why does water sample of higher dilutions have more colonies on TSA plates? - (bacterial enumeration) (Nov/22/2006 )
Currently i am facing a problem while doing my final year project (on bacteria enumeration)in sch... and i can't figure out the following problems:
1. i have adopted the use of pour plate and spread plate method to innoculate the bacteria on TSA plate, however the growth on the agar plates is weird and inconisistent for different experiments. some results showed that those sample of higher dilution factors has more colonies growth than those of lower dilution factor.
2. Also, some dilution factors in betwwen has no growth on the agar plates, and growth appeared again in the later dilution.
why is it so? what are the possible reasons?
(note:the problem should not be due to the lab techniques as the lab technician saw how we carried out our experiment and she said it should not be a problem.)
is it the serial dilution! 
in this case, your sample is diluted till you reduce other bacteria you are not interested in...so mostly the more dilution you do, the more "pure" (not exactly ) is your sample ..
1. i have adopted the use of pour plate and spread plate method to innoculate the bacteria on TSA plate, however the growth on the agar plates is weird and inconisistent for different experiments. some results showed that those sample of higher dilution factors has more colonies growth than those of lower dilution factor.
2. Also, some dilution factors in betwwen has no growth on the agar plates, and growth appeared again in the later dilution.
why is it so? what are the possible reasons?
(note:the problem should not be due to the lab techniques as the lab technician saw how we carried out our experiment and she said it should not be a problem.)
I would have a look at your water sample microscopically. If your organisms are clustering together, then a microcolony of organisms will appear as one colony when plated out. Not sure if that might explain whats going on but it wont take long to see if you are getting flocculation microscopically. If it is a problem you may need to vortex longer (1-2 minutes) or perhaps add a non-lethal amount of detergent like SDS.
reply to strawberry,
i guess what u mentioned is rather impossible as my main objective is to enumerate bacteria which is the colonies growth, regardless of what bacteria it is, so as long as there's growth it will be counted. and it is weird to have colonies growth at the last few dilution if in between don't have as serial dilution is carried out.
reply to ML_1975,
i'm sorry that i can't provide you with the microscpoic view. as we are just doing the plating directly after serial dilution... ...
***Thanks for all your replies. THey are greatly appreciated! 
do leave a message if you all think there are other possible reasons.
no offence, but you need to trouble shoot what you are doing otherwise you will get the same result over and over again. There is no point asking for assistance and then saying that you can't do it. Of course, its your work and you can do what you like but troubleshooting is one of the more important components of science......
Another possibility is a toxic susbtance in your water that when is diluted allows your bacteria to grow. Hence more growth in the higher dilutions.
i guess what u mentioned is rather impossible as my main objective is to enumerate bacteria which is the colonies growth, regardless of what bacteria it is, so as long as there's growth it will be counted. and it is weird to have colonies growth at the last few dilution if in between don't have as serial dilution is carried out.
reply to ML_1975,
i'm sorry that i can't provide you with the microscpoic view. as we are just doing the plating directly after serial dilution... ...
***Thanks for all your replies. THey are greatly appreciated!
do leave a message if you all think there are other possible reasons.