For isolating mycobacteria from clinical specimens, optimally a liquid medium along with one or more solid media should be used. Middle Brook 7H9 broth is the most commonly used liquid medium for mycobacterial isolation. Solid media used for mycobacterium culture can be divided in to two categories; egg based and agar based. The most often used egg based media being Lowenstein Jenson (LJ) medium and agar based media are Middle Brook 7H10 and 7H11.Mycobacterial growth in these media is usually observed within a time period of 3 to 56 days.
Blood agar is a non-selective media for wide range of pathogenic and non-pathogenic bacteria and is widely used in clinical laboratories. 5% Sheep blood agar slant tubes could also be used to isolate mycobacteria from specimens after NaOH and N-acetyl-L-cystein decontamination. Inoculate 0.2ml of decontaminated suspension to agar slants. Inoculated blood agar slants could be incubated at 37◦C and 30◦C for 28 days. Slants are to be monitored for three times a week upto eight weeks for any growth and the isolates are Ziehl Neelsen stained to look for characteristic AFB and identified by phenotypic analyses.
Advantages of using Blood agar slants:
· Cost effective
· Possible to incubate at different temperatures (M.marinum grew in blood agar incubated at 30◦C and other species of mycobacterium at 37◦C)
· M.haemophylium grew only on blood agar media.
· Small amount of inocula is enough to promote growth on blood agar than other specific medium for mycobacteria.
· Requires no special equipments like BACTEC TB 460 for monitoring the mycobacterial growth.
· Readily supports growth of all Mycobacteria except M.ulcerans.
· M.ulcerans, a fastidious organism rarely isolated from diseased tissue grew only on Middle Brook 7H10 agar and could not be isolated in blood agar
· Being a highly sensitive medium, is susceptible to contamination.
· Mycobacteria cannot be isolated from CSF, Blood and Bone marrow specimens on blood agar.
Reference: Drancourt M, Raoult D.,Cost-effectiveness of blood agar for isolation of mycobacteria. PLoS Negl Trop Dis. 2007 Nov 28;1(2):e83.
- Ms. Sweetha
keywords: TB, Mycobacterium, Mycobacteria, Blood agar, Chocolate agar, TB culture, AFB culture, Blood agar plate , TB culture blood agar, 7H9, 7H11, LJ slants,