It’s sooner than you think.

If you’re someone who repeatedly uses the same sponge to wipe your kitchen counters, table and dishes, replacing it only every month or so, a new study published in Scientific Reports may make you want to change your cleaning habits ASAP.

A team of German researchers analyzed 14 used kitchen sponges, collected from households in southwestern Germany, and discovered that they harbored a surprisingly high amount of bacteria. You may be wasting your time by even trying to clean your sponge; the study found that the best thing you can do is replace your sponge once a week.

Previous research has shown that kitchen sponges contained the second highest amount of coliform bacteria—a broad class of bacteria that’s found in human and animal feces—out of the whole house. (The number-one spot: drain traps.) It has also shown that sponges contain pathogenic bacteria—the type that can lead to disease—like E. coli and salmonella.

In fact, kitchens have more microbes than toilets—mainly because that’s where the sponges are. Sponges are “the biggest reservoirs of active bacteria in the whole house,” the authors write.

Unfortunately, the new study showed that regularly sanitized kitchen sponges did not contain significantly less bacteria than unclean ones, so it’s probably best to invest in a weekly sponge if you want to have a cleaner kitchen.

Written by Kate Samuelson, first published on health.com