Steam sterilization biological indicators

The biological indicators are made up of standardized bacterial spores inoculated on different types of support which, in addition to allowing their appropriate use, emulate the process that leads to the reduction of the microbial load.

The biological indicators for sterilization with steam appear as:

Strips, discs, threads, rods, etc. inoculated with the spores.
Self-contained devices containing the strip with the spores and the ampoule with the culture broth.
Glass ampoules containing spores suspended in culture broth.
Vials of spores in aqueous suspension.

Spores used:

Geobacillus stearothermophilus 7953
Bacillus subtilis 35021

Biological indicators for steam sterilizationare regulated by:

ISO 11138-1:2017 Sterilization of health care products — Biological indicators Part 1: General requirements
ISO 11138-3:2017 Sterilization of health care products — Biological indicators Part 3: Biological indicators for moist heat sterilization processes
ISO 11138-5:2017 Sterilization of health care products — Biological indicators Part 5: Biological indicators for low-temperature steam and formaldehyde sterilization processes
EP and USP

Focus: validity of genetic tests on Geobacillus stearothermophilus

In recent years, some customers have started performing genetic tests on the spores contained in biological indicators. This has highlighted the difficulty of obtaining reliable and unambiguous results with regard to Geobacillus stearothermophilus, which shows extremely high genetic similarity to other Geobacillus species, including G. thermoleovorans and G. kaustophilus, as well as others. Many species share highly conserved genetic regions; in Geobacillus, genetic markers often lack the variability required to distinguish species with 100% certainty.

To achieve exact differentiation, a multifaceted approach is often required, combining genetic, biochemical, and phenotypic analyses, thereby enabling precise differentiation of Geobacillus species. However, this approach is complex and costly.

Comunicazione Verifica Genetica Geobacillus stearothermophilus

Biological Indicators Steam Crosstex

Biological Indicators Steam Etigam

Biological Indicators Steam True Indicating

Strips, paper discs, threads

The strips are paper strips inoculated with Geobacillus stearothermophilus spores used to optimally verify the steam sterilization of dry loads.

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 steam 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. Broths with pH indicators are available which, by changing color with bacterial growth, make it possible to validate a significant shortening of the incubation time.

For particular uses, mini strips, paper discs and cotton threads can be used.

“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 24 hours or 10 hours and there is no manipulation of the spores since the strip remains inside the plastic tube, where it is incubated. The indicator must be positioned inside the load and subjected to the sterilization cycle, during which the steam 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.

Glass ampoules

The glass ampoules contain the spores suspended in a culture broth with color change, therefore rapid incubation. They are used to verify the steam sterilization of liquid loads (e.g. culture media) but also dry ones (e.g. glassware). At the end of sterilization, the ampoule should be left to incubate as is to check for any color change).

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.