It is difficult to determine in advance the best number of serial dilution. In the case of contaminated plants I used to apply the third and fourth dilution respectively the seventh and eight.

Thanks Attila. But your answer is not very clear. What do u mean by applying " the third and fourth dilution respectively the seventh and Eight"

Hi, I hope now you understand what I meant. 

Ten fold serial dilutions up to10^-8 needs to made. 100 µl of the third and fourth dilution respectively the seventh and eight dilution have to be plated onto agar medium with a glass rod, then incubated. 

If I understand you, I will make about eight ten fold serial dilutions, plate third, fourth, seventh and eighth dilution for incubation and then find the average for the colony counts. 

Hi,

Depending on the incubation of preparation. Maintain consistent incubation period, then do tenfold dilutions.  For example: E. coli, 20 hours of incubation at 30-35 will yield 10^9, but Pseudomonas only yield 10^8. 

Dear Johnson

Greetings

Please see different papers below for - number of dilution appropriate for best results during microbial enumeration

https://www.researchgate.net/profile/Ajar_Nath_Yadav/publications

https://www.researchgate.net/publication/263297581_Diversity_and_phylogenetic_profiling_of_niche-specific_Bacilli_from_extreme_environments_of_India

https://www.researchgate.net/publication/270590458_Culturable_diversity_and_functional_annotation_of_psychrotrophic_bacteria_from_cold_desert_of_Leh_Ladakh_India

https://www.researchgate.net/publication/270571229_Prospecting_cold_deserts_of_north_western_Himalayas_for_microbial_diversity_and_plant_growth_promoting_attributes

Thank You

Ajar Nath Yadav