Streptococcus

            Gram positive, catalase negative, nonsporing, spherical or ovoid cells. Cell division occurs in one plane, therefore, they are arranged in pairs or chains. Most strains produce capsules composed of hyaluronic acid. Generally nonmotile. Majority are aerobes and facultative anaerobes, some are obligate anaerobes, some require CO₂ for growth. Also growth can be enhanced by the addition of glucose, blood or serum. Their G + C content is 30 – 46 mol%. Streptococci form normal flora of man and animals. They inhabit respiratory tract, live harmlessly as commensals.

Important disease causing species are : Streptococcus pneumoniae

                                                          Streptococcus pyogenes

                                                         Streptococcus mutans

Based on the haemolytic properties, streptococci can be classified into 3 groups :

Αlpha haemolytic streptococci – Colonies surrounded by a narrower zone of haemolysis, with nonhaemolysed RBCs in an inner zone and complete haemolysis in outer zone. Green discolouration of haemolysed zone, due to partial lysis of RBC by bacterial enzymes. They are widely foundly in upper respiratory tract of humans, opportunistic pathogens. Eg. S. salivarius.

Beta haemolytic streptococci – Produce a clear zone of complete haemolysis in which no RBC is visible. This lysis is due to the production of Streptolysin O and S. Pathogenic streptococci comes under this group. Eg. S. pyogenes

Gamma haemolytic streptococci – Do not produce haemolysis. Eg. S. faecalis

Streptococcus pneumoniae (Pneumococcus)

            It is a nonmotile, nonsporing, gram positive coccus. Characteristically it occurs in pairs of flame shaped cocci, about 1 μm in diameter, rounded ends being adjacent to each other. The cell wall contains an antigen C, similar to the carbohydrates. In cultures they usually appear in chains like streptococci. It is capsulated which can be demonstrated by Quellung reaction, ie., treatment with type-specific anticapsular serum which combines with capsular polysaccharide antigens and renders refractility and increases the visibility of the capsule.

            Aerobe and facultative anaerobes, need 5-10% CO₂, optimum temperature 37⁰C, pH 6.5 – 8.3, grows only in enriched media. On blood agar, after 18-24 hrs’ incubation at 37⁰C, virulent strains with abundant capsular polysaccharide produce small, moist, mucoid, transparent colonies. The colonies surrounded by 2-3mm, zone of α haemolysis. On further incubation, colonies develop a central depression because of autolysis with raised rim. Under anaerobic conditions, β haemolysis due to the liberation of oxygen sensitive, streptolysin O by the bacteria.

            They ferment glucose, sucrose, lactose with acid production. Pneumococci are bile soluble, bile salts lyse pneumococci. It can be killed by moist heat at 55⁰C in 10 minutes. They are sensitive to a wide range of antibiotics. Optochin Sensitivity Test – is used for the identification of pneumococci which distinguishes from other streptococci. Optochin is a quinine derivative that inhibits the growth of pneumococci, but not other viridans streptococci.

Antigenic structure -   Pneumococci possess capsular antigen, somatic antigen (pneumococcal C substance), M protein, C-reactive protein, which adds its antigenicity, also streptolysin toxin production.

a.       Capsular antigen : The polysaccharide capsule, this polysaccharide diffuses into the culture medium or infective exudates and tissues, it is also known as specific soluble substance(SSS). The capsule of the pneumococcus is essential for virulence, its role being to protect the organism from phagocytosis.

b.      Somatic antigen : Species – specific carbohydrate hapten, referred as pneumococcal C substance. Chemically, this is choline-containing ribitol teichoic acid.

c.       M protein : Type specific protein antigens

d.      C– reactive protein : CRP is an abnormal protein. It precipitates with C antigen of    pneumococci, hence the  name. Its production is stimulated by bacterial infection, inflammation, malignancy and tissue destruction.

