Methods
quantitative PCR (qPCR) and fluorescence microscopy, to quantify the level of chromosome replication present during Escherichia coli propagation in the mouse peritonitis model. We find that the methods complement each other and allow for quantification of growth rate, both on a population average and on a single-cell level. We demonstrate the presence of heterogeneous growth rates within bacterial populations propagating during infection. Also, no growth cessation was observed during the apparent stationary phase in vivo, and, by comparison of growth dynamics at different anatomical sites, we demonstrate that E. coli is unlikely to grow independently intravascularly. These findings provide novel insight into bacterial growth during host infection, and underscore the importance of pinpointing the primary site of infection in septicaemia of unknown origin and ensuring antibiotic availability at this site.