Modified Middlebrook 7H9 broth for the rapid detection of mycobacteria

To compare the growth of acid-fast bacilli on classically formulated Lowenstein-Jensen media and Middlebrook 7H11 agar to modified Middlebrook 7H9 broth.

Microbiology culture results were evaluated to determine the number of days required to detect growth of acid-fast bacilli from clinical specimens inoculated on different media.

Three thousand one hundred fifty-seven acid fast bacilli cultures were performed from May 1996 through May 1999. There were 140 positive cultures. Initial growth was detected only on Lowenstein-Jensen medium and/or Middlebrook 7H11 agar for 108 cultures, Lowenstein-Jensen medium and/or Middlebrook 7H11 agar and 7H9 for 12 cultures, and only modified Middlebrook 7H9 broth for 20 cultures. Of the 20 cultures that were initially detected only in 7H9, 11 grew in a mean time of 21.1 days compared with 32.5 days for the later growing solid media. The remaining nine grew in a mean time of 26.0 days but did not grow on solid media within the eight-week incubation period. Mycobacterium avium grew in a higher proportion of broth than solid medium cultures, suggesting a preference for the broth.

The modified Middlebrook 7H9 broth was inexpensive to prepare, allowed for quick and easy growth interpretation, required no special appliances, e.g., ultraviolet light, and allowed for faster growth of some cultures. Although Lowenstein-Jensen medium and Middlebrook 7H11 agar remained the main-stays of our routine acid fast bacilli culture protocol, modified Middlebrook 7H9 broth provided earlier or additional positive results in 14% of the positive cultures. We recommend the use of modified Middlebrook 7H9 broth as described in this paper for improving early detection of acid fast bacilli by culture.