Horizontal gene transfer is common among bacteria, even among very distantly related ones. This process is thought to be a significant cause of increased drug resistance when one bacterial cell acquires resistance, and the resistance genes are transferred to other species. Transposition and horizontal gene transfer, along with strong natural selective forces have led to multi-drug resistant strains of "S. aureus" and many other pathogenic bacteria. Horizontal gene transfer also plays a role in the spread of virulence factors, such as exotoxins and exoenzymes, amongst bacteria. A prime example concerning the spread of exotoxins is the adaptive evolution of Shiga toxins in "E. coli" through horizontal gene transfer via transduction with "Shigella" species of bacteria. Strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed. and integrons. Still, they are more frequently a chromosomal-encoded barrier to MGE than an MGE-encoded tool for cell infection.