Microbial growth in a water polisher

A resistivity display can stay flattering while a water polisher grows a film you cannot see in the number. The words for that film are easy to swap, and the swaps hide different failures. This glossary is for reading a water-system report, a sanitisation instruction, and a certificate without treating colony counts, endotoxin units, and megohm readings as one fact.

Ionic purity, organic carbon, and endotoxin answer different questions. That split is the point of laboratory water types and where they fail. Water that then becomes a buffer still needs a pH record, which is preparing a buffer and checking pH. Salts you dissolve into that water are in the reagents and chemicals catalogue. Flasks that should not arrive pre-contaminated are in beakers and flasks. A system or a consumable is specified on a quotation request, not by the word pure.

Biofilm

A biofilm is microorganisms attached to a surface and held in a matrix of polymeric material they produce. In a polisher the surfaces are resin beds, storage tanks, the inside of tubing, the threads of a dispenser, and the bowl of a final filter. The bulk water can look clear because most of the community is on the wall, not in the glass you just filled.

The confusion is to equate "no floaters" with "no film". A tank that is filled on Monday and left until Thursday is a stagnant volume. Stagnant water and a dead leg are the usual design clues that a film will have somewhere to sit. Organisms in the film are also harder to kill than the same organisms suspended in a rinse, which is why a quick flush is not a sanitisation. Sanitisation method and frequency come from the manufacturer of the modules you own. A chemical that is kind to one resin can damage another. Do not borrow a dose from a different manual.

Warm rooms help growth. So does a recirculation pump that stopped when the power failed and was not restarted until the next shift. Those are reasons to inspect and to follow the restart instruction, not a growth-rate you can look up for your particular tank.

Colony-forming unit

A colony-forming unit, written CFU, is a count of colonies on a chosen medium after a chosen incubation. One colony may have come from one cell or from a clump. Cells that are viable but will not grow on that agar never appear. The number is meaningful inside the method: sample volume, medium, temperature, and time all belong in the sentence.

The confusion is to treat CFU per millilitre of dispensed water as the population of the biofilm. A first draw from a quiet dispenser can be richer than a sample taken after a flush, or poorer, depending on whether the film is shedding. A plate that shows nothing does not prove a sterile loop. It proves that this plating found nothing. Alert and action numbers, if your laboratory uses them, should be copied from the system manual and from your own quality procedure. This page does not assign a universal limit.

When a count rises, the next check is where the sample was taken, whether the point-of-use device is overdue, and whether a dead leg was included. Repeat the same sampling method. A new method and a new limit on the same day does not tell you whether the system changed.

Endotoxin

Endotoxin, in this context, is lipopolysaccharide from the outer membrane of gram-negative bacteria. It is not a living cell, and it is not removed by killing the cell. Cells that die in a tank can leave endotoxin behind. It is heat-stable under ordinary autoclave conditions, which is why a later sterilisation of a buffer does not unwind contamination that the water already contributed. The water pillar makes the same point for the meter: resistivity does not see it.

The confusion is to hear "we sanitised" and infer "endotoxin is gone". Sanitisation can drop the viable count and still leave a residue, or it can disturb a film and shed a pulse of material into the next litres you dispense. Whether your assays care is a separate question. A plasmid prep may be indifferent. A cell-culture reagent or a lysate assay may not. Name the assay before you name the water.

Endotoxin unit

An endotoxin unit, written EU, is an activity unit in a lysate-based endotoxin assay, scaled to a reference standard endotoxin. It is not a gram of "endotoxin" in general. Different lipopolysaccharides produce different activity per unit mass, so a casual conversion from EU to nanograms does not travel from one sample to another. Report the number with the method, the standard, and the sensitivity of the assay you actually ran.

The confusion is to treat EU as a contaminant you can see, smell, or infer from resistivity. You cannot. A certificate that states an endotoxin claim is evidence about the product under the conditions of that claim. Water taken from a colonised dispenser the following week is a different material. If you need a number for a reagent you prepared, you measure it, or you accept that you do not have one. Do not copy the feed-water certificate onto the buffer label.

Point-of-use filter

A point-of-use filter is the device at the dispenser, often a membrane rated to retain bacteria, sometimes with an additional claim. Used within its interval and changed as the maker says, it can reduce the cells that reach your flask. It does not repair a tank upstream. It can itself become a colonised surface when it is left in place, especially if it stays wet and warm.

The confusion is to point at the filter, read the resistivity, and call the water sterile. Resistivity is an ionic measurement. Sterility is a process and, when claimed, a test. A final filter also does not confer an endotoxin specification unless that device was built and documented for endotoxin reduction. When a culture fails and the ionic number still looks typical, inspect the filter's install date and the sanitisation record before you blame the medium recipe.

Dead leg

A dead leg is a branch of pipe that does not see routine flow, so water sits in it. A tee leading to a rarely opened valve, a long stub to a gauge, or a spare outlet capped "for later" are the usual shapes. Hygienic-design practice judges the stub by how long it is relative to its diameter and by whether flow actually scours it. The drawing of your system is the place to measure that length. A number copied from a different code, without the drawing, is not an inspection.

The confusion is to sanitise the main loop and assume the stub followed. Sanitant that never enters the leg does not touch the film in the leg. The next time someone opens that valve, the leg joins the water you collect. A stagnant tank is the large version of the same idea: a volume that is not turned over. Design clues, not a resistivity trend, are how you find them.

What to do when the number and the assay disagree

If resistivity is still in the range the manual calls acceptable and a culture or a lysate assay has become noisy, do not spend the week recalibrating the conductivity cell first. Ask where biofilm could sit: tank turnover, a dead leg, an overdue point-of-use filter, a dispenser that drips and stays wet. Then look at the sanitisation record and do what that system's manufacturer specifies. A borrowed bleach ritual can swell a resin and give you a new ionic problem on top of the biological one.

If the plate count rose and the endotoxin assay did not, you have two results, not a contradiction to average. Cells and free lipopolysaccharide do not have to move together. If both rose after a sanitisation, consider a shed from a disturbed film and follow the manual's rinse-out before you prepare a sensitive batch. If the count is high only in a first draw, the dispenser path is the branch, not the entire plant.

Sampling belongs in the note: time of day, first draw or flushed, who changed the filter. A result without that sentence cannot be compared with next month. BMBL and the WHO laboratory biosafety manual are about handling organisms safely, not about setting your polisher's alert limit. They do remind you that a water system which has grown a film is a microbiological question, and the waste from a sanitisation is a chemical question. Follow the laboratory's waste rules for the sanitant you were actually told to use.

A warm room and a stopped pump

Heat shortens the time a stagnant tank can sit before growth is the default expectation. Humidity does not sterilise a wet loop. After a power cut, assume recirculation and ultraviolet lamps, if you have them, need a deliberate restart, and follow the manual's idle-restart steps before you fill a carboy for the week's buffers. Write the idle time in the log. The buffer you prepare from the first litres after a restart should not be the buffer you give to the most sensitive assay until the checks the manual names have been recorded.

What an enquiry should ask

Name the feed water, the daily volume, and whether the assays care about ions, organic carbon, bacteria, particles, or endotoxin. Ask for the sanitisation method the manufacturer requires, the interval they state, and which modules a dead-leg review is supposed to include. Ask which document shows the resistivity cell's position relative to the tank and the final filter. Ask for the point-of-use device's claimed retention and its change criterion. You are not asking a seller to promise a microbial count for your future tank. You are asking for the design and the maintenance sheet your laboratory will be able to follow.