The answer;
Through hands;
The primary mode of transmission of microorganisms to food contact surfaces is your hands.
Explanation;
Hands are contaminated through the touching of other parts of the body, using the restroom, Handling or caring for animals, coughing, sneezing, blowing nose, handling raw meat/poultry, Handling garbage/dirty dishes, Handling money, among other ways.
Contaminated hands in turn contaminate food and food contact surfaces.