Presumed autoclave failure due to false-positive spore strip tests

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.