Survival in Macrophages Figure 6. Functional sigB and agr operons, but not sarA, are indispensable for S. aureus to survive phagocytosis by hMDMs. Macrophages were allowed to engulf defined strains of S. aureus for 2h at a MOI of 25, and the intracellular survival of bacteria on consecutive days post-phagocytosis was monitored by enumeration of the CFU of cell lysates (see Fig. 3A legend). The data shown is representative of at least three separate experiments, performed in triplicate, using hMDMs derived from different donors. Bars represent mean CFU value 6SD. A. 8325-4, a natural rsbU defective strain lacking a functional SigB. B. SH1000, a derivative of strain 8325-4 with a restored rsbU gene and SigB activity. C. Newman sigB mutant D. Newman agr mutant E. Newman sarA mutant F. Newman wild-type doi:10.1371/journal.pone.0001409.g006 to reduce the initial intracellular bacterial load by several orders of magnitude, in conjunction with a maintained ability to produce reactive oxygen species. Eventually, however, macrophages succumbed to the infection and were killed. In stark contrast to epithelial, endothelial and mesothelial cells, enterocytes, keratino- cytes, osteoblasts, lymphocytes [17,63–65] and especially human monocytes [66,67], the macrophages infected by S. aureus did not undergo apoptosis. At numerous time points several tests were PLoS ONE | www.plosone.org 9 January 2008 | Issue 1 | e1409