Fig. 1. Histidine auxotrophy is bactericidal and impedes pathogenesis of Mtb.

a Comparison of the growth kinetics of H37Rv, ΔhisD and chisD in the presence and absence of external histidine shows the inability of the ΔhisD to multiply in the absence of external histidine supplementation ab initio (n = 9 independently grown bacterial cultures; mean and SEM). b Bacillary counts corresponding to the growth kinetics show a decline of ΔhisD when no external histidine is supplemented ab initio (n = 9 independently grown bacterial cultures; mean and SEM). c A decline in bacterial counts of ΔhisD (>5-fold) over a course of 48 h post infection in Primary macrophages (PM); completely rescued in chisD hint at the indispensability of HisD in ex vivo infection (n = 4 independently acquired and cultured primary cells; mean and SEM; *P-value < 0.05). d Bacillary load in B6 mice lungs over a period of 9 weeks depicting ~10 and 100-fold declines in bacterial counts of ΔhisD at days 28 and 63, respectively, compared to H37Rv and chisD (n = 8 individual mice per time point; mean and SEM; **P-value < 0.005). e The disease rapidly progresses in H37Rv and chisD infections, as is evident from a large number of lesions on the external physiology of the lungs. ΔhisD shows minimal infection at day 28 and almost clears out by day 63 (pictures are representative of nine individual mice lungs samples). f Histopathology of lungs depicting decreased air spaces and higher number of lesions on days 28 and 63 in H37Rv and chisD-infected mice. ΔhisD infected lungs have fewer granulomatous formations by day 28 with almost clean lungs on day 63 (pictures are representative of nine individual mice lungs samples).