Methods of microbial control

Setting the dial to 121 °C proves only that the chamber reached the set point somewhere; it does not prove that the steam penetrated the load, that the air was expelled, or that the organisms inside are dead. Validation therefore runs on three levels. Physical monitoring uses thermocouples threaded into the most difficult part of the pack, a pressure gauge and a chart or data logger that records the whole cycle, because the operator needs the temperature inside the load rather than in the jacket. Chemical indicators such as autoclave tape and internal strips change colour once the required temperature has been held, which confirms that the pack itself was exposed but says nothing about kill. Biological monitoring is the criterion: vials or strips carrying a defined population of Geobacillus stearothermophilus spores, an organism that is harmless to people and far more heat-resistant than Mycobacterium tuberculosis, are placed in the coldest, densest corner of the load and incubated afterwards. Failure of those spores to grow is the evidence of sterility; growth means the cycle failed even if every gauge read correctly. The reason a well-stocked laboratory does not rely on the gauge is the biphasic kill curve described by Gillespie and Ainsworth: a single exponential rate does not describe spore death, so the survival of a small resistant fraction cannot be excluded by temperature alone. Load configuration is the commonest cause of a false sense of security. A full chamber, an over-tight pack, a closed container, a load that is still wet at the end of the cycle or a residual air pocket all create a protected site where saturated steam never arrives and the exposure is shorter than intended. For that reason the routine checks are simple but not optional: the indicators are read, the printout is filed, the packs are stored dry and the shelf life is counted, and an unopened package may be judged sterile only while its wrapper is intact and dry. In hospital practice the nurse is the last quality-assurance step. Before an instrument is used on a patient, the pack should be inspected for a torn or damp wrapper, the chemical indicator should have changed as specified, and the batch record should be available; a load that failed biological monitoring is recalled and must never be released.

Pasteurisation is a public-health instrument aimed at named pathogens rather than at microbial life in general, and the temperatures were chosen to hit the most heat-resistant of those pathogens with a margin. The target that set the original holder method was Coxiella burnetii, the cause of Q fever, and the standard it produced is 63 °C for 30 minutes; the high-temperature short-time process that dairies actually use is 72 °C for 15 seconds, and ultra-high-temperature treatment, 135–150 °C for a few seconds, pushes the product into a different category altogether because it kills spores and allows months of unrefrigerated storage. The history matters because it explains the odd temperatures: Louis Pasteur and Claude Bernard showed in the 1860s that heating wine and beer to about 60 °C prevented souring without ruining the taste, and the process was named for him; the dairy industry later raised the temperature until the pathogen target was covered. What survives is what makes the milk non-sterile. Thermoduric bacteria such as enterococci, micrococci and some Bacillus species survive, and while they are not usually dangerous they multiply at refrigeration temperature and coagulate or sour the milk. Alkaline phosphatase, the enzyme assayed to check that pasteurisation was actually performed, is destroyed by the process and its absence is the test of adequacy, whereas heat-stable lipases and proteases from psychrotrophic bacteria are not destroyed and go on degrading the fat and the protein during cold storage, so a milk that was pasteurised correctly can still turn bitter. That is why the shelf life of pasteurised milk is measured in days under continuous refrigeration at about 4 °C and collapses rapidly above 8 °C, why a warm delivery van or a fridge door left ajar shortens it, and why the product must be dated and rotated. It is also why pasteurised milk is emphatically not a sterile product: it cannot be used for neonatal parenteral nutrition, it is not interchangeable with UHT milk, and it cannot be left at room temperature as though it were canned.

The gap between 121 °C and 170 °C is not an accident of equipment; it follows from how heat kills and from what each form of heat does to the cell. Moist heat kills chiefly by coagulating proteins, and water is an excellent conductor, so saturated steam condenses on the cooler load, releases latent heat and drives the surface to the chamber temperature almost immediately. Dry heat acts much more slowly: it kills partly by oxidising cell constituents rather than by coagulation, and air conducts poorly, so the load must be held at a higher temperature for a long time to accumulate the same damage. This is why the standard moist cycle is 121 °C for 15 minutes at about 15 psi, or 134 °C for 3 minutes at about 30 psi, while the standard dry-heat reference is 160 °C for 2 hours or 170 °C for 1 hour. The supplied notes give dry heat as 170 °C for 120 minutes on lines 1736–1737; the correct standard pair is 160 °C for two hours or 170 °C for one hour, and the notes’ combination overstates the time needed at 170 °C. Moist heat also penetrates more predictably, which is why it is preferred for fabrics, liquids, media and instruments, while dry heat is reserved for glassware, metal, oils, powders and sharp items that steam cannot reach or that corrosion or moisture would damage. Behind both methods sits one quantitative rule: microbial killing by heat is approximately exponential, so the population falls by a constant proportion per unit time rather than by a constant number. The D-value, the decimal reduction time, is the time at a stated temperature needed to reduce the population by 90 %, or one log; a sterilisation cycle is then designed to deliver enough log reductions to bring an initial bioburden down to a defined probability of a surviving organism, conventionally one in a million for a sterilised item. Two consequences follow. First, the cycle time depends on the bioburden and on how resistant the contaminating spores are, so heavily soiled instruments require longer, and organic matter such as blood or pus must be removed before sterilisation because it shields organisms and can even protect them chemically. Second, temperature and time are interchangeable to a degree: a higher temperature with a shorter holding time can achieve the same log reduction, which is the whole logic of flash cycles. For the nurse this arithmetic explains two habits that otherwise look like fussiness: never assume that a short cycle at the right temperature is automatically enough, and never place a cold or wet instrument into the chamber, because the moisture left on the surface will delay the rise of the whole load to the required temperature.