Biological indicators are recognized in the art as providing an accurate and precise means for testing the effectiveness of a sterilization procedure. Conventional biological indicators gauge the effectiveness of a sterilization cycle by monitoring the survival of a test microorganism that is many times more resistant to the sterilization process than most organisms that would be present by natural contamination. The biological indicator is exposed to a sterilization cycle and then incubated under conditions that will promote the growth of any surviving test microorganisms. If the sterilization cycle fails, the biological indicator generates a detectable signal that the biological specimen survived. The detectable signal may, for example, be luminescent, fluorescent, color or radiation.
One well-known type of biological indicator uses spores from bacteria or fungi to test the effectiveness of a sterilization procedure. Biological indicators of this type commonly employ a nesting tube arrangement, in which a smaller inner tube that contains spore growth media nests within a larger tube having a gas-permeable cap. The inner tube is made of glass or some other frangible material and is completely sealed. The outer tube is made of a compressible plastic. The spores are impregnated upon filter paper or some other appropriate carrier, which is located between the walls of the outer and inner tubes during the sterilization procedure. The sterilant enters this void through the cap and contacts the spores during the sterilization procedure. Afterward the inner tube is broken, exposing the spores to the growth media. The biological indicator is then incubated in conditions that promote the growth of viable spores. If spores survive the sterilization cycle, a pH indicator in the growth medium will change color, indicating that the sterilization cycle has failed. Although accurate, biological indicators that rely on the growth of spores are slow, commonly requiring between 1 and 7 days to give final results. During the incubation period, the goods must be quarantined, and a large amount of space must be committed to their storage.
More recently biological indicators have been developed that measure the effectiveness of sterilization cycles by monitoring an enzyme whose activity can be correlated with the viability of at least one microorganism commonly used to monitor sterilization efficacy. In contrast to biological indicators that measure spore growth alone, enzyme indicators provide fast results, often in as little as one to three hours. If the sterilization procedure works properly, the enzyme will be inactivated during the cycle. If, however, the sterilization procedure fails, the enzyme will retain its activity and react with a substrate to form a detectable product. Enzyme biological indicators include a source of active enzyme and a substrate that reacts with the enzyme. The enzyme and substrate are separated from each other by a physical barrier during the sterilization cycle, and are mixed together afterward. The formation of an enzyme-substrate product is detectable as a fluorescent or color change. Enzyme biological indicators are described in U.S. Pat. No. 5,073,488, which is incorporated in its entirety herein by reference.
Enzyme biological indicators may be used alone or as part of a dual biological indicator. Dual biological indicators are enzyme biological indicators that measure both enzyme activity and spore growth to determine the effectiveness of a sterilization cycle. The enzyme system gives rapid results, which are then confirmed by measurement of spore outgrowth. In a dual biological indicator the spores themselves may be the source of active enzyme. 3M.TM. Attest.TM. 1291 and 1292 Rapid Readout Biological Indicators, available from 3M Company, St. Paul, Minn., are dual indicators that measure both enzyme activity and the growth of live spores.
Although enzyme biological indicators are both rapid and accurate for most uses, it has been observed that when certain pre-vacuum, or vacuum assisted, steam sterilization protocols are followed the enzyme may be prematurely inactivated, leading to false indications of sterility or false negatives. When these sterilization procedures are used, dual indicators may show positive signs of spore growth even though the indicator showed no signs of enzyme activity. This is not a problem in the United States, where steam is introduced to the sterilization chamber before a vacuum is drawn. However, in Europe, where pre-vacuum sterilizers often draw a vacuum before steam is introduced, it is known that the enzyme is sometimes inactivated before the spores are killed. When this occurs, the enzyme is unable to form an enzyme-substrate complex that indicates that the sterilization cycle has failed, and contamination goes undetected. Although the precise mechanism underlying this problem is not known with certainty, it is believed that in European pre-vacuum cycles condensed sterilant contacts the enzyme and inactivates it.
There is a need for a protective housing for a biological indicator that will prevent condensed sterilant from contacting the biological indicator and inactivating it prematurely, but that will allow noncondensed sterilant to enter the housing and contact the biological indicator.