[Biosynthesis of folic acid in Eimeria tenella (Coccidia)]

The inhibitory analysis has shown that E. tenella does not include ready folic acid into metabolic process but synthesizes its metabolite, 7:8-dihydrofolic acid (DHFA), from its precursor, para-aminobenzoic acid (PABA), which enters the host's organism. PABA can be fully substituted in the process of metabolism by sulphadimesin (SD) at the molar ratio PABA: SD=1: 16.7. As a concurrent antagonist of PABA SD is 6 times stronger in the parasite-host system of Coccidia than in bacteria in cultural medium. The increase in PABA content in relation to SD more than 1: 16.7 intensifies the viability of Coccidia that is expressed in higher lethality of chicks, which reaches its maximum at PABA : SD=1 : 1.25. A further increase in the content of PABA does not affect the host's lethality. The decrease in PABA in relation to SD lower than 1 : 16.7 reduces the viability of Coccidia. Nucleic bases G, A transforming in the organism into corresponding nucleotids and aminoacids, 1-metionin and glycine, which are the products of biosynthesis with a participation of DHFA, do not stimulate the viability of E. tenella that apparently is associated with the non-capability of the host to assimilate timidin.