Blood agar, chocolate agar, and Mycobacterium tuberculosis.

We have read with interest the recent paper by Drancourt et al. in which they reviewed the use of blood agar medium for the primary isolation of Mycobacterium tuberculosis (2). They state that to their knowledge “no comparative study comparing the efficacy of blood-based agars and egg-based agars has been carried out, and even the ability of blood agar to support growth of M. tuberculosis was forgotten.” In December 2000, we published a letter (PubMed PMID: 11198009) concerning the isolation of M. tuberculosis on both blood and chocolate agar media from a synovial fluid sample following prolonged incubation (5). In recent years, we have been routinely recovering M. tuberculosis in our laboratory from blood samples that have been cultured initially in liquid media and then subcultured onto chocolate agar and from skin biopsy samples directly cultured on to chocolate agar. The use of blood agar media for the recovery of M. tuberculosis was reported early in the last century but has been removed from contemporary microbiology manuals (4, 6). However, there have been more recent reports, including one from 1998 in which Arvand et al. isolated M. tuberculosis from a lymph node when investigating a diagnosis of cat scratch disease (1). Even earlier, a comparative study of different media conducted in 1977 suggested that penicillin blood agar would be at least as good as, if not better than, LöwensteinJensen medium for recovering M. tuberculosis (3). The study of Drancourt et al. with clinical samples is very useful in once again highlighting the ability of these media to grow M. tuberculosis, in particular when this is not the organism being sought. Moreover, we agree with Drancourt et al. in highlighting the importance of handling in a secure manner culture media which are not specific for mycobacteria but require prolonged incubation, as we have already stated in our paper. Sealing the agar plates with adhesive tape (Micropore surgical tape; 3M, St. Paul, Minn.) is a simple way to avoid risks.