Sulfonamid is an antimicrobial agent, why has it no effect on the growth of Rickettsia?
Sulfonamides competitively inhibit the enzyme dihydropteroate synthase, which is involved in the tetrahydrofolate synthesis pathway in most bacteria. Rickettsia lack this enzyme target, and thus possess a natural resistance to the antibiotic. As tetrahydrofolate is an essential compound, it is believed that Rickettsia possess an alternative enzymatic process which is not susceptible to sulfonamides, or the bacteria simply acquire the compound (or its immediate precursors) from their environment.
richettsiae lacks folP gene which encodes for dihydropteroate synthase and folA which encodes dihydrofolate reductase enzymes. This shows that richettsiae lacks the target of sulfonilamides. therefore sulfonilamides are