Streptococcus pyogenes

 Streptococcus pyogenes

GENERAL CHARACTERISTICS

·       Gram’s classification – Gram positive bacteria

·       Shape – Spherical (Cocci)

·       Arrangement – Chain form often having Diplococcal arrangement.

·       Motility – Non-Motile

·       Capsule – Present (Hyaluronic acid capsule)

·       Endospores - Absent

·       Respiration – Aerobic/Facultative anaerobic

·       Optimum Temperature - 37 °C

·       Optimum pH – 7.3 to 7.4

·       Streptococcus type (Lancefield classification) – Group A

·       Habitat – Throat and Skin

·       Discovered by Theodor Billroth (Austrian Surgeon) in 1874.

PATHOGENICITY OF Streptococcus pyogenes

DISEASE TRANSMISSION

            Streptococcus pyogenes infections are mainly transmitted by

a)     Person-to-person spread by respiratory droplets.

b)     Through breaks in skin after direct contact with infected person, fomite, or arthropod vector.

INCUBATION PERIOD

1 to 3 Days

VIRULENCE FACTORS OF Streptococcus pyogenes

(i)    Antigenic structure

a)     M – Protein (Adhesins)

b)     T – Substance (Differentiation of Streptococcus pyogenes from other Streptococcus)

c)     Nucleoprotein (P Substances) 

(ii)  Enzymes (Spreading factors)

a)     Streptokinase (Fibrinolysin)

b)     Deoxyribonuclease

c)     Hyaluronidase

(iii) Toxins

a)     Pyrogenic Exotoxins (Erythrogenic toxins) (Type A, B and C)

b)     Hemolysins (Streptolysin O and Streptolysin S)  

PATHOGENESIS OF Streptococcus pyogenes

·  Streptococcus pyogenes has multiple mechanisms for avoiding Opsonization (molecules enhancing Phagocytosis) and Phagocytosis.

·       The Hyaluronic acid capsule is a poor immunogen and interferes with Phagocytosis.

·    The M - proteins also interfere with Phagocytosis by blocking the binding of the complement component C3b, an important mediator of Phagocytosis.

·    Finally, all strains of Streptococcus pyogenes have C5a peptidase on their surface. This serine protease inactivates C5a, a chemoattractant of Neutrophils and Mononuclear phagocytes, and protects the bacteria from early clearance from infected tissues.

CLINICAL DISEASES CAUSED BY Streptococcus pyogenes

I) SUPPURATIVE DISEASES

(i)    Pharyngitis

·      Reddened Pharynx (part of the throat behind the mouth and nasal cavity) with exudates (fluid emitted by an organism) generally present.

·       Cervical Lymphadenopathy (abnormal size of Lymph nodes) can be prominent.

(ii)  Scarlet fever

·     Diffuse Erythematous rash (redness of skin and mucus membrane) beginning on the chest and spreading to the Extremities (limb or appendage of the body, particularly the hands and feet).

·       Complication of Streptococcal pharyngitis

(iii)  Pyoderma

·       Localized skin infection with vesicles progressing to Pustules.

·       No evidence of Systemic disease 

(iv)   Eryslpeals

·       Localized skin infection with pain, inflammation, lymph node enlargement and systemic symptoms.

(v)  Cellulitis

·       Infection of the skin that involves the Subcutaneous tissues

(vi)   Necrotizing Fasclitis

·       Deep infection of skin that involves destruction of muscle and fat layers.

(vii) Streptococcal Toxic Shock Syndrome

·       Multiorgan systemic infection resembling Staphylococcal toxic shock syndrome.

II) NON - SUPPURATIVE DISEASES 

(i)    Rheumatic fever

·   Characterized by Inflammatory changes of the Heart (pancarditis), Joints (arthralgias to arthritis), Blood vessels, and Subcutaneous tissues.

(ii)  Acute Glomerulonephritis

·   Acute inflammation of the Renal glomeruli with Edema, Hypertension, Hematuria and Proteinuria.

LABORATORY DIAGNOSIS OF Streptococcus pyogenes

MICROSCOPIC EXAMINATION

·     Gram stating – Violet coloured Gram positive cocci arranged in form of Chains.

·     Motility test – Non-Motile.

COLONY MORPHOLOGY ON CULTURE MEDIUM

·     Blood agar – Grayish white, transparent to translucent, matte or glossy; smooth; flat; large zone of Beta hemolysis.

·     MacConkey agar – Pink coloured Lactose fermenting colonies.

BIOCHEMICAL TESTS

a)     Catalase test - Negative

b)     Oxidase test - Negative

c)     Urease test - Negative

d)     Indole test - Negative

e)     Methyl Red (MR) test - Positive

f)      Voges Proskauer (VP) test - Negative

g)     Citrate utilization test – Negative

h)    Starch Hydrolysis – Negative

i)      Casein Hydrolysis – Negative

j)      Alkaline phosphatase activity – Positive

k)     Arginine Dehydrolase – Positive

l)      Hyaluronidase test – Positive

m)   Neuramidase test – Positive

n)    Bacitracin test – Sensitive

o)     PYR Test – Positive

p)     CAMP Test - Negative

Bacitracin Test

        Streptococcus pyogenes can be differentiated from other non-group A β-hemolytic streptococci by their increased sensitivity to Bacitracin.

PYR Test

       The PYR test is a rapid colorimetric method often used to distinguish Streptococcus pyogenes from other β-hemolytic streptococci with a similar morphology and tests for the presence of the enzyme Pyrrolidonyl aminopeptidase. This enzyme hydrolyzes L-pyrrolidonyl-β-naphthylamide (PYR) to β-naphthylamide, which produces a Red color when a Cinnamaldehyde reagent is added. The test can be performed on paper strips that contain dried Chromogenic substrates for the Pyrrolidonyl aminopeptidase within a few minutes.

SEROLOGICAL TEST

·     Anti-Streptolysin O (ASO) Test - Positive

ANTIBIOTIC THERAPY AND TREATMENT

• Streptococcus pyogenes are susceptible to Penicillin (Benzylpenicillin or Penicillin G or oral Penicillin V).
• For penicillin allergic patients, Erythromycin is a drug of choice.
• In some cases, Clindamycin or Vancomycin is also recommended.