Pink slime is a bacterium, not a mold

Search for the pink film in a humidifier tank and almost every result calls it pink mold. It is not mold. It is Serratia marcescens, a bacterium, and the distinction is not pedantry: it explains why the film reappears within days of cleaning, why it also turns up in shower corners and toilet bowls, and why the standard mold advice under-treats it.

What Serratia marcescens is

It is a Gram-negative bacterium in the Enterobacteriaceae family, and it produces a red-pink pigment called prodigiosin, which is what you are seeing. It is one of the more historically documented bacteria in the literature precisely because that pigment makes it so visible - it has been mistaken for blood on food for centuries.

It is widespread in the environment and it does well in exactly the conditions a humidifier provides: standing water at room temperature, with a supply of the phosphorus and fatty compounds it metabolizes. That is why it shows up in humidifier tanks, shower corners, tile grout, toilet water lines and pet bowls, and why it is not a sign that your house is unusually dirty.

Why the mold framing leads people wrong

Three practical consequences follow from the correction.

1. It returns faster than mold. A bacterium doubling in a warm tank re-establishes a visible film within days, not weeks. If you clean thoroughly and the pink is back on Thursday, that is not a failed clean, that is bacterial growth rate.
2. Vinegar is the wrong tool. Vinegar is a descaler. It removes the mineral deposits the film clings to, which helps, but it is not what dislodges a biofilm. The EPA's recommendation for water-contact surfaces is a 3% hydrogen peroxide solution followed by thorough rinsing, and mechanical scrubbing with a brush.
3. Scale makes it worse. The EPA notes that scale acts as a breeding ground for microorganisms. Rough mineral deposits give a biofilm somewhere to anchor that smooth plastic does not, which is why hard-water households see the pink film more often.

Is it dangerous?

Here is where we stay carefully inside what we can support. Serratia marcescens is classified as an opportunistic pathogen: it is chiefly a concern for people who are immunocompromised or in hospital settings, and it is not generally regarded as a threat to healthy adults through household contact.

What we can say with a citation, and what actually matters here, is the EPA's own position on humidifiers generally: microorganisms often grow in tanks containing standing water, and breathing mist containing these pollutants has been implicated as causing a certain type of inflammation of the lungs. A humidifier is a device for aerosolizing the contents of its tank into the air of the room you sleep in. That is reason enough to take the film seriously without anybody needing to overstate the organism.

Removing it

Stopping it coming back

• Empty and dry the tank daily. The EPA's instruction, and the single most effective preventive measure. Standing water is the condition; remove the condition.
• Clean every third day, not weekly.
• Do not top up. Adding fresh water to old water gives you old water. Empty, rinse, refill.
• Use low-mineral water. Less scale means less anchor. See which water to use.
• Dry it between uses if the machine can. Levoit's Superior 6000S has a Dry Mode that dries the wicks; Canopy runs a fan whenever it detects moisture inside. Both attack the standing-water condition rather than the organism.
• Buy a machine you can reach inside. A wide opening and dishwasher-safe parts are worth more than any antimicrobial claim. See easiest to clean.

Actual mold, when you do have it

Black, green or gray and fuzzy rather than pink and slimy. It grows more slowly, and in a humidifier it usually appears on a wick or filter rather than on smooth tank plastic. A wick with visible mold is not cleaned, it is replaced - see filter care. If the smell persists after a full clean and a new wick, the growth is somewhere in the water path you cannot reach, and the machine has reached the end of its useful life. See mold symptoms.