Bacillus stearothermophilus spores are used as a biological indicator for autoclave efficacy because they are highly resistant to heat and steam sterilization. Their spores can survive harsh conditions, making them an ideal marker to confirm that the autoclave cycle has effectively destroyed all microbial life. If these spores are killed during the process, it ensures the sterilization was successful, guaranteeing patient safety by preventing surgical site infections. Other organisms listed lack the necessary heat resistance to serve as reliable indicators in steam sterilization validation.

Reference: Medical Microbiology, Jawetz, Melnick & Adelberg, 27th Edition.