Is a probiotic the opposite of an antibiotic?

Yes, antibiotics kill bacteria indiscriminately, while probiotics are administered to nurture the growth of specific bacteria. Somewhat in the same way that antibodies can start killing their own host indiscriminately, whereas probodies are proteolytically activated antibodies designed to remain inert, until locally activated by diseased tissue (IE. The destruction of the local organs trips the probody defense mechanisms).