Mucosal associated invariant T (MAIT) cells are characterized by an invariant TCR α chain and restriction by the MHC-related protein MR1. This study shows that MAIT cells are activated by bacteria and yeasts, but not viruses, in both humans and mice. Activation requires a cognate interaction between the invariant TCR and MR1, which can present a bacteria-derived ligand. In humans, MAIT cell numbers decrease in patients with bacterial infections such as tuberculosis. In mice, MAIT cells protect against infections by Mycobacterium and Escherichia coli. MAIT cells are evolutionarily conserved innate-like lymphocytes that sense and help fight off microbial infections. MAIT cells are another lymphocyte subset that expresses an evolutionarily conserved invariant TCRα chain. They are selected on the highly phylogenetically conserved MHC class I related molecule, MR1. MAIT cells are abundant in human blood, the intestinal mucosa, and mesenteric lymph nodes. Human MAIT cells display a memory phenotype early in life, but are small in numbers and naïve in cord blood, suggesting MAIT cells expand after birth and acquire their memory phenotype in the presence of commensal flora. Supporting this hypothesis, MAIT cells are not detectable in germ-free mice. However, their number is increased in transporter associated with antigen processing and invariant chain double-deficient mice. Here we show that MAIT cells respond to antigen presenting cells (APC) cocultured with bacteria in an MR1-dependent manner both in humans and mice. This activation is induced by a wide variety of bacteria and yeasts, but not by viruses. APCs acquire their stimulatory capacity very quickly after infection and a cognate interaction between the iTCR and MR1 is required. Moreover, this interaction has antibacterial potential in vivo, as MR1-deficient animals are more susceptible to Mycobacterium abscessus and Escherichia coli infection. Finally, in humans, the number of MAIT cells is reduced in peripheral blood from patients with infectious diseases such as tuberculosis, while they are detected at the site of infection. We conclude that MAIT cells are evolutionarily conserved innate-like T cells with anti-microbial properties.Mucosal associated invariant T (MAIT) cells are characterized by an invariant TCR α chain and restriction by the MHC-related protein MR1. This study shows that MAIT cells are activated by bacteria and yeasts, but not viruses, in both humans and mice. Activation requires a cognate interaction between the invariant TCR and MR1, which can present a bacteria-derived ligand. In humans, MAIT cell numbers decrease in patients with bacterial infections such as tuberculosis. In mice, MAIT cells protect against infections by Mycobacterium and Escherichia coli. MAIT cells are evolutionarily conserved innate-like lymphocytes that sense and help fight off microbial infections. MAIT cells are another lymphocyte subset that expresses an evolutionarily conserved invariant TCRα chain. They are selected on the highly phylogenetically conserved MHC class I related molecule, MR1. MAIT cells are abundant in human blood, the intestinal mucosa, and mesenteric lymph nodes. Human MAIT cells display a memory phenotype early in life, but are small in numbers and naïve in cord blood, suggesting MAIT cells expand after birth and acquire their memory phenotype in the presence of commensal flora. Supporting this hypothesis, MAIT cells are not detectable in germ-free mice. However, their number is increased in transporter associated with antigen processing and invariant chain double-deficient mice. Here we show that MAIT cells respond to antigen presenting cells (APC) cocultured with bacteria in an MR1-dependent manner both in humans and mice. This activation is induced by a wide variety of bacteria and yeasts, but not by viruses. APCs acquire their stimulatory capacity very quickly after infection and a cognate interaction between the iTCR and MR1 is required. Moreover, this interaction has antibacterial potential in vivo, as MR1-deficient animals are more susceptible to Mycobacterium abscessus and Escherichia coli infection. Finally, in humans, the number of MAIT cells is reduced in peripheral blood from patients with infectious diseases such as tuberculosis, while they are detected at the site of infection. We conclude that MAIT cells are evolutionarily conserved innate-like T cells with anti-microbial properties.