HPX and infection: Our findings that de novo-synthesized heme is important for M. tuberculosis survival is also supported by prior studies indicating that many enzymes of the M. tuberculosis heme biosynthetic pathway were upregulated in TB patient lungs compared with in vitro growth conditions (52) and that the granuloma contains many host heme-scavenging proteins, including hemopexin and haptoglobin, signifying that the host restricts heme availability at the site of infection (53).