What is known from such studies is that bacteria subjected to heat are killed at a rate that is proportional to the number of organisms present. The process is dependent both on the temperature of exposure and the time required (at this temperature) to accomplish the desired rate of destruction. Thermal calculations thus involve the need for knowledge of the concentration of microorganisms to be destroyed.
D-values differ amongst various types of microorganisms. In Figure 1, there are three survivor curves drawn. Two have the same slope but start from different initial populations – this represents how, for one particular pure culture of a microorganism treated under particular specified conditions, the D-value will always (within limits) be the same. The other, much steeper, survivor curve represents a different microorganism, more sensitive to the sterilisation treatment and hence with a smaller D-value.
Irrespective of the method of sterilisation, bacterial endospores are more resistant to inactivation than Gram-positive bacteria and Gram-positive bacteria are more resistant than Gram-negative bacteria. Moreover, resistance is to a lesser extent a function of the environment in which the microorganism is present during the inactivation process, e.g., suspended in water, dried on a filter medium, etc. Environmental effects are not predictable - they must be determined empirically. For instance, there is data (often difficult and confusing to interpret due to heat transfer factors playing a part) to suggest that thermal D-values for bacterial endospores may be up to two times greater when dried on rubber than when suspended in water.
Predicting Sterility Assurance Levels (SALs) for items with different types of contaminants, and even with several types of contaminants at the same time, is not simple. The SAL is a term used to describe the probability of a single unit being non-sterile after it has been subjected to the sterilisation process.
For instance, a population of items each initially contaminated by 103 endospores would require a 9 log inactivation to provide a 10-6 SAL. If the spores had D121-values of 0.5 minutes, this would require a sterilisation treatment of 4.5 (i.e., 9 x 0.5) minutes at 121°C. However, were the items to be contaminated with 102 microorganisms of a Gram-negative species with a D121-value of 0.05 minutes, the same 4.5-minute treatment at 121°C would provide a SAL of 10-88 ([4.5/0.05] -2).
Steam sterilisation (moist heat)
The most widely used sterilisation method in the pharmaceutical industry is steam sterilisation in autoclaves (moist heat in the form of saturated steam under pressure). This is primarily applicable to the terminal sterilisation of products, stainless steel items and equipment not intended for single use. Sterilisation occurs as the latent heat of condensation is transferred to the load causing it to heat rapidly. Steam sterilisation is nontoxic, inexpensive, rapidly microbicidal, sporicidal and is efficient at heating and penetrating fabrics. The penetrating ability of the heat is the reason why steam sterilisation is so widely used as a terminal process for drug products in glass ampoules, vials, syringes and plastic containers.
It is also used for sterilising closures, filters, manufacturing equipment, cleaning equipment, and product holding vessels. Whilst most medical and surgical devices used in healthcare facilities are made of materials that are heat stable and can therefore undergo heat processing, there has been a move towards low-temperature sterilisation methods such as ethylene oxide and radiation as the biopharmaceutical industry embraces single-use disposable items. However, for sterile medicinal products, steam sterilisation remains the most widely used method for products that can be terminally sterilised.