Sterile, Sterilization and Sterility – the Language of Keeping Things Contamination-Free

John Batts – Senior Technical Trainer, Masterflex Bioprocessing

When it comes to biopharmaceutical manufacturing, one thing that is of prime importance is maintaining a fluid pathway free of microorganism contamination. It’s common to hear terminology like sterile, sterilization and sterility. But what do these terms mean – and how can they help us better understand how to achieve the goal of removing the risk of contamination with the products being produced?

Let’s look at the official definitions for these terms, and then delve further into their meanings to help differentiate them from one another.

  • The first term is sterile. Something is considered sterile when it is “free from viable (live) microorganisms”.
  • The second term is sterilization. Sterilization is “a validated process by which an article, surface, or medium is freed of all living microorganisms, either in the vegetative or spore state”.
  • The third term is sterility. Sterility is “the state of being free from viable microorganisms”.

Obviously, these three terms are inter-related but let’s look at each a little more closely.

Sterile

Being sterile is an absolute term, an ideal that is virtually impossible to achieve. Microorganisms – things like viruses, fungi and bacteria – abound in our world, typically living undetected on most surfaces. They can be found on eyelashes, skin cells, dust particles – not to mention on common surfaces, such as clothing and cardboard and plastic packaging. Most of the time, the presence of these microorganisms has no noticeable impact on our everyday lives – in fact, in many cases, it is beneficial that these microorganisms are present.

There are times, however, when the presence of microorganisms can be detrimental. This is especially true in biopharmaceutical research and production, where scientists are often working with specific strains of biological material that have been designed to produce some biotherapeutic product. In those instances, the presence of foreign biological material stemming from microorganisms could compromise the efficacy and safety of the final product.

Sterilization

To remove those microorganisms, sterilization techniques are often employed. Many people gravitate toward autoclaving as a preferred method. However, while being a tried-and-true technique, there are numerous limitations to autoclaving that make it cumbersome to employ in large-scale production environments. It is commonplace for most biopharmaceutical companies to employ one of several other sterilization techniques – each with its own advantages and limitations.

One common sterilization technique is the use of ethylene oxide (EOor EtO). Ethylene oxide is a penetrating gas that can migrate well onto product surfaces, even when large batches are present. Additionally, EO is typically chemically compatible with most surfaces, but EO can leave residuals behind on the surface of the products going through sterilization. EO is dangerous to work with;, being both flammable and explosive. Because of its sensitivity to temperature and relative humidity, it is an overall complex process that can take days to achieve maximum efficacy.

With the challenges EO brings, more attention has been placed on radiation-based methods for sterilization. These alternative methodologies offer several advantages, including fewer complex setups, shorter run times, and the ability to work with nonpermeable packaging (which allows finished products to be sterilized in their final packaging more effectively). One of these methods is known as Electron-Beam (or E-Beam). With E-Beam technology, the finished product is exposed to a concentrated, highly charged stream of electrons generated by an accelerator. This stream of electrons alters chemical bonds, which damages the DNA of any microorganisms that might be present, destroying their ability to reproduce. While E-Beam technology is very effective, it does have limitations – including the size of the batches that can be processed as well as not being suitable for products that have complex geometries or are composed of high-density materials.

To overcome the limitations with E-Beam technology, one of the most widely accepted and used sterilization techniques is known as Gamma Irradiation. Gamma Irradiation is a penetrating form of electromagnetic radiation. Like E-Beam, with Gamma Irradiation, the result is damage to the contaminants’ DNA and cellular structures. Depending upon the level of radiation exposure, death of the contaminating microorganisms can also occur. Because of the kind of energy produced, Gamma Irradiation can be used to fully penetrate even highly dense products, making it effective for use with large product batch sizes. Further advantages include not being dependent upon either specific chemicals or heat, nor being hindered by product geometries. There are limitations as well; the primary ones being that the radiation source must be replenished and revalidated from time to time and the fact that the radiation produces undesirable changes in many medicinal products. Because of this, Gamma Irradiation is more typically used with physical products, like tubing assemblies, fittings, etc.

As discussed, the goal of these sterilization techniques is to help a finished product to achieve full sterility and be labeled as sterile. However, it is functionally impossible to guarantee 100% of all microorganism contaminants are fully eliminated through the sterilization process. As such, rather than referring to something achieving full sterility, the degree of sterility is expressed in terms of its Sterility Assurance Level.

Sterility Assurance Level (SAL)

The Sterility Assurance Level – or SAL – is the probability of a single viable microorganism occurring on an item after sterilization. In the medical device marketplace, the target SAL is 10-6, which indicates a 1 in 1,000,000 likelihood of an organism surviving at the end of the sterilization process. While not fully sterile, with a SAL of 10-6, there is virtually no risk of contamination remaining on the finished product, and thus there is virtually no risk of harm to any humans who might encounter that finished product either.

Ensuring products are contamination-free can be a complex process. While there is no “one size fits all” solution, there are numerous and effective sterilization techniques that can be employed to finished products to ensure the target level of sterility is achieved, thus ensuring the safety of those using and benefitting from the finished product.

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