D-specific C carbohydrate; are capable of growing at 10`C and 45`C and in at 6.5% sodium chloridesoln and cause variable hemolysis. They are parts of the normal flora of the intestinal tract of man and animals and may cause disease when introduced into the tissues; are occasionally associated with food poisoning. They are resistant to most antibiotic and chemotherapeutic agents.
Lactic streptococci
The members of this group elaborate group N-specific C, carbohydrate, and their hemolytic ability is variable. They don't produce disease but are present in milk and cause normal coagulation of milk ("souring").
This classification of streptococci based on hemolysis is far from satisfactory, for the following reasons:
Conventionally classified with the aerobic - streptococci, but are considered in a separate category.
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More than 20 antigenic extracellular products are elaborated by Group A streptococci:
A. Streptokinase (fibrinolysin) is produced by many strains of beta-hemolytic streptococci. It transforms the plasminogen of human serum into plasmin, an active proteolytic enzyme that digests fibrin and other proteins.
B.Streptodornase (streptococcal deoxyribonuclease) is an enzyme that depolymerizes DNA. Purulent exudates owe their viscosity largely to deoxyribonucleoprotein. Streptodornase is employed therapeutically to liquefy viscous exudates.
C.Hyaluronidase is an enzyme that splits hyaluronic acid, an important component of the ground substance of connective tissue. Thus, hyaluronidase acids in spreading infecting microorganisms (spreading factor). Hyaluronidases are specific for each bacterial or tissue source.
D. Erythrogenic toxin is soluble and is destroyed by boiling for 1 hour. It causes the rash that occurs in scarlet fever. Only strains elaborating this toxin can cause scarlet fever. Erythrogenic toxin is elaborated only by lysogenic streptococci. The test for the presence of scarlet toxin is the Dick Test. One-tenth milliliter of standardized, diluted erythrogenic toxin is injected intradermally. Similar material, heat inactivated, is used as a control. In the absence of significant concentration of antitoxin in the blood, an area of erythema and edema is observed in 8-24 hours. In the first few days of scarlet fever the Dick's Test is generally positive.
E.Some streptococci elaborate a diphosphopyridine nucleotidase into the environment. This enzyme may be related to the organism's ability to kill leukocytes. Proteinases and amylase are produced by some strains.
F.Hemolysins - many streptococci are able to hemolyze red blood cells in vitro in varying degrees. (See preceding test)
Beta-hemolytic group A streptococci elaborate 2 hemolysins
a. Streptolysin O is a protein (molecular weight 60,000) that is hemolytically active in the reduced state (available SH groups) but rapidly inactivated when oxidized. It combines quantitatively with antistreptolysin O, an antibody that appears in animals or humans following infection with any streptococci that produce streptolysin O. This antibody blocks hemolysis by streptolysin O.
b. Streptolysin S is the agent responsible for the hemolytic zones around streptococcal colonies on blood agar plates. It is not antigenic. However, sera of humans and animals frequently contain a nonspecific inhibitor that is independent of past experience with streptococci.
pyogenic streptococci are divided into the following groups:
Group A Streptococcus pyogenes
Although Streptococcus pyogenes has great public health significance in humans it is primarily adapted to that species and is an infrequent cause of animal disease. It occasionally causes bovine mastitis in cows, infected by their milkers. It can be isolated from infected wounds, probably as a direct result of contamination by a human. When found, the infected animal should be isolated and specific instructions given to the owner to protect workers.
Group B Streptococcus agalactiae (syn. streptococcus mastitidis)
This bacteria causes a significant percentage of the cases of chronic catarrhal mastitis in cattle. In agalactiae mastitis, the udder does not become hot, red and swollen but the infection progresses insidiously with both tissue and secretion changes appearing slowly. The milk may appear completely normal to the eye or it may show flakes, masses of stringy fibrin, blood or clotted areas. The inflammation in the udder progresses through proliferation of interstitial tissue resulting in fibrosis of the infected gland. Although these hard masses often cannot be detected visually, they can be easily detected by gentle palpation.
Streptococcus, agalactiae, Streptococcus dygalactiae, and Streptococcus uberis are three species of streptococci which are biochemically and serologically different; however, they are referred to as "mastitis streptococci". Str.dysgalactia (Lancefield Group) is less frequent in mastitis cases; Str. uberis may be associated with an acute mastitis which later becomes chronic. Hotis test is used for the detection of Str. agalactiae infection, but is less accurate in detecting other streptococci such as Str. dysgalactiae. The CAMP test is used in typing mastitis streptococci.
Group C Streptococcus equi.
This organism causes strangles in horses. Epizootics of strangles constantly occur in horse populations. The organism is spread by close contact and by contaminated water and feeds. There is evidence that infected horses remain resistant carriers of the organism and are sources of the infection. Pathogenic Str. equi is often a secondary invader in respiratory diseases such as influenza. This is most prevalent in young horses. It may be found in genital tract of the mare. Septicemia, pyemia, or pneumonia may occur and may result in death of the animal.
Immunity
Immunization with bacterins and vaccines have not given completely satisfactory results. The use of a herd autogenous bacterin is more successful than the use of polyvalent bacterins.
Diagnosis:
The identification of the organism from lesions arecharacteristics of the disease. Serological reactions have not been used in the diagnosis of the disease.
Streptococcus equisimilis - is pathogenic for man and not for horses. It produces bone lesions in swine.
Streptococcus zooepidemicus - causes pyogenic infections in all species of animals including mastitis in swine.
Group E Streptococcus lentus - causes pyogenic infections in swine. It is not found in human infections.
Group F Streptococcus anginosus - This is found only in humans and mostly in reproductive tract.
Group G has two types of colonies:
(A) Minute "colony G" - It is similar to Str. anginosus known only from human sources.
(B) Large "colony G" - common infection in dogs.
Group H Streptococcus sanguis - human pathogen.
Group K, L, M and O - not much information is available.
Viridans Group - the members of the group are like Streptococcus pyogenes.
No C substance or Lancefield group present.
Streptococcus salivarious - found in the mouth, throat and lungs of man. Few cases of endocarditis have been reported.
Streptococcus bovis - present in mouth and throat of cattle. In humans, it causes subacute bacterial endocarditis.
Streptococcus equinus - present in feces of horses.
Streptococcus thermophilus - grows between 45-50`C and are not present in animal body except in milk.
Streptococcus mitis - is present in human throat, and causes subacute bacterial endocarditis in man. It is thought that this is a variant of Streptococcus salivarious.
Streptococcus acidominimus - urinary tract infections of man, and vagina of cattle. Not pathogenic.
Streptococcus uberis - causes mastitis in dairy cattle. Grows between the temperatures of 10-45`C. May or may not have Lancefield group antigen.
Lactus Group they grow at 10`C. Non-hemolytic:
Streptococcus lactis - milk souring streptococci, non-pathogenci.
Streptococcus cremoris - produces longer chains and is non-pathogenic. Sometimes this is used in commercial "starters" either alone or with Str. lactis
Enterococci Group will grow at 10-45`C, can tolerate a pH of up to 9.6, and 6.55% salt:
Streptococcus faecalis - present in intestinal tract of man, cattle, horse, sheep and goat. Str. liquefaciens is a variant of str. fecalis.
Streptococcus zymogenes and durans are Beta-hemolytic variants of streptococcus fecalis. These organisms are resistant to heat but are killed by pasteurization.
Note: Please see references and questions at the end of Chapter 25.
