A child exposed to cowpox develops immunity to smallpox. This demonstrates:

A. 160°C for 2 hours: This temperature and duration correspond to dry heat sterilization rather than steam sterilization in an autoclave. Dry heat sterilization is commonly performed in hot air ovens for glassware or metal instruments that cannot tolerate moisture. Autoclaves rely on moist heat under pressure, which allows sterilization at lower temperatures compared with dry heat.

B. 71.6°C for 15 seconds: This condition corresponds to high-temperature short-time (HTST) pasteurization, commonly used in the dairy industry to reduce microbial load in milk. Pasteurization reduces pathogenic microorganisms but does not destroy all microbes or bacterial spores. Therefore, it does not achieve sterilization.

C. 100°C at 10 psi for 30 minutes: Boiling water reaches 100°C at atmospheric pressure and can kill many vegetative bacteria and viruses. However, many bacterial endospores, such as those produced by Bacillus and Clostridium species, can survive this temperature. True sterilization requires higher temperatures achieved through pressurized steam.

D. 63°C at 5 psi for 30 minutes: This temperature corresponds to low-temperature long-time (LTLT) pasteurization, also known as batch pasteurization. Like other pasteurization methods, it reduces microbial numbers but does not eliminate all microorganisms or spores. As a result, it cannot achieve sterilization.

E. 121°C at 15 psi for 15 to 30 minutes: These are the standard minimum conditions for steam sterilization in an autoclave. Pressurized steam at 121°C effectively denatures proteins, destroys cellular structures, and kills all forms of microbial life, including highly resistant bacterial endospores. This method is widely used for sterilizing laboratory media, surgical instruments, and medical equipment.

A. Pilus: During bacterial conjugation, a specialized structure called a sex pilus (or conjugation pilus) forms a direct connection between two bacterial cells. The donor cell transfers a copy of plasmid DNA or other genetic material through this pilus to the recipient cell, allowing for horizontal gene transfer and the spread of traits such as antibiotic resistance.

B. Capsule: The bacterial capsule is a polysaccharide layer that surrounds the cell wall and provides protection against phagocytosis and desiccation. It does not play a role in the transfer of DNA between bacterial cells, and therefore is not involved in conjugation.

C. Flagellum: The flagellum is a whip-like appendage that enables bacterial motility. While important for movement and chemotaxis, it is not used for DNA transfer during conjugation. DNA exchange occurs specifically through the pilus, not through flagella.