When a drug is used on a species of bacteria, those that cannot resist die and do not produce offspring, while those that survive potentially pass on the resistance gene to the next generation (vertical gene transmission).
Cando se utiliza un fármaco nunha especie de bacteria, as que non poden resistilo morren e non producen descendencia, mentres que as que sobreviven poden pasar o xene que as fai resistentes á seguinte xeración (transmisión xenética vertical).