Question

Information provided:

lab facts/results:

•do a direct antigen test on swab sample of pharyngeal area (use tongue depressor also)

•patient name: Tim Barton

•gender: male

•patient age : 6 years old

•doctor : Dr. F. Bacald

•tentative clinical diagnosis : throat infection (pharyngitis)

•symptoms: sore throat, difficulty swallowing, fever 102°F (39°C), swollen neck glands, white spots on tonsils

•medication: penicillin for 10 days

•child better after 48 h, recovers very well

QUESTIONS:

1. If the direct antigen test is negative, what is the best way to proceed? (2pt)

2. If the swab sample was cultured for initial identification of streptococci, which medium should the lab have used and in which condition should it be incubated?  (2 pts)

3. On this medium, what would be the appearance (colonial characteristics) and colour of the colonies of the organism causing the child’s pharyngitis? (3 pts)

4. How would the identification of the bacterial agent be confirmed? (1 pt)

Answer 1

1. some cases, if you have symptoms of a strep infection but your test comes back negative, your doctor may order a throat culture. ... This means that if your test is negative, you could still have an infection from another type of bacterium or virus.

throat swab culture, or throat culture, is a test commonly used to diagnose bacterial infections in the throat. These infections can include strep throat, pneumonia, tonsillitis, whooping cough, and meningitis. ... Streptococcal bacteria are very contagious. They can be spread through airborne droplets.

A rapid antigen detection test uses antibodies that attach to a specific antigen to detect its presence. If the sample contains the pathogen, the antigens will attach to the labelled antibodies during the rapid antigen detection test and allow the doctor to identify the pathogen causing the illness.

2. Blood agar is commonly used to isolate not only streptococci, but also staphylococci and many other pathogens. Besides providing enrichments for the growth of fastidious pathogens, Blood agar can be used to detect hemolytic properties. ... Reactions on blood agar are said to be beta, alpha, gamma.

Blood agar plates are routinely used in the clinic to test for pathenogenic bacteria in throat swabs. These throat pathogens are often Gram positive cocci that may be hemolytic, producing exotoxins called hemolysins that destroy blood cells.

3. Streptococci are generally grown on agar media supplemented with blood. This technique allows the detection of β-hemolysis, which is important for subsequent identification steps, and enhances the growth of streptococci by the addition of an external source of catalase. Selective media for culturing Gram-positive bacteria (such as agar media that contains phenylethyl alcohol, or Columbia agar with colistin and nalidixic acid) also provide adequate culturing conditions for S. pyogenes. Optimal incubation conditions for the vast majority of streptococcal strains include a temperature range of 35°C to 37°C in the presence of 5% CO2 or under anaerobic conditions. These conditions are optimized for culturing streptococcal species that belong to the viridans group, but they may not be ideal for growing S. pyogenes.

Special procedures have been developed to optimize the identification of S. pyogenes in throat cultures. When properly performed and interpreted, culturing throat swabs on a 5% sheep blood agar with trypticase soy base incubated in air remains the gold standard and reference method for the diagnosis of S. pyogenes acute pharyngitis .

4. Streptococcus pyogenes bacterial agent confirmed.