Diagnostics & problems
Biofilm is a colony of bacteria inside a slime shield they build themselves. That shield blocks sanitiser - which is why a sprayed, wiped surface can still be harbouring bacteria.
The short answer
Biofilm is a layer of bacteria bonded to a surface inside a slimy shield they build themselves. That shield blocks sanitiser, so a spray-and-wipe kills only the exposed cells and leaves the colony protected underneath - and it regrows within hours. Biofilm only comes off with mechanical cleaning that physically breaks the shield.
What biofilm is
When bacteria settle on a damp, food-soiled surface, they do not stay as loose single cells. They anchor down and secrete a matrix around themselves - the extracellular polymeric substance, a slime of sugars, proteins and DNA that glues the community to the surface and to each other. That matrix is the whole problem. It is a physical and chemical barrier that stops sanitiser reaching the cells inside, so the bacteria survive doses that would kill them out in the open.
This is why a surface can be sprayed, wiped and still be harbouring bacteria. The sanitiser knocks out the exposed film and looks like it worked, but the protected colony underneath is untouched - and because a fragment of biofilm regrows the whole structure, it can be back within hours. Biofilm can shelter organisms you very much do not want in a kitchen, including Listeria, Salmonella and E. coli.
Where it lives and how it goes
Biofilm forms wherever it is moist and there is organic matter to feed on: drains and floor gullies, grease traps, the seals and undersides of equipment, ice machines, cloths and the corners a routine wipe skips. Because the matrix shields the bacteria from chemistry alone, you cannot sanitise your way through it. It has to be physically broken and lifted - mechanical agitation from a brush or scrub, usually with an enzymatic or alkaline detergent and proper dwell time to loosen the matrix, and only then a sanitiser on the genuinely clean surface underneath.
Biofilm is the mechanism sitting under several familiar complaints: the drain that smells sour again days after cleaning, the drain flies that keep hatching, the sanitiser that "did not work". They are all the same story - a matrix protecting a colony that surface cleaning never reached.
The limit
Sanitising is necessary but it is not sufficient. The EPS matrix shields the bacteria, so chemistry on its own leaves the colony intact and biofilm regrows from what is left. Only mechanical deep cleaning - breaking and lifting the matrix, then sanitising the clean surface beneath - actually removes it, and partial removal simply resets the clock. When a smell, a fly problem or a failed swab keeps coming back after cleaning, biofilm is usually why, and it is why the fix has to reach deeper than a spray and a cloth.
Questions
Biofilm is a community of bacteria that has bonded to a surface and wrapped itself in a self-made slime matrix. It forms in moist, food-soiled spots - drains, gullies, grease traps, equipment seals, ice machines and cloths. The matrix glues the bacteria in place and shields them, which is what makes biofilm far harder to remove than loose surface bacteria and why it keeps causing problems that ordinary cleaning does not fix.
Because the bacteria are protected by their matrix - the extracellular polymeric substance - which acts as a physical and chemical barrier that sanitiser struggles to penetrate. A spray-and-wipe kills the exposed cells and looks effective, but the colony sheltered inside the matrix survives. To actually remove biofilm you have to break that shield mechanically first, then sanitise the clean surface underneath.
Fast - it can regrow within hours if any of it is left behind, because a surviving fragment reseeds the whole structure. That is why partial cleaning does not hold and why a sour drain or a swab failure returns days after you thought it was dealt with. Full mechanical removal is what breaks the cycle; anything less just resets the clock.
Anywhere it is damp and there is organic matter to feed on: inside drains and floor gullies, in grease traps, under and around equipment seals, in ice machines, and in cloths and awkward corners a routine wipe skips. These are also the spots behind recurring smells, drain flies and failed hygiene swabs - which is often the first sign biofilm is present.
By breaking the matrix physically and lifting it - mechanical agitation from a brush or scrub, usually with an enzymatic or alkaline detergent given proper dwell time to loosen the slime, and then a sanitiser on the genuinely clean surface beneath. Chemistry helps, but agitation is the part that does the work. A deep clean is built around exactly that sequence, which is why it clears biofilm that daily cleaning cannot.
Phoenix Duct Clean · by the numbers
A deep clean breaks and lifts biofilm from drains, gullies, seals and equipment with agitation and the right detergents, then sanitises the clean surface beneath - so it does not grow straight back.