Blood Agar


Blood agar contains 5% sheep blood, is used to grow fastidious organisms, and can differentiate between different species based on their hemolytic properties.  Many of these fastidious organisms require particular nutrients found in blood and are considered more difficult to grow.  Blood agar is considered a differential agar and can be used to differentiate between different Strep spp.



 Streak plate of Streptococcus pyogenes (incubated @ 37 degrees C for 48 hrs) showing beta hemolysis due to hemolysin O and S, complete lysis of red blood cells, around the isolated colonies. Image taken using transmitted light.  S. pyogenes causes Strep throat.



Streak plate isolation of Streptococcus mitis on blood agar demonstrating alpha hemolysis seen as a greenish color around the growing colonies due to a reduction of the hemoglobin, oxidation of iron in the hemoglobin,  to methemoglobin in the surrounding agar. Image taken using transmitted light.




Enterococcus faecalis grown on blood agar for 48 hrs at 37 degree's C. Small, pin point, circular colonies can be seen with gamma hemolysis (no hemolysis of the blood agar). Colonies were white with a glossy translucent appearance.



(a)


(b)
(a)  Three organisms inoculated onto blood agar, straight line inoculation, to demonstrate hemolysis. From top to bottom: Streptococcus pyogenes: beta, complete lysis of red blood cells, clear area around colony growth. Streptococcus bovis: alpha, incomplete lysis of red blood cells, green area around colony growth. Enterococcus faecalis: gamma, growth with no blood cell lysis. Image taken using transmitted light.
(b)  Close up of S. pyogenes showing beta hemolysis and a zone of clearing around the growing colonies




Bacitracin test done on a lawn of Streptococcus pyogenes grown on blood agar. The zone of inhibition around the bacitracin disc, approx 14mm measuring the entire length of the zone, indicates sensitivity. The zone of inhibition is red because the red blood cells did not lyse. Grown for 24 hrs at 37 degree's C.  The bacitracin test can help differentiate S. pyogenes from other Strep or Staphylococcus spp.






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All images copyright Sturm

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