The resistant bacterial cells illustrate what characteristic of life?

Adaptation

Explanation:

Adaptation refers to the process of modification of structural, physiological, behavioral, traits of organisms in order to improve their survival under the changed surroundings.  

The processes such as mutation impart genetic variations among the individuals of the population which are then acted upon by natural selection.  

Use of antibiotics by the human to control the bacterial population led to natural selection of the bacteria consisting of antibiotic resistance genes. Owing to increased survival rate of these bacteria, their frequency increased in the population over several generations.