Amprolium is not an antibiotic, it is a thiamine analog (thiamine-like molecule) used as a coccidiostat (meaning it does not kill coccidia, it just reduces their rate of reproduction)
http://en.wikipedia.org/wiki/Amprolium
Basically it allows the chicks to develop resistance to the coccidia without being overwhelmed by them, but does not kill coccidia or any bacteria that may be present.
http://en.wikipedia.org/wiki/Amprolium
Basically it allows the chicks to develop resistance to the coccidia without being overwhelmed by them, but does not kill coccidia or any bacteria that may be present.