I am helping out with an experiment with multiple bacterial cultures (>20 species). I need to a very specific concentration (cells/mL or CFUs*/mL) for this purpose. Unfortunately it appears that standard growth curves (OD** vs. CFU/mL) are not available for these cultures. I am not in a position to generate them afresh either given the sheer number of cultures I have to work with and how laborious the growth curve experiments are.
Given these constraints, are there other reliable (and needless to say fast and accurate) ways through which I can find out how many cells/mL (or CFUs/mL) I have in a given bacterial culture at a given OD? All the more better if the method is parallelizable.
I came across this study (Robust estimation of bacterial cell count from optical density.) where they suggest using silicon microspheres with dimensions similar to E. coli to reliably estimate ODs, but I don't know what the cell sizes for the bacteria I am working with are. In this case, what are some easy and accurate ways in which I can estimate the average size of these bacterial species?
* - CFU - colony forming unit
** - OD - optical density