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I am doing an experiment in which I am growing S. mutans in agar dishes, and I am not sure how I would measure the growth of the S. mutans. I am also not sure if I would do this by measuring cell mass or by cell count. Any ideas?

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The most simple way is seeding the plates with a suspension of bacteria ensuring that you spread the solution properly. Then you can count the number of colonies, wich would be equal to the number of single cells.

If you want to mesure the growth speed, usually it's simpler to just measure the diameter of the colonies, always ensuring you inoculate the plates with the same amount of inoculum.

Lastly, it's even easier to estimate the growth if your culture is un liquid media and you measure the optic density with an spectophotometer.

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I don't understand the effect of not "always ensuring you inoculate the plates with the same amount of inoculum." –  YAK Sep 25 '13 at 16:33
    
If you inoculate a plate with 100bacteria/ml and a second plate with 1000bacteria/ml, the second one will have more colonies, even if the effect of the experiment would have the opposite effect. –  Miguel Ángel Naranjo Ortiz Sep 25 '13 at 19:39
    
Thank you so much. And thank you for clarifying that statement. I was about to ask the same question. Also, I understand the second part, but I am still confused on what you mean by "seeding the plates with a suspension of bacteria ensuring that you spread the solution properly. Then you can count the number of colonies, which would be equal to the number of single cells." I'm sorry if this is a silly question, it's just that I'm in 8th grade and pretty new to this. I am doing this for science fair, actually. –  Sohum Sep 26 '13 at 1:23
    
Imagine you have a liquid media with a concentration of 100 bacteria/ml and you inoculate a plate with 1 ml. In theory, every single cell will grow to form a colony, so you must have around 100 colonies. If you don't spread it properly, all 100 colonies will appear clustered, and you won't be able to identify single colonies. That's why you must spread the inoculum yo form an homogenous layer over the agar, normally using an inoculation loop. –  Miguel Ángel Naranjo Ortiz Sep 26 '13 at 6:28

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