1. Herbicide: as described in the article, “while a bacteria alone might have been killed by an antibiotic, when exposed to an herbicide, a resistance gene is turned on, in effect “‘immunizing’ the bacteria to the antibiotic.””
2. Dust: “antibiotic-resistant bacteria are being spread from cattle yards through dust laden with excrement. Given the winds and droughts in some areas, dust bowls could spread the resistant bugs for hundreds of miles.”
3. Heavy metals: “heavy metals, added to feed as growth promoters, can also select for antibiotic resistance. Pollution with heavy metals—even at very low levels— can promote bacteria with multiresistance plasmids (small bits of extrachromosomal DNA)”.
Additionally, some emerging markets are the home of companies without environmental consideration. It is known that some India-based drug companies are dumping waste in water. Without proper sanitation, it is a huge health concern for the population.
This Forbes article is informative and insightful about what might cause antibiotic resistance and how we could modify our behaviors in order to control this issue.