A spore test only reads true at the temperature its own organism needs: Geobacillus stearothermophilus belongs at 55-60 °C, while Bacillus atrophaeus belongs at 35-37 °C. Incubating either strip outside its window produces a result you cannot trust, and a single shared incubator makes that error easy to repeat quietly for months. The organism inside the tube, not the tube itself, sets the temperature you must run, and getting that wrong is one of the quietest errors in a reprocessing room. This article separates the two organism groups, explains how a wrong block distorts the result, and shows how to keep both temperature ranges straight in one clinic.
Which organism sits in which spore test?
Steam cycles are monitored with Geobacillus stearothermophilus spores, a thermophile that germinates near 55 °C and above. Low-temperature processes such as dry heat and ethylene oxide use Bacillus atrophaeus, which grows at body temperature. The two organisms are also read differently, so the label on the tube, not its shape or colour, is the deciding factor. Read the label before you load the block, and store the two types apart so a 55 °C strip never waits beside a 37 °C strip in the same drawer. A dental or medical clinic running steam only will live at 55-60 °C. A site that also runs a low-temperature process has two temperature worlds under one roof and needs to keep them apart.
What happens if a strip is incubated at the wrong temperature?
A thermophile dropped into a 37 °C block will not germinate, so a load that actually failed can read as a pass. Run B. atrophaeus at 55 °C and the same failure appears in reverse, with a dormant control adding confusion about whether the incubator or the strip is at fault. Neither direction triggers an alarm. The well simply stays quiet, or stays overgrown, and the technician reads whatever the well shows. The cost of that silence is real: the load is released, the sterilizer is logged as passing, and the underlying problem keeps running until a different check happens to catch it. Two operators can share the same incubator and never notice the mismatch, because a well reports what the organism did and not what temperature the block was set to.
How do you keep two temperatures straight in one clinic?
Name the ranges explicitly in your written procedure and label the incubators. Write the ranges on the inside of the incubator lid as well as in the manual, because the lid is read many times a day and the manual is read once. A sticker on the door stating the set temperature costs nothing and removes the need to rely on memory. Colour-coding the wells helps only if the label survives the shift.
| Organism | Incubation temperature | Paired process |
|---|---|---|
| Geobacillus stearothermophilus | 55-60 °C | Steam autoclave |
| Bacillus atrophaeus | 35-37 °C | Dry heat, ethylene oxide |
| Positive control (either) | Same as its test batch | Batch verification |
Should the positive control travel with every batch?
Yes, and it should sit in the same block at the same temperature as the test strips. A control incubated somewhere else, or in a second block you assume is matched, defeats the comparison it exists to provide. Label the control with the batch, run it every processing day, and record both wells on the same line. A control that grows on every batch is a small daily cost, and it is the only way to show that a quiet test well means the load passed rather than the test never worked. If the control fails to grow, the batch tells you nothing about the load, and you treat the run as invalid rather than passing it.
Ordering for a clinic, lab or care home? Wholesale and multi-site ordering covers case pricing and account setup, and the B2B wholesale collection lists the lines stocked for institutional buyers.
→ Start with a trial: CliniEco 24-hour Biological Indicator — 5-Pack Trial (CA $12.99, shipping included). For multi-site or high-volume teams, the wholesale and multi-site ordering page covers account setup and case quantities.
Related reading
Bacillus atrophaeus vs Geobacillus stearothermophilus · Growth control and incubation temperature · Sterilization compliance hub
Running two processes? CliniEco 24-hour biological indicator incubator (precision dry block, 24 wells) keeps a steady block temperature for both strip types, and the hub above links the rest of the monitoring range.
Frequently Asked Questions
Can I run a steam spore test at 37 °C if that is the only block I have?
No. The thermophile will not germinate reliably at that temperature, so a passing reading proves nothing about the load.
Do both organism types look the same in the tube?
They can look similar at a glance, which is why the printed label and the paired process on the record are what identify them.
How often should incubator temperature be checked?
Check it against your written procedure, and recheck whenever a control reads unexpectedly or the unit is moved.
Can one incubator hold both organism types at once?
Only if it can maintain two separate set temperatures. A single-temperature block cannot serve both, so most mixed sites run a dedicated block per range.
CliniEco Medical is a licensed medical device establishment (MDEL #35334).
0 comments