I isolated bacterial species from marine samples. Then I proceed with cross-streak assay against human pathogens. First, I inoculated the marine isolates for two days and then streaked the bacterial pathogen perpendicular to the isolate.
On the first day of incubation I didn't observe any significant growth on the plate, whereas by the second day the pathogens had grown well. What does that imply?
The possible reasons why you did not observe significant growth on the plate on the first day may be due to the number and age of cells. The number of cells for both must be standardized and in the log phase. If you have done that then you could say that your bacterial isolates may have inhibitory activity but not bactericidal.for the pathogen.
I have also experienced this kind of situation during my cross-streak assay. I am also searching for the antibacterial compound from the marine isolates. However my target bacteria to be inhibited is the fouling bacteria (marine bacteria). I've made a negative control during the cross-streak assay whereby the marine isolate was substituted with sterilized seawater/dH20. Then my target bacteria (fouling bacteria) were streaked perpendicularly to the marine isolate. After two days, I've compared the bacterial growth of the control with the one with the marine isolate. If the control set shows luxuriant growth of the target bacteria, then you can read your result in your test plate.
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