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\section{Parameters}  We classify During  the relationship between host cells and bacteria in one of three ways: host cells with no bacteria $H_u$, with invasion process, a bacterium can either be  attached bacteria $H_a$ and with to a host cell $B_a$,  invaded bacteria $H_x$. a host cell $B_x$, or neither (i.e. swimming around looking to attach) $B_u$.  These are mutually exclusive, so the total number of host cells bacteria  is $H_{\rm $B_{\rm  tot} = H_u B_a  + H_a B_x  + H_x$. B_u$. This is, of course, ignoring bacterial replication and xenophagy.  Similarly, a bacterium can either be unattached/uninvaded $B_u$, We classify the relationship between host cells and bacteria in one of three ways: host cells with no bacteria $H_u$, with  attached $B_a$ or bacteria $H_a$ and with  invaded $B_x$ bacteria $H_x$. A host cell may have both internal (invaded) and external (attached) bacteria so we defined another parameter $H$ which is simply the number of host cells  with a one or both. The  total $B_{\rm number of host cells is therefore $H_{\rm  tot} = B_u + B_a H  + B_x$. Here we are discounting any bacterial replication or xenophagy. H_u$.  An important and controllable quantity when performing invasion assays is the ratio of bacteria to host cells, or cells at inoculation. This is called  the multiplicity of infection (MOI) $ m and in the above terms $m  = B_{\rm tot} / H_{\rm tot}$.