Mycobacterium and Aerobic Actinomycete Culture: Are Two Medium Types and Extended Incubation Times Necessary?

Abstract

Mycobacterial cultures are historically performed using a liquid medium and a solid agar medium with an incubation period of up to 60 days. We performed a retrospective analysis of 21,494 mycobacterial and aerobic actinomycetes cultures performed over 10 months to determine whether two medium types remain necessary and to investigate whether culture incubation length can be shortened. Specimens were cultured using Bactec MGIT liquid medium and Middlebrook 7H11/S7H11 solid medium with incubation periods of 42 and 60 days, respectively. Time-to-positivity and the identity of isolates recovered from each medium were evaluated. A total of 1,205/21,494 cultures (6%) were positive on at least one medium. Of the 1,353 isolates recovered, 1,110 (82%) were nontuberculous mycobacteria, 145 (11%) were aerobic actinomycetes, and 98 (7%) were Mycobacterium tuberculosis complex. Assessing medium types, 1,121 isolates were recovered from solid medium cultures, 922 isolates were recovered from liquid medium cultures, and 690 isolates were recovered on both media. Liquid cultures were positive an average of 10 days before solid cultures when the two medium types were positive (P < 0.0001). Isolates detected on solid medium after 6 weeks of incubation included 65 (5%) nontuberculous mycobacteria, 4 (0.3%) aerobic actinomycetes, and 2 (0.2%) isolates from the M. tuberculosis complex. Medical chart review suggested that most of these later-growing isolates were insignificant, as the diagnosis was already known, or they were considered colonizers/contaminants. This study reaffirms the need for both liquid medium and solid medium for mycobacterial and aerobic actinomycetes culture and demonstrates that solid medium incubation times may be reduced to 6 weeks without significantly impacting sensitivity.T raditionally, mycobacterial and aerobic actinomycetes cultures are performed using both liquid and solid media with extended incubation times of up to 60 days recommended for optimal recovery (1). The use of two types of media is recommended by the College of American Pathologists (CAP) accreditation program (MIC.32250) and the Clinical and Laboratory Standards Institute (CLSI) (1, 2).Many of the studies that support these recommendations were performed 10 to 20 years ago (3-6). Since that time, much has changed in mycobacteriology, including the transition of many laboratories from the use of radiometric liquid medium to mycobacterial growth indicator tubes (MGIT; Becton Dickinson, Franklin Lakes, NJ) or VersaTREK (TREK Diagnostics/Thermo Fisher, Oakwood Village, OH) medium and the description of many new species of mycobacteria (7,8).As laboratories enter into an era with a significant emphasis on cost-effective health care, it becomes increasingly important to examine traditional laboratory practices to ensure that they meet the needs of a changing health care environment and to look for cost-savings opportunities. The purpose of this study was to determine whether one type of medium can be eliminated or if incubation times can be s...

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