How did you determine the best Multiplicity of Infection?

Best MOI will have to be titrated for your experimental setup (e.g. duration of infection), cell type etc so as to have every cell in culture take up atleast one bacteria as well as a desired phenotype is observed in reasonable time.
When starting with new experiment it will be good to use a range of MOI's like 1, 3 and 10 and check for cellular viability at each MOI alongside your desired measurable output.
Hope this help.