Say you have a bacterial isolate and you need to check its antibiotic resistance profile. In this case, you need to do an antibiogram study. Ideally, you’ll need to make bacterial lawn on Mueller-Hinton agar medium for doing antibiogram. For that purpose, you prepare bacterial culture from stock in nutrient broth and place it in shaking-incubator. When the culture is young enough, that means when the bacteria on the beginning of its growth phase, you use the culture to make lawn on Mueller-Hinton agar plate. How do you know the culture is young? The common practice is to measure its spectrophotometric absorbance at 600nm. If the culture’s optical density or OD is near to 0.1, then it is perfect for making a lawn. Very simple procedure.
But this can be a cumbersome job when you have to handle many bacterial cultures rather than one. If bacterial samples you are working with are of different species, then their binary fission time will vary. For some bacteria cell division rate is higher and it’s culture will approach to 0.1 OD fast. For other, the rate will be slow and thereby their culture will take time to get that point. Continue reading “Optical Density Hack: Use Visual Comparator in Antibiogram”