It acts as a buffer to protect sperm cells from their own toxic byproducts, as well as protecting the sperm from cold shock and osmotic shock during the chilling and shipping process (the semen is chilled to reduce metabolism).
[1] Special freezing extender use also allows cryogenic preservation of sperm, which may be transported for use, or used on-site at a later date.
Dual-sugar extenders typically have similar ingredients, with an additional sugar, sucrose.
Ticarcillin (often used in combination with clavulanic acid under the designation Timentin), amikacin sulfate, penicillin, and gentamicin are commonly used.
When private donors ship chilled semen outside of the formal regulatory environment, and fertilization is accomplished by allowing sperm to swim through the reproductive tract without the help of procedures such as IVF, then it is possible to achieve better results without antibiotics.