Pathogenicity –

e.       Sinusitis : Swelling of sinus cavity linings slows or prevents drainage and leads to pressure and severe pain. Mucus accumulates and fosters bacterial growth. Secretions consisting of mucus, bacteria and phagocytic cells collect in the sinuses. Treatment with Penicillin.

f.       Bronchitis : Linked to smoking, air pollution, inhalation of coal dust and heredity. Patients coughed sputum contains mucus, organisms and phagocytic cells. Infection spread to alveoli from bronchioles and cause pneumonia. Diagnosis by culturing sputum.

g.      Classical pneumonia :  Pneumonia, an inflammation of the lungs tissue by Pneumococcus also, other bacteria, virus, helminths, chemicals, radiation etc., Infection by the inhalation of bacteria which is in the upper respiratory tract, enters the lower respiratory tract by a deep breath or by mucus. It causes lobar pneumonia if one or more lobes of the lungs are affected. Fibrin deposits cause blockage in air spaces. Inflammation of pleural membranes, pleurisy, causes painful breathing.

h.      Bronchial pneumonia begins in the bronchi, spread into the surrounding tissues toward the alveoli. It appears as a secondary infection and no fibrin deposits. This is common in elderly and debilitated patients and is sometimes called “ the oldman’s friend”, and painful death occurs. They are transmitted by respiratory droplets and by carriers. The onset of pneumococcal pneumonia is sudden after a few days of mild upper respiratory symptoms. The infected person suffers violent chills and high fever. Chest pain, cough and sputum containing blood, mucus follows. Fever may end 5-10 days after onset when untreated, or within 24 hours after antibiotics are given. Penicillin is the drug of choice. The patient after recovery, immunity exists for a few months only against particular serotype, so infection developed by other serotypes. Prevnar, an anti – S. pneumoniae vaccine introduced for young children to prevent pneumonia and ear infections.

i.        Streptococcal meningitis : is the most serious of pneumococcal infections. It is usually secondary to other pneumococcal infections such as pneumonia, otitis (ear infections), sinusitis. Organisms generally spread via blood from lung, sinus, ear infections.  

Laboratory diagnosis – Demonstration of pneumococci in sputum, exudate, blood and CSF by Gram staining, culture and by demonstration of pneumococcal antigen by coagglutination. The sputum is homogenized by agitating the specimen for 30 minutes in a mechanical shaker with an equal quantity of distilled water and a small number of glass beads. Gram stained smears are prepared from homogenized sputum and examined. If the sputum is unobtainable, as in young children, a serum-coated laryngeal swab is taken and processed. The organisms isolated are identified by their morphological and biochemical characters. Typing can be done by Quellung rection. Isolation can also be done by intraperitoneal inoculation in mice.

            Since many healthy individuals carry pneumococci in their throats, demonstration of organisms in sputum or throat culture is not necessarily indicative of the disease. But if pneumococci are isolated from patient’s blood, the diagnosis of a severe pneumococcal infection can be made with certainity. In case of suspected meningitis, a centrifuged deposit of CSF should be examined immediately in a Gram film.

            Chemotherapy can be done by treatment with penicillin. In case of infection with penicillin resistant strains or if the patient is penicillin sensitive then cephalosporins, erythromycin, tetracycline, vancomycin, chloramphenicol, sulphonamides can be used.

Streptococcus pyogenes

            Spherical or oval(0.6-1.0 μm), arranged in chains(due to the cocci dividing in one plane only and the daughter cells failing to separate completely), nonmotile, nonsporing, some strains have capsules of hyaluronic acid, which may be demonstrable during the first two hours of growth and because many strains produce hyaluronidase later during the growth cycle, capsules may not be seen in older cultures. Since hyaluronic acid is a normal component of connective tissue, no anticapsular antibodies are formed.

 It is an aerobe and facultatively anaerobe, temperature 37⁰C, growth only in media enriched with blood, serum, carbohydrates. On blood agar small, circular colonies, low convex discs. Virulent strains give granular colony, avirulent glossy colony. Ferments sugars, catalase negative. It is destroyed by heat, 54⁰C for 30 minutes. Survive in dust, if not exposed to sunlight, inactivated by antiseptics.

Antigenicity – The capsule, if present, inhibits phagocytosis. The cell wall is composed of protein and lipoteichoic acid, middle layer of group-specific carbohydrate and an inner layer of peptidoglycan. PG has some thrombolytic activity. Fimbriae project through the capsule. It produce several exotoxins and enzymes which contributes to its virulence. M protein inhibits phagocytosis, C polysaccharide have a toxic effect. Streptolysin O and S present.

            S. pyogenes have haemolysins, pyrogenic toxins, streptokinase, deoxyribonuclease, NADase, hyaluronidase etc., which are the toxins and virulence factors.

Pathogenicity

Scarlet fever : sometimes called scarlatina. Strains of the organism that cause scarlet fever have been infected by a temperate phage that enables them to produce an erythrogenic, red producing, that causes scarlet fever rash. Patients who have exposed to the toxin previously have antibodies, therefore, neutralise it, they develop strep throat without scarlet fever rash. Three types of toxin, so persons develop scarlet fever once from each toxin. Penicillin is the effective antibiotic. Carriers shed infective organisms from the nasopharynx, fomites are also an important source.

Erysipelas : also called St. Anthony’s fire. The disease begins as a small bright, raised, rubbery lesion at the site of entry (wounds, surgical sites). Lesions spread as streptococci grow at lesion margins, producing toxic products and enzymes such as hyaluronidase. The organism spread through lymphatics and cause septicaemia, abscesses, pneumonia, arthritis, death if untreated.

Pharyngitis : or Sore Throat is an infectionof pharynx, familiarily known as Strep throat, common in children 5-15 years old. It is acquired by inhaling droplet nuclei from active cases or healthy carriers. Dogs and other family pets can also be carriers. Contaminated food, milk and water also can spread the disease, so infected persons and carriers should not handle food. In strep throat the throat typically becomes inflamed and the adenoids and lymph nodes in the neck swell. The tonsils become tender and develop, white, pus-filled lesions. Symptoms are chills, headache, acute throat soreness, especially upon swallowing and often nausea and vomiting, high fever. The absence of cough and nasal discharge helps to distinguish strep throat from common cold.

Diagnosis made by throat culture using a throat swab. If delayed S. pyogenes interact with immune system and give rise to rheumatic fever. Treatment with Penicillin.

Rheumatic fever : is a multisystem disorder by β-hemolytic S. pyogenes. Onset of the disease usually occurs 2-3 weeks after a strep throat. Symptoms include fever, arthritis, rash, damage to the heart valve confirms rheumatic fever. Weeks or months later subcutaneous nodules appear, especially near elbows. Heart damage is due to the immunological events. Certain streptococcal strains have an antigen, similar to heart cell antigens. Antibodies that bind to one antigen will bind to the other, or cross reactive. Lymphocytes become sensitized to the antigen and attack heart tissue as well as the streptococci. The resulting heart damage can be fatal.

Toxins & Virulence factors -  S. pyogenes forms several exotoxins and enzymes which contributes to its virulence. M protein acts as a virulence factor by inhibiting phagocytosis. Also C polysaccharide of capsule has a toxic effect.

j.        Haemolysins – Streptococci produce two haemolysins, streptolysin O and S. Streptolysin O is oxygen labile. It is inactive in the oxidised form. On blood agar, O activity seen on subsurfaces. It is also heat labile. It is lethal on intravenous injection and has a cardiotoxic activity. It has leucotoxic activity also. Streptolysin S is oxygen stable, responsible for haemolysis seen around streptococcal colonies on the surface of blood agar plates. It is soluble in serum, but inhibitede by serum lipoproteins.

k.      Pyrogenic toxin : Erythrogenic toxin, the primary effect of toxin is induction of fever, seen in scarlet fever.

l.        Streptokinase (fibrinolysin) : This toxin promotes the lysis of human fibrin clots by activating a plasma precursor. It is an antigenic protein. It facilitates the spreading of infection by breaking down the fibrin barrier around the lesions.

m.    Nicotinamide Adenine Dinucleotidase (NADase) : This acts on the coenzyme NAD and liberates nicotinamide from the molecule. It is antigenic.

14. Hyaluronidase : This enzyme breaks down the hyaluronic acid of tissues, which favors the spread of infection along the intercellular spaces. It is antigenic. Streptococci possess a hyaluronic acid capsule and elaborate hyaluronidase – self destructive process.

Streptococcus mutans – Gram positive cocci in chains. They are g haemolytic. It causes tooth decay or dental caries, the enamel dissolution. It occurs in countries where the consumption of refined sugar is more. If unchecked the decay pass through enamel, dentin, pulp cavity and eventually causes abscess in the bone that supports tooth. The acids formed by bacterial sugar fermentation dissolve enamel, after which protein digesting enzymes breakdown any remaining material.

Dental caries are treated by removing decay and filling the cavity with resins, amalgam. It can be prevented by reducing the intake of sugary, sticky foods, brushing regularly with plaque- removing pastes.