Macrophages, key cells in the innate immune system, can distinguish some pathogenic bacteria from those that are beneficial, but the recognition process is not fully understood. Previous research has shown that host cells respond when their intracellular fluid, or cytosol, is contaminated by bacterial flagellin, but researchers may now have demonstrated that macrophages can detect virulence factors, molecules secreted by pathogens to compromise healthy cells. Edward Miao et al. (pp. 3076-3080) analyzed macrophages infected with strains of disease-causing Gram-negative bacteria, including Salmonella and Escherichia coli, and determined that the NLRC4/ Ipaf sensor pathway recognizes a specific component of the molecular rod that many pathogens use to invade host cells.
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