Why was Tetracycline more effective than streptomycin?
Wе dіd аn experiment іn class, еνеrу group іn thе class found thаt tetracycline wаѕ more effective thаn streptomycin іn inhibiting growth οf E coli. Cουld іt hаνе bееn bесаυѕе thе particular strain οf e coli wе used mау hаνе hаd a resistance tο streptomycin?
both have the same mechanism inhibiting bacterial growth – they interfere with protein synthesis. A common mistake is a wrong concentration of the antibiotic in the agar – Tetracycline resistance for example is a mechanism that "pumps" the antibiotic out of the cell before it can act but after a certain treshold the pump wont be able to get enough out so the cell is inhibited – the antibiotic still works after beeing pumped out so a lot of energy of the bacteria goes in the defence and they won t grow well even when resistant
streptomycin resistance is a chemical modification (an adenylyltransferase ) and the antibiotic is destroyed (similar to the beta lactamases and penicillin derivates) so there is no stress for the bacteria
my guess is that you got a resistant strain if one plate worked very differently
also both are heat sensitive so the temperature of the agar when you added the antibiotic before pooring the plates might have been too high for the strep plates
It could have been. If no one in your class observed any zone of inhibition on your streptomycin plates, and all growth was similar to the control nutrient agar plates, that would be my conclusion.
It is possible that E.coli was resistance, also consider the inoculam size you use to spread it on agar.