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We classify the relationship between host cells and bacteria in one of three ways: host cells with no bacteria $H_u$, with attached bacteria $H_a$ and with invaded bacteria $H_x$. A host cell may have both internal (invaded) and external (attached) bacteria so we define another parameter $H$ which is simply the number of host cells with one or both. The total number of host cells is therefore $H_{\rm tot} = H + H_u$.  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, $m = B_{\rm tot} / H_{\rm tot}$.