Understanding microbiological test results

I would like to understand more about the microbiological testing, but I am not sure where to start (other than buying a microbiology science textbook and jumping in with both feet). 

Does anyone have any useful resources? 

I received my testing results back on my first formulation this week, and my 'bile tolerantgra  negative bacteria' results were less than 1000, but greater than 100. I'd like to get it below 100, but I don't know how to determine whether this count is caused by contamination during production (k.em if my SOP needs updating), or if maybe my actual jningredients could be causing the issue. 

If anyone has any experience in this, that'd be really helpful. I want to get it right and then send for retest, but right now don't know where to start. 
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