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An important and controllable quantity when performing invasion assays is the ratio of bacteria to host cells at inoculation. This is called the multiplicity of infection (MOI) and in terms of previously defined parameters, $m = B_{\rm tot} / H_{\rm tot}$.  A well-studied feature of invasion by \textit{Salmonella} is the formation of epithelial cell membrane ruffles, triggered by effectors secreted via the bacterium's type III secretion system 1 (TTSS-1). We call the total number of ruffles $R$, and the number of ruffles per cell $\tilde{R}$. host cells with at least one ruffle $H_r$.