I'm planning to isolate Aspergillus in a commercialized peanut butter from public market, I want to check the number of population of Aspergillus and compare it to the standard population that is approved by FDA. Please help me.
Dear, Jade,
You can prepare a suitable dilution, for example 10-5 by using chain of five dilutions. Add 1 ml of the last dilution (10-5) to a plate with sterilized medium and incubated it at 25 C for 3-4 days. After that, you must count the fungus colonies. Finally, you can calculate the number of spores per 1 ml according to the following equation:
No. of spores/1ml =No. of colonies x inverse dilution (105)
It could be useful to add antibiotics such as streptomycin (40 mg/mL), or chloramphenicol, or penicillin (20U/mL) to the medium to avoid (or at least to control) the growth of bacteria, because you are working with a unsterilized sample. You can adjust the volume of the suspension you prepare following the advice of professor Salih in consideration of the diameter of the Petri dish you are using (e.g. if you are using a 150mm diameter Petri dish you can plate 1 ml of the last dilution, if you are using a 90mm diameter Petri dish you can plate 0.2 ml of the last dilution and modify accordingly the equation).
Best regards