Does antimicrobial use cause antibiotic resistance?
No. The use of antimicrobial agents simply provides the selective pressure that allows small numbers of previously resistant bacteria to become dominant. There are two subtly different ways that this can occur, and I’ll try and explain this to you. Suppose you have a mixture of organisms in a tube. You have 99 cells of E. coli (an organism that is naturally sensitive to most antibiotics) and 1 cell of Pseudomonas aeruginosa (an organism that is naturally resistant to many antibiotics). If you now add some antibiotics to the tube and allow the organism to grow, the E. coli are inhibited and cannot grow, but the Pseudomonas is resistant and grows happily to become the dominant organism in the mixture, when it was initially outnumbered 99 to 1. This type of selection of resistant types of bacteria from a mixture of bacterial types happens frequently in biological systems. Alternatively, suppose you have a tube of 100 cells of E. coli. As the cells grow and divide to become 200 cells, one
