Isolation of Mycobacteria from Clinical Specimens by Use of Selective 7H11 Medium

Abstract

7H11 agar containing carbenicillin, amphotericin B, polymyxin B and trimethoprim lactate (selective 7H11 or S7H11) was used for the selective isolation of mycobacteria from clinical specimens. This medium as previously described contained 100 μg. carbenicillin per ml. It was found that reducing the concentration of carbenicillin to 50 μg. per ml. made S7H11 less inhibitory to certain strains of mycobacteria. However, some strains of M. kansaii, M. intracellulare, and M. gordonae still do not grow well on S7H11. Of 3,134 clinical specimens (mostly sputum) received, processed, and plated on 7H11 agar and the S7H11 medium, 508 positive specimens were isolated. Of these, 402 were positive on both types of media, 30 were positive on 7H11 only, 19 were positive on S7H11 only, 52 were contaminated on 7H11 but positive on S7H11, while only 5 were contaminated on S7H11 and positive on the plain medium. Thus, the total positive specimens on 7H11 was 437 and total positive specimens on the selective medium was 473. Used in conjunction with nonselective media, S7H11 agar appears to be a valuable culture medium for use in diagnostic mycobacteriology laboratories.