Enhanced isolation of MOTT on egg media of low pH

The influence of alternative carbon sources, i.e. glycerol and pyruvate, and medium pH on the growth of 33 mycobacterial strains, including MOTT (n = 18), M. tuberculosis (TBC) (n = 14) and BCG, was experimentally studied. This was followed by culturing of clinical specimens (n = 4706) on three medium versions for primary isolation of mycobacteria. The decrease of medium pH below 6.5 enhanced the growth of isolated strains of M. avium complex (MAI) and M. malmoense. Pyruvate had a further beneficial effect on half of them; it was, however, inhibitory to some strains of M. malmoense, MAI and TBC as well as to BCG. In primary isolation, 74 specimens were positive for TBC and 37 for MOTT. The number of MOTT isolates of potential clinical significance was 11 on unacidified and 23 on acidified media (pH 6.2) (p less than 0.05). M. malmoense and five of 11 MAI isolates were exclusively detected on acidified media. TBC was isolated in equal frequency on all media, but it was detected on the acidified versions one to four weeks earlier in 20 of 55 (36%) specimens positive on all three media versions. The results indicated that acidified media of pH 6.2 offer enhanced growth conditions for MOTT, especially MAI and M. malmoense, without interfering with the growth of TBC. The growth enhancement obtained with pyruvate for some strains and with glycerol for some others makes it necessary to use both media versions in parallel.