Is It Mold or Is It Bacteria? How to Tell What’s Growing in Your Home (Alternate Perspective)

You find a strange spot under the sink at 9 p.m.
You turn your head like a confused golden retriever.
You think, “Well… that wasn’t there yesterday.”

So what is it? Mold? Bacteria? Something in between?

Mold behaves like a long-term tenant. It likes to build colonies, spread outward, and establish itself on materials with texture—drywall, wood, grout, cardboard. It often looks fuzzy, powdery, or dusty as it matures. It grows slowly at first and then suddenly seems to appear everywhere at once.

Bacteria, on the other hand, behaves like a transient visitor. It appears quickly, looks wet or shiny, and often forms irregular, slimy patches. It thrives in areas where water constantly cycles through—dishwashers, humidifiers, refrigerator drip trays, or sink overflows. Instead of growing outward in a fluffy pattern, bacteria tends to look like a spill that won’t dry.

The environment offers clues too. Mold grows where moisture persists. Bacteria grows where moisture moves. Mold develops on structure. Bacteria clings to surfaces in contact with water.

If the mystery substance is dry, textured, or expanding: likely mold.
If it's slick, glossy, or smelly: likely bacteria.

Still unsure? Many homeowners are. A surface sample taken with the Detekt Home Mold & Bacteria Test Kit can help confirm whether mold is present so you can take the right next steps with confidence.

Sometimes your eyes can only get you so far. A little science goes a long way.

After you test, check out Detekt's extensive visual identification guide.

By A. Anagnos, Biomedical Engineering Specialist