LABORATORY DIAGNOSTICS OF STREPTOCOCCAL INFECTIONS

Actuality of the theme. Infection with group A streptococci (GAS) can result in a range of symptoms: no illness; mild illness (strep throat or a skin infection such as impetigo); severe illness (necrotizing faciitis, streptococcal toxic shock syndrome).

Severe, sometimes life-threatening, GAS disease may occur when bacteria get into parts of the body where bacteria are usually not found, such as blood, muscles, or the lungs. These infections are termed "invasive GAS disease." Two of the most severe, but least common, forms of invasive GAS disease are necrotizing fasciitis and Streptococcal Toxic Shock Syndrome. Necrotizing fasciitis (occasionally described by the media as "the flesh-eating bacteria") destroys muscles, fat, and skin tissue. Streptococcal toxic shock syndrome (STSS), causes blood pressure to drop rapidly and organs (e.g., kidney, liver, lungs) to fail. STSS is not the same as the "toxic shock syndrome" frequently associated with tampon usage. About 20% of patients with necrotizing fasciitis and more than half with STSS die. About 10%-15% of patients with other forms of invasive group A streptococcal disease die. Group B strep infection is fatal in about 20% of infected men and non-pregnant women and about 5% to 15% of infected newborns. Babies who survive can be left with speech, hearing, and vision problems as well as mental retardation.

Primary objectives: to be able to carry out and evaluate microbiological diagnostics of streptococcal infections.

QUESTIONS FOR DISCASSION

1. Classification of Streptococci. Biological properties of Streptococcus spp.

2. Streptococcal infections (streptococcal pharyngitis or tonsillitis, scarlet fever, rheumatic fever, glomerulonephritis, sepsis and pneumoniae), microbiological diagnostics.

3. Epidemiology and pathogenesis of the diseases caused by Streptococcus. Specific features of immunity in such cases.

4. Basic principles of streptococcal infections; prophylaxis and treatment.

PROCEDURE OF THE PRACTICAL SESSION

Task 1. Study the preparation from pure cultures of Streptococcus pyogenes, Streptococcus spp. microscopically in the material taken from the patient and in pure culture, draw it in the copy book.

Task 2. Study the growth of S. pyogenes and S. pneumoniae in blood meat infusion agar.

Task 3. Study the main antimicrobial drugs used for treat­ment, prevention and diagnostics of suppurative diseases and write them down in the copy book.

RECOMMENDATIONS FOR PRACTICAL WORK

Task 1.

Streptococcus pyogenes in pleural puncture biopsy material is Gram-positive cocci in twisted chains (in small number).

S. pyogenes in pure culture, Gram stain is gram-positive cocci in twisted chains. There are in one plane streptococci form chains of different length.

Many bacteria (e.g., S. pneumoniae [pneumonia]) contain a gelatinous covering called a capsule. In the clinical laboratory, demonstrating the presence of a capsule is a means of diagnosis and determining the organism’s virulence, the degree to which a pathogen can cause disease. To study the structure a special method of staining – Burri-Gins method is used.

In a negative method of living bacteria staining by Burri's technique, the bacteria remain unstained in a dark field. In a drop of Indian ink diluted with distilled water 1 to 10 the culture to be tested is inoculated and spread uniformly with a loop or the edge of the glass slide. The smear is air-dried. Nigrosin, Congo red, and other dyes may occasionally be utilized instead of Indian ink.

When Gins' staining fix an air-dried preparation made by Burri's technique and with a pipette put 2-3 drops of alcohol on it and bum it. Pour Preiser's fuchsin on the cooled slide to act for 3-5 minutes, wash the smear with water and dry it.

In this case, the bacteria are stained red, whereas unstained capsules are distinctly outlined against the dark background of the preparation. Sometimes, small colorless zones can be observed around those stained bacteria that do not form capsules. They are called false capsules and are due to inadequate drying or fixation of the smear.

Task 2.

S. pyogenes and S. pneumoniae on blood meat infusion agar forms the small, semitranslucent gray-white colonies of streptococci surrounded by zones of β-hemolysis. Streptolysins produced by S. pyogenes cause the lysis of red blood cells in vitro, producing β-hemolysis (a clear zone of hemolysis with no color change) on blood agar. Two types of beta lysins: streptolysin O and streptolysin S are producing. The former is oxygen-labile, while the latter is oxygen-stable. Streptolysin O is demonstrating only in deep colonies on the blood agar medium. Since most strains of S. pyogenes produce both types of lysins, surface of hemolysis is generally observed.

Different streptococci can select gemolysins:

Colonies with α-hemolysis on blood agar are surrounding by a green zone. This “greening” is caused by H₂O₂, which converts hemoglobin into methemoglobin.

Colonies with β-hemolysis on blood agar are surrounding by a large, yellowish hemolytic zone in which no more intact erythrocytes are present and hemoglobin is decomposed.

Colonies with γ-hemolysis indicate the absence of macroscopically visible hemolytic zones.

Task 3.

O-streptolysin is enzyme using for diagnostic. The streptolysin O (a streptococcal hemolytic exotoxin) is oxygen-labile; the other antigen being oxygen stable streptolysin-S. Both enzymes are involved in producing hemolysis, i.e., digestion of blood, particularly beta-hemolysis. Anti-streptolysin O antibody can be detected in an antistreptolysin O titre (Antistreptolysin O titre (AS(L)O titer or AS(L)OT) - titer is a measure of concentration). Titre testing employs serial dilution to obtain approximate quantitative information from an analytical procedure that inherently only evaluates as positive or negative) of (serum) antistreptolysin O antibodies is a blood test used to assist in the diagnosis of a streptococcal infection or indicate a past exposure to streptococci).

Pneumococcal vaccine consists of antigens of most actual subtype of S. pneumoniae. It is used for specific prevention of streptococcal pneumonia. Immunity after vaccination develops in 10-15 days and saved to 5 years. A requirement in a vaccination (preventions) against a pneumococcal infection increases and in connection with the growth of resistant to causative agents looked after in the entire countries of the world to penicillin, macrolids and other antibiotics - as a result of their frequent setting at infecting of S. pneumoniae.

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Date: 2016-01-14; view: 1103