I think the best for you is to do your own curve according to your strain. You start a culture and every hours or 2 hours you spread on plates and you measure the OD as long as you can. You draw your curve and you will see...Generally for Staph 1 OD 550nm= 5.8 10^8

Dear Mohab

if i remenber well od=1 is about 2x10^8 cells but you can simple check it by plating and counting some cells at different od.

if you need it for further analysis you dont need to growth the bacteria up to od of about 10 the cells which is quite high and difficult to reach but you can just growth it in higher volume uo to od 0.6-1 ( which are in exponential phase and viable ) centrifuge the culture remove the media resuspend the cells in a small volume of pbs, measure the od and than dilute it up to od10.

ciao

Manuele