Why does microwaving a kitchen sponge often fail to effectively sanitize it and potentially worsen the situation?

It can select for heat-resistant bacterial strains that subsequently multiply and dominate the sponge.

Microwaving is a common but flawed method for sterilization because it does not uniformly eliminate all microbial life. While the heat generated can kill many bacteria, it often fails to reach the temperatures required to eradicate the most resilient or heat-tolerant strains. Furthermore, by killing off the more vulnerable, non-resistant bacteria, the process effectively removes competition for resources within the sponge. This allows the surviving, heat-resistant bacteria to multiply rapidly, filling the newly available space and potentially leaving the sponge in a more dangerous condition than before the cleaning attempt.