Biological indicators

Sterilization Indicators – Biological Indicators

Strips, paper discs

The strips are paper strips inoculated with spores of Geobacillus stearothermophilus, Bacillus atrophaues or Bacillus pumilus, used to optimally verify the sterilization of dry loads with various sterilizing agents, respectively steam, ethylene oxide/dry heat and radiation.

The strip is usually enclosed in a paper sachet, which must be placed inside the load and subjected to the sterilization cycle, during which the sterilizing agent comes into contact with the spores.

Once finished, the strip must be removed from the sachet and incubated for 7 days in Soybean Casein Digest Broth (TSB). Any cloudiness of the medium indicates bacterial growth and therefore sterilization failure. There are broths with a pH indicator that changes color with the growth of the spores, and which can be validated for a significant shortening of the incubation time.

For particular uses, mini strips, paper discs and cotton threads are available.

“Self-contained” biological indicators (SCBI)

A valid alternative to the strips is given by the "self-contained" biological indicators, in which a paper disk with the spores is placed inside a small plastic test tube with a perforated cap, together with a glass ampoule containing a broth. of culture with color change indicator.

The advantages of SCBI are two: the post-sterilization incubation time is shortened to 48 or even 24 hours (in the case of steam it can even reach 10 hours) and there is no manipulation of the spores since the strip remains inside the test tube of plastic, where it is incubated. The indicator must be positioned inside the load and subjected to the sterilization cycle, during which the sterilizing agent comes into contact with the spores. At the end, the test tube is crushed, breaking the internal flask, which then releases the broth, moistening the disk. We proceed with the incubation and verification of any color change.

Steel discs and SCBI

For sterilization with hydrogen peroxide, Geobacillus stearothermophilus or Bacillus atrophaues spores inoculated on a steel support are usually used, both in the classic version (steel disk in a sachet) and in the self-contained version.

Spore suspensions

Spores suspended in aqueous solution for particular needs, for example verifying the sterilization of thin or complex-structured medical devices.

Proceed by inoculating an aliquot of the suspension onto the sample and allowing it to dry. The object must then be packaged as usual and subjected to sterilization. At the end, the object must be incubated in culture broth to check for any bacterial growth.

It should be considered that the D-value indicated on the certificate refers to the tests performed on the suspension inoculated on paper strips and that the resistance could change significantly depending on the type of support on which the suspension is inoculated. In fact, spores can adhere to surfaces, forming densities in areas that appear porous or wrinkled at a microscopic level. This aspect is even more evident if you intend to inoculate the suspension in a liquid, whose effects on the resistance of the spore can be completely unexpected.