How do i determine the minimum lethal dose of antibiotics against bacteria?



Answers:
First you have to determine the bacteria strain.
The lab will do this. Its called a "Culture and Sensitivity"
You can't do it yourself without access to culture media, an incubator and innoculation discs.
That's a difficult thing to do. In a given population of bacteria, there are always bacteria dying and new ones coming to life; the minimum lethal dose would be hard to determine since you couldn't tell for sure if it was the antibiotic that killed them or age. What I believe is the prevailing method is to measure the LDX where X = some number, usually 50. The LD50 is the point at which half of the target population die. This number is much more useful because it provides an average. If we were going for LD1, it would be a very low number since a lot of bacteria are probably weak and dying already. LD100 is extremely high because the antibiotic doesn't affect everything the same, and killing everything takes a very high dose.

Also, although I strongly encourage experimentation, if you plan on carrying out an experiment like this, you need to look up and employ proper containment measures. We have a very big problem currently with bacteria becoming resistant to some of the best antibiotics we have, and treating bacteria with low doses of antibiotic is the perfect way to breed resistant strains, so don't do it unless you know they won't get out.
You certainly don't have to determine the strain of bacteria first. There are several microbiological methods that may be of interest to you: determination of MIC (minimum inhibitory concentration) and determination of MBC (minimum bactericidal concentration). You can find a quick review of these methods here: http://www.bmb.leeds.ac.uk/mbiology/ug/u.

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