How would you test the efficacy of a dry antimicrobial product (powder form)?. The dry product is applied to dry samples. I am trying to see which dosage works the best so I have to test different dosage rates for example 15 kg of antimicrobial powder per 1 ton of product, 20 kg of antimicrobial powder per 1 ton of product, etc.
I have tried adding the dry powder in different percentages to a broth of bacteria (with known cfu/ml) and testing the recovery after specific exposure times but this showed an increase in bacteria numbers. I think this was because the broth I used to culture the bacteria buffered the antimicrobial powder out of it's effective pH range.
I am thinking of trying the experiment above in saline (isotonic but acidic) so as to not to take antimicrobial powder out of it's effective pH range.
I have tried making up different percentage solutions (in water to eliminate the buffering effect that the broth had) and then done disk diffusion using said solutions but these had no effect (i.e. did not produce any zones of inhibition).
I am then thinking of adding the antimicrobial powder to molten agar and making cast plates. But I'm suspect it won't would work because the agar could have a buffering effect, such as in the case of the broth, and negate the effects of the powder.
Would my best bet be to artificially infect a dry sample with bacteria and then treat it with the antimicrobial powder and see if I can recover the bacteria?
As you said it is possible to simulate (artificially infect) your product, however, it is still subject to the variability of the intrinsic and extrinsic properties of your products for microbial growth. It is hard to maintain those entire factors in simulation. So it is better if you could check all other techniques on in vitro culture media before conducting the artificial infection and checking for microbial load/risk reduction. The following literature might help:
I agree with the comments given above. In addition, how did you prepare the mixture of an antibiotic and a powder. Did you apply geometric dilution for mixing of the antibiotic powder with the bulk powder? Unless you use geometric dilution mixing, you will not achieve uniform mixture.
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