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Presumed autoclave failure due to false-positive spore strip tests
Abstract:
The sterilizing processes in autoclaves and ethylene oxide sterilizers are challenged on a regular basis with a controlled inoculum of spores from two Bacillus species. Within a two-day period in March 1980, the seven autoclaves at this 550-bed hospital appeared to have failed in their function of killing spores on 18 out of 46 test strips. A shut-down of the autoclaves and a massive investigation failed to identify any mechanical, physical, or human failures. However, after 48 hours, it was found that the broth used as growth medium contained a contaminant, Bacillus coagulans, that resulted in broth turbidity at 55 degree C. Incubating an uninoculated tube of trypticase soy broth (TSB) for quality control at 55 degree C in addition to the usual 37 degree C quality check is a recommended safeguard against such occurrences.
Insights
Hospital autoclaves failed to kill spores due to a contaminated growth medium, Bacillus coagulans. A modified quality control incubation temperature is recommended to prevent future sterilization failures.
Area of Science:
- Microbiology
- Sterilization Processes
- Hospital Infection Control
Background:
- Autoclaves and ethylene oxide sterilizers are critical for hospital sterilization.
- Regular spore challenge testing verifies the efficacy of sterilization processes.
- Sterilization failures can lead to significant patient safety risks.
Purpose of the Study:
- To investigate the cause of apparent sterilization failures in hospital autoclaves.
- To identify factors contributing to false positive results in spore challenge tests.
- To recommend improvements in quality control procedures for sterilization monitoring.
Main Methods:
- Sterilization processes were challenged with Bacillus species spores.
- Autoclave performance was monitored using biological indicator test strips.
- Growth medium used for spore viability testing was analyzed for contaminants.
- Quality control procedures for growth medium incubation were reviewed.
Main Results:
- Seven autoclaves showed apparent failures on 18 of 46 test strips within a two-day period.
- No mechanical, physical, or human errors were identified as the cause of autoclave failures.
- Bacillus coagulans, a contaminant, was found in the growth medium, causing turbidity at 55°C.
- This contaminant interfered with the accurate assessment of spore kill in the tests.
Conclusions:
- Contamination of growth medium with Bacillus coagulans can lead to false positive sterilization test results.
- Standard quality control incubation at 37°C may not detect this specific contaminant.
- Implementing an additional quality control incubation at 55°C for trypticase soy broth (TSB) is recommended.
- This enhanced quality control measure can prevent future misinterpretation of sterilization efficacy.