* Mutation: Random changes in the DNA sequence.

* Horizontal gene transfer: The transfer of genetic material between bacteria, which can occur through conjugation, transformation, or transduction.

* Recombination: The exchange of genetic material between homologous chromosomes.

The only option that doesn't contribute to genetic variation in bacteria is vertical gene transfer: The transfer of genetic material from parent to offspring.

Vertical gene transfer is a fundamental part of reproduction in all organisms, including bacteria. However, it doesn't introduce new genetic variation into a population. Instead, it simply passes existing genetic information down to the next generation.

Therefore, the answer is vertical gene transfer.