Final answer:
Horizontal gene transfer through conjugation is a primary method by which bacteria spread antibiotic resistance. Other methods include transformation and transduction. Transposons also aid in horizontal gene transfer across species, contributing to genetic diversity.
Step-by-step explanation:
The mechanism of horizontal gene transfer naturally responsible for the spread of antibiotic resistance genes within a bacterial population is conjugation. Other methods include transformation, where bacteria take up DNA from the environment, and transduction, where a virus transfers genes between bacteria. In conjugation, a pilus or mating bridge allows the direct transfer of DNA between two bacterial cells. This could lead to the acquisition of antibiotic resistance genes from a nonpathogenic to a pathogenic bacterium. Additionally, horizontal gene transfer can occur through the movement of transposons, which are DNA sequences that can change their position within a genome. This process can be accelerated by retroviruses and leads to increased genetic diversity.