Water activity is measured on a scale of
0.0 to 1.0 with 1.0 = pure water. 
The lower the water activity, the drier the substance and the less likely it is for that substance to serve as a growth medium for microbes.  

Water activity is important because it affects the safety, quality, and texture of food. 
Water activity is used to control the growth of microorganisms like bacteria, yeasts, and mold. 
Water activity is used to control the stability of dehydrated and semi-moist foods. Examples of water activity in food: Fresh fruits and vegetables: 0.99, Fresh meat: 0.99, Milk: 0.99, Bacon: 0.97, and Bread: 0.95. 
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Water activity (aw) is a measurement of how much water in a substance is available for microorganisms to grow.

Higher water activity substances tend to support more microorganisms. bacteria usually require water activity values of at least 0.91 compared to fungi which need 0.6 or so.
 Every microorganism has a limit of water activity below which it will not grow.

Most bacteria thrive with a higher water activity.
Certain fruits and vegetables with higher moisture content are more susceptible to bacterial spoilage.

how do temperature, moisture and UV light affect microbial growth?

physical growth factors and microbial growth

Ultraviolet light inactivates microbes by
​ disrupting the structure of their DNA making them unable to replicate. 

moisture content

 temperature