the experimental design is wrong. food contains bacteria in different types u didn't count specially S.aureus.  

your aim is't clear what do you want to study?

Effect of plant extract in different conc on S.aureus in food? so food sample should be free from other bacteria -----  by irradiation for example.

or the effect of bacterial load in food on the effect of plant extract on S aureus reduction? so different concentration of bacterial load and s. aureus is required with constant conc of plant extract

I'm not sure what MSA is.  Expanding on what Gehan Kassem stated above, I see several problems.  1) why not start with demonstrating efficacy of the suspect antimicrobial on the pure culture of S. aureus?, 2) the levels of S. aureus you seeded with is extremely high (1 ml of 1.5x 10E+8 cfu + 10-100 ul of extract).  Even if you observed no inhibition, one could easily say you overwhelmed the inhibitor with excess seed inoculum far above practical levels.  I would check a high level of inhibitory extract against different inoculum levels.....if you find one that works, then you can go back and start cutting back the inhibitor levels, sort of like finding an MIC level. Once you've got that worked out with pure culture, then see if you get similar inhibition in a food matrix.  But like Kassem said, if you inoculate non-sterile food, how will you determine your inoculum from the indigenous bacteria in the food?  One way I have done that is to use B-D antibiotic disc assay using about 20 antibiotics (antibiotics impregnated on paper discs), placed on a lawn of your intended inoculum strain, find out what antibiotics it is naturally impervious to, include several of them in plating media to make it a 'selective media' (predetermine what levels do not affect plate counts of your intended inoculum), and then you can recover/plate your inoculum strain from a contaminated food (while inhibiting the background flora).....you should test the media by plating un-inoculated food to make sure any background counts obtained, if any, are well below the inoculum level.

Your method is faulty. First establish the antimicrobial properties on of the extract on S. aureus using well in agar diffusion method. Determine the MIC and MBC. Then try the preservative potential in the food sample using 103 cfu/ml of the culture. Different concentrations of the extracts should be prepared. Make sure the food sample is treated to eliminate all other bacteria before inoculating with your choice organism (S. aureus). Plate out samples of inoculated food daily to determine the microbial population remaining (whether there is reduction or increase in bacterial number). If there is reduction, calculate rate of reduction.

thank you everyone for your responses, it was quite helpful, actually  i had already carried out the antibacterial activities of the extract on s.aureus only and wanted to see if the same scenario will play out if it were in food. so, i crushed the prepared food, weighed 10g in 100ml of water and measured 1ml each into tubes then added (10ul not 1ml of 1.5x10^8 s.aureus, sorry ) then in each tube added different conc. of my extract from 10-100ul then prepared Mannitol salt agar in which i poured d diff.concentrates in a pour plate method. after which the colonies where counted. i expected the colonies to decrease with increasing concentration of extract. there was also controls; positive control with known antibiotic, s.aureus and food only, food only.

how do i interpret my results statistically or is the design still wrong?

Conduct the normal ANOVA on the microbial counts obtained. First convert the count to log 10 and use that for your ANOVA (Analysis of Variance). Separate the means using Fisher's Least Significant Difference.Do these first lets see what happens.

Thank you very much.

Thank you very much.