North American Sterilization & Packaging Company
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Phone:1.973.209.4388    1.800.392.6310       Fax:1.973.209.6374        Email: info@naspco.com

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DETERMINING THE APPROPRIATE MICROBIAL CHALLENGE

A biological indicator (BI) is an "inoculated carrier contained within its primary pack providing a known resistance to the relevant process."⁴ There are many different types of BIs but the most common include:

• Commercial units supplied with the manufacturer's certification, such as bacterial spore strips individually packaged in glassine envelopes or pieces of filter paper impregnated with a certified population of a challenge organism that has a known resistance to EtO, such as the spores of Bacillus subtilis var. niger.
• A liquid bacterial spore suspension commercially sold and certified by the manufacturer. The suspension is placed on or in the product, which is then referred to as inoculated product.
• A liquid bacterial spore suspension made by a device manufacturer using a commercial, certified strain of a bacterium.
• Strains of microorganisms that have been isolated from a device manufacturing facility. These strains represent the most resistant organisms found in or on the devices and are typically used for the combined BI/bioburden method of validation.

It is crucial to ensure that the type of BI used to validate or routinely monitor a given sterilization process is the most appropriate indicator for that process. In addition to identity, quantitation, resistance, storage, general directions for use, and disposal conditions, the manufacturers of BIs are required to provide information regarding the optimal culturing conditions, such as temperature and type of growth media.

Irrespective of which BI is chosen, the methods used to recover the challenge organism must be validated. This recovery is expressed in terms of the percent recovery of the original inoculum. These recovery studies can be especially challenging when using liquid spore suspensions because of the interaction between the suspension and the material onto which it is inoculated. The material substrate can alter the resistance characteristics of the inoculum because of such anomalies as spore clumping or the physical sheltering of spores in certain sites within the product.

The goal is to kill the microbes, which means disabling their ability to reproduce even in their most favorable growth conditions as described by their manufacturer. There have been records of seemingly killed microbes that regenerated when conditions become favorable.⁵ The user must validate that the incubation time under the prescribed conditions is sufficient to recover delayed growth of the organisms after exposure to a given sterilization process. For routine processing, this time period is typically 7 days unless validated for a shorter time period in accordance with current national requirements.⁶ In such cases, periodic checks should be run to confirm that the shorter time period yields equivalent recoveries to those obtained from the longer incubation period. It is also important to ensure that the incubation time is sufficient to recover growth from injured organisms exposed to sublethal cycles. In some cases, this may mean using a 14-day incubation period. This incubation period is also required by the U.S. Pharmacopeia for product sterility testing.⁷

Just as the BI must provide a defined resistance to a specified process, it is necessary to prove that the inherent bioburden on the product does not have a greater resistance than the BI. Characterizing bioburden involves quantitation, identity, and resistance of the bioburden. Several methods can be employed to determine which BI is appropriate for a specific situation.

• Challenge organisms that make up BIs typically resist a particular sterilization process far more than do common bioburden organisms. The resistance of the bioburden cannot be adequately evaluated by quantitation only, yet it must be determined if the products to be processed might contain organisms that are more resistant than the challenge organism. After the microbial identifications and quantitation of the bioburden have been completed and analyzed, comparative resistance determinations of the most resistant bioburden component or product must be calculated. A literature review is also required.
• When microbial identifications of the bioburden are not performed, at least one sublethal cycle should be run to compare the relative inactivation rates of the bioburden with that of the challenge organism. Product sterility testing, after exposure to at least one sublethal cycle under appropriate experimental conditions, can ensure that the product's bioburden is not more resistant than the challenge organism.
• If the quantity, identity, and resistance of the product's bioburden are known, it might be possible to validate and routinely monitor the sterilization process by combining BI and bioburden methods. It must be demonstrated that the BI's degree of challenge to the sterilization process is adequate to ensure that the process will attain the desired SAL for the bioburden. Combining the BI and bioburden methods to determine the appropriateness of the BI can be time-consuming and result in additional testing costs. However, the required sterilization parameters can be more accurately determined, which can result in reduced processing time and reduced exposure to the sterilant.

BIs can be configured in many different ways depending on the cycle development method chosen.

Inoculated Product. The actual product, configured and packaged as it is intended to be sold, can be inoculated with spores of a microorganism such as Bacillus subtilis var. niger. Direct inoculation usually uses spores suspended in liquid, then placed on the product and dried. The product's surface characteristics will affect the distribution of spores and may lead to a difference in resistance behavior compared with other challenge systems.⁴ It is, therefore, important to achieve an even distribution of spores on the product's surface. Indirect inoculation involves placement of a carrier, such as filter paper that has been impregnated with spores, in the product or its package.

Inoculated Unit. A carrier, such as a filter paper strip or disk, can be inoculated with a population of a resistant organism, such as Bacillus subtilis var. niger, that has been extensively characterized and certified by the manufacturer. The resistance of this inoculated carrier must be compared to that of the inherent bioburden of the product being validated or the equivalent simulated product. An inoculated unit is usually used when there is the potential that the bioburden on the product is more resistant than the indicator organism and is required for the combined bioburden and BI cycle development method.

Inoculated Simulated Product. A simulated product that comprises the most difficult to sterilize portions of a device or that configuratively represents a device family can also be directly or indirectly inoculated. This simulated product must present the greatest challenge to the process in order to be considered an adequate microbial challenge. Each unit must contain a certified inoculum either in liquid form or on a carrier.

Natural Product. The inherent bioburden on the product can also be used as the microbial challenge during validation and for routine monitoring when the absolute bioburden method is employed for cycle development (see "EtO Cycle Development Approaches", below).

All validation methods for EtO sterilization require that the BI used for validation and to monitor routine cycles must be more resistant than the bioburden of the product and be placed in a location that is more difficult to sterilize. Comparative resistance testing is an effective means of selecting the BI and its location in the product that presents the greatest challenge to the sterilization process. Such an assessment should be made prior to validation as part of determining the appropriateness of the BI. These studies are usually carried out in small chambers that are capable of delivering rapid ramp rates, e.g., the times required to achieve specific pressure set points.

Products should be exposed to cycles in which the only variable is the gas exposure time period. The data obtained from this testing can be used to justify the choice of a specific actual or simulated product to inoculate and use for the BI. If the design of the product is such that a BI unit cannot be placed in the part of the device that is the most difficult to sterilize, the product should be inoculated with a liquid spore suspension to provide a known number of viable spores. The spore suspension, materials, and techniques used should comply with ISO 11138, parts 1 and 2.^8,9

Many device manufacturers include an additional objective in their validation plan that involves the use of external BI monitoring systems. Often referred to as process challenge devices (PCDs), they assess the lethality of the EtO process after the cycle has been designed. The PCDs are geometrically distributed around the load rather than in internal locations in the case cartons. Direct comparisons can then be made between the sterility test data obtained from these external PCDs and the BIs placed in internal locations. A PCD must be shown through comparative resistance studies to provide more of a challenge to the process when it is placed in external locations in the load than do the the BIs placed in internal locations. They usually, therefore, bear no resemblance to the product. Examples of external PCDs are spore strips double-packaged in plastic bags, in sealed plastic tubing, or in syringes. There are also commercially available PCDs that are sold as ready-to-use packaged systems. It is advisable during the validation studies to evaluate different PCD configurations during the comparative resistance studies to determine the best candidate. To monitor routine sterilization cycles, it must be shown at the time of validation that the PCDs in the external locations comply with the same requirements for resistance to sterilization.

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