Surrounding animals including shellfish, wild fish and crustaceans absorb these residues at levels greater than what is acceptable for human consumption. Furthermore the potential for resistant bacteria from animals to transfer genetic elements which confer resistance to bacteria pathogenic to humans exacerbates the problem. Bacteria may become resistant to a given antibiotic by either mutation or by acquiring a gene from other bacteria that are already resistant. Mutation under normal conditions does not happen very frequently. However, under the selective pressure of antibiotics killing non-resistant bacteria, mutation is more common.
Resistant bacteria become dominant through competitive selection.
Bacteria may also become resistant through gene transfer by routes known as conjugation , transformation or transduction where resistant bacteria transfer the genetic material necessary to become resistant to another type of bacteria. Eating meat contaminated with resistant bacteria Eating eggs or milk that have not been properly pasteurised or cooked, contaminated by resistant bacteria Eating contaminated fruits or vegetables which have been grown in manure contaminated with resistant bacteria Eating food preservatives contaminated with resistant bacteria Resistant bacteria may directly infect humans, making them contract an illness not treatable by one or multiple antibiotics. Alternatively, resistant bacteria can contaminate humans by transferring the genetic information for resistance to bacteria naturally occurring in the human gut. This is to say that when bacteria become resistant to an antibiotic, they have the ability to become resistant to many other antibiotics similar in structure to or in the same family as this antibiotic. The rising spread and emergence of resistant bacteria is progressively rendering present antibiotics less and less useful.