Description of Wylie-Stanley Agar for the Recovery of Mycobacterium abscessus

Abstract

Background: The microbiology of cystic fibrosis (CF) is complicated by the presence of several species, including Mycobacterium abscessus, which are highly resistant to antibiotics. Conventional selective bacteriological methods employ antibiotics which favor the growth of one bacterial component over others in a mixed population. For in vitro studies examining multiple species, for example, in dual biofilm models, it is difficult to successfully separate M. abscessus from nontuberculous mycobacterial (NTM) species. Therefore, it was the aim of this study to develop a selective agar medium that was able to isolate M. abscessus from a pool of other highly-resistant Gram-negative organisms, which would be useful to microbiologists performing co-culture experiments and which require re-isolation of the NTM organism. Methods: Wylie-Stanley agar (WSA) was developed consisting of glucose, 16 g/l; yeast extract, 30 g/l; peptone, 6.8 g/l; and agar, 20 g/l along with selective supplements including chloramphenicol, 50 mg/l; ceftazidime, 32 mg/l; colistin, 24 mg/l; trimethoprim, 21.3 mg/l; sulfamethoxazole, 106.7 mg/l; and novobiocin, 50 mg/l. This medium was (i) challenged with 10 non-NTM species (27 isolates) of common Gram-negative and Gram-positive organisms associated with CF and (ii) compared to Columbia Blood Agar and Middlebrook 7H10 Agar for the isolation of M. abscessus organisms from mixed cultures of NTM organisms and Pseudomonas aeruginosa and Stenotrophomonas maltophilia. Results: This medium was highly specific for the growth of M. abscessus organisms and grew all NTM organisms. WSA medium did not allow the growth of any of the non-NTM species. When mixed cultures of M. abscessus species and P. aeruginosa and S. maltophilia were inoculated onto WSA medium, only the NTM organism could be grown successfully, highlighting the specificity of this medium. In contrast, both Columbia Blood Agar and Middlebrook 7H10 Agar allowed the growth of both NTM and non-NTM organisms. Conclusion: While the specificity was high, the sensitivity of WSA was low, and therefore, we do not advocate employment of WSA medium for the primary isolation of M. abscessus organisms from CF sputum, rather for the purposes of separating M. abscessus populations of organisms from other highly-resistant organisms, including P. aeruginosa and S. maltophilia, which would be useful to microbiologists performing co-culture experiments and which require re-isolation of the pure M. abscessus organism.