In vitro comparison of NALC-NaOH, tween 80, and C18-carboxypropylbetaine for processing of specimens for recovery of mycobacteria.

A recent article (C. G. Thornton et al., J. Clin. Microbiol. 36:1996-2003, 1998) reported a new specimen-processing method for improved recovery of mycobacteria. This method used C18-carboxypropylbetaine (CB-18) and increased both smear and culture sensitivity. The companion article (C. G. Thornton et al., J. Clin. Microbiol. 36:2004-2013, 1998) described initial improvements to this method. Additional significant parameters of the CB-18 processing method are identified herein. First, eliminating the incubation step was shown to further improve culture sensitivity. Subsequently, recovery of several mycobacterial isolates by the CB-18 method was compared to a contemporary processing method that combines NALC and NaOH (NALC-NaOH) and a Tween 80-based method. Recovery of the tuberculous isolates following NALC-NaOH processing averaged 20% and ranged from 1.6 to 45%, whereas recovery of the nontuberculous isolates averaged 11% and ranged from 0.1 to 55%. Recovery of the tuberculous and nontuberculous isolates by the Tween 80-based method ranged from 22 to 92% and 27 to 93%, respectively, with averages of 58 and 65%, respectively. Recovery of the tuberculous and nontuberculous mycobacteria following CB-18 processing averaged 86 and 73%, respectively, with ranges from 61 to over 100% and from 43 to over 100%, respectively. Other parameters of the CB-18 method were also examined, including recovery versus CB-18 concentration and the relationship between CB-18 concentration and the tuberculocidal effect. The tuberculocidal effect was time dependent but independent of concentration, whereas recovery was directly proportional to concentration. Increasing the CB-18 concentration to 4 mM provided quantitative recovery on solid medium; however, higher concentrations of CB-18 were not compatible with liquid culture. Examination of the relationship between increasing CB-18 and lecithin concentrations suggested that lecithin could not overcome the deleterious effects of CB-18 in liquid culture at these higher concentrations.