The minimum number of microbes needed to cause infection is:

A. Periplasmic flagella: Spirochetes have flagella located within the periplasmic space between the inner and outer membranes. These internal flagella, also called axial filaments, rotate to produce a corkscrew-like motion, allowing the bacterium to move efficiently through viscous environments such as mucus or connective tissue. This unique mechanism is characteristic of genera like Treponema and Borrelia.

B. Pili: Pili are hair-like appendages on the surface of many bacteria used primarily for attachment to surfaces and for conjugation (gene transfer). Pili do not contribute to the corkscrew motility seen in spirochetes.

C. Capsules: Capsules are extracellular polysaccharide layers that protect bacteria from phagocytosis and desiccation. While important for virulence and immune evasion, capsules do not play a role in bacterial motility or the corkscrew movement of spirochetes.

Physical methods of microbial control, such as autoclaving, dry heat, or filtration, can achieve sterilization by completely eliminating all forms of microbial life, including bacterial spores. In contrast, chemical methods, like alcohols, chlorine, or phenolic compounds, primarily disinfect surfaces by killing or inhibiting vegetative bacteria, fungi, and some viruses, but they do not reliably destroy highly resistant spores. The effectiveness of chemical disinfectants depends on concentration, contact time, and the presence of organic material. Therefore, while physical methods can sterilize under controlled conditions, chemical methods are generally limited to disinfection rather than true sterilization.