Make sure your strain of E. coli is sensitive to ampicillin at the concentration used. Do this by an MIC determination. In other words, determine the minimum ampicillin concentration needed to prevent growth of the culture. Then use that concentration as the control.

If your seaweed extracts don't inhibit growth of E. coli, don't be too surprised. It can be difficult to inhibit the growth of wild-type Gram-negative bacteria (that is, with active efflux pumps and normal outer membranes). Try using a Gram-positive bacterium instead. It's usually easier to get growth inhibition. Or use a strain of E.coli lacking efflux pumps (acrB or tolC) and/or with compromised outer membrane structure (imp).

Don't assume that your seaweed extracts contain antibacterial compounds. They may not.

Thank you so much for your reply , I greatly appreciate it . I am using a control strain ATCC E.coli 29522 ? I was hoping if i saw promising results to move on to wild gram negitive and positive strains of bacteria. Would you suggest using more volume of bacteria ie 100ul or placing the plate in a shaking incubator overnight ?

There is a standard methodology for broth microdilution MIC measurements. It can be found at the CLSI web site, but it isn't free. The culture is started with a certain number of MacFarland units. The plate is not shaken. It is incubated at a certain temperature for a certain amount of time. Here is a protocol I found on-line:

http://www.antimicrobialresistance.dk/data/images/protocols/mic%20broth%20dilution%20sensititre%20010405,%20modified%2010-2010.pdf

Thank you so much for your help , I greatly appreciate it.