Middlebrook 7H11 reduces invalid results and turnaround time of phenotypic drug susceptibility testing of M. tuberculosis

Phenotypic drug-susceptibility testing (pDST), which relies on the growth inhibition in the drugcontaining media, remains a challenge for fastidious Mycobacterium tuberculosis complex (MTBc) isolates due to insufficient growth on the growth controls (GC). Middlebrook 7H11 (M7H11) medium contains casein hydrolyzate, which may promote their growth. In this study we tested whether M7H11 reduces invalid results due to insufficient growth on the GCs and the turnaround time (TAT) of pDST for MTBc compared to Middlebrook 7H10 (M7H10) and how it differs between rifampicin- and isoniazid-susceptible (non-MDR ), multidrug-resistant (MDR) and MDR with additional resistance to fluoroquinolones (pre-XDR) MTBc. We compared the occurrence of invalid initial and final pDST results due to insufficient growth on the GCs, colony forming unit (CFU) count of the 1% GC as an indicator of growth abundance, and the percentage of pDSTs
that had a valid result after four weeks of incubation as an indicator of growth speed of MTBc on M7H11 and M7H10. For MDR and pre-XDR isolates, compared to M7H10, M7H11 significantly reduced the occurrence of initial invalid pDST results due to insufficient growth (from 8.1% to 2.2%) and increased growth speed of MTBc with 18% of the isolates having a net gain of two weeks in TAT. In addition, the growth abundance of MTBc on M7H11 was significantly higher
compared to M7H10 (16 CFU on M7H10 vs 27 CFU on M7H11) irrespective of drug-resistance profiles. Our study findings suggest that the M7H11 medium should be preferred over M7H10 for pDSTs of MTBc.