Cipro is an antibiotic that happens to be effective against anthrax bacteria, as well as many other types of bacteria. For example, it will kill E. coli bacteria. It is helpful in treating bacterial infections that cause everything from bronchitis to gonorrhea.
According to the Bayer site, Cipro works in the following way:
..it inhibits bacterial nuclear DNA synthesis, so that bacteria rapidly die. The target is the enzyme DNA gyrase (topoisomerase II), which is responsible for the supercoiling and uncoiling of the DNA. Supercoiling of the DNA allows the long DNA molecule to fit into the cell. Uncoiling of the structure is the initiative step for replication, transcription and repair of the DNA. Thus, prolonged inhibition will eventually lead to the death of the cell.
In other words, inside E. Coli bacteria and anthrax bacteria is an enzyme, called topoisomerase II, that helps the cell to wind DNA into a compact structure and then unwind it when needed. Cipro blocks topoisomerase II and prevents it from doing its job. A bacterial cell that has Cipro in it can no longer uncoil its DNA in order to create enzymes or reproduce. The bacteria containing Cipro eventually die.
There are reports that Soviet scientists created antibiotic-resistant strains of anthrax. One easy way to accomplish this would be to grow large quantities of anthrax and then treat it with Cipro to see if any of the cells lived. Then you would allow those few living cells to reproduce. These would be Cipro-resistant cells. However, they would not be resistant to other antibiotics that happen to work against anthrax.
For more information on Cipro, anthrax and related topics, check out the links below.