Question

What is the clinical reason that throat cultures are performed on blood agar ?

microbiology question

Answer 1

• A throat culture is a technique for identifying disease causing micro organisms in material taken from the throat.
• Most throat culture are performed to identify infections caused by group A beta- hemolytic streptococci which cause strep throat infection.
• The primary purpose of a throat culture is to isolate and identify organism from throat that cause infection of the posterior pharynx and tonsillar areas.
• The bacterium that most often cause a sore throat is Streptococcus pyrogenes or Group A beta-hemolytic streptococcus.
• It is also performed to identify people who are carrier of the organism that may cause meningitis ( Neisseria meningitides, Streptococcus pneumoniae) and whooping cough (Bordetella pertussis).
• Other bacteria may cause pharyngitis include Group B, C and G streptococci.
• Neisseria gonorrhoeae, Corynebacterium diptheriae, Hemophilis influenzae, Mycobacterium pneumonia.
• The physician or nurse should indicate if sny disease organism other than strep are suspected because some bacteria require special culture media and growth conditions.
• S.pyogenes, Group A beta-hemolytic streptococcus is cultured on a growth medium called blood agar.
• Agar is a gel that is made from the cell wall of red algae.
• Blood plates are made from agar that contain a low carbohydrate nutrient such as trypticase soy and 5% sheep red blood cells.
• When the thoat swab is wiped across blood agar plate.
• An innoculating loop is used to streak the plate and stab the agar .
• This process seperate the bacteria so that individual colonies can be isolated.
• Blood agar allows differentiation of streptococci based upon the characteristic hemolysis that they produce.
• Beta hemolytic strep releases products into the medium called beta hemolysin which lyses the RBC and cause a clear zone to foem around colonies.
• Alpha strep releases alpha hemolysin which causes green discoloration to the blood around the colonies.
• Gamma hemolysin ( no hemolysis produced) refers to no zone of discoloration around the colonies.
• Blood agar is non selective and permits the growth of normal throat flora as well as other potential pathogens.