Pathogenic bacteria gain resistance to antibiotics primarily due to changes in their plasmids. Plasmids are small, circular, double-stranded DNA molecules that exist independently of the bacterial chromosome. They often carry specific "R-factors" (resistance genes) that allow bacteria to survive in the presence of antibiotics by producing enzymes that inactivate the drugs or by modifying cellular targets. These plasmids can be easily transferred between different bacteria through horizontal gene transfer (conjugation), leading to the rapid spread of antibiotic

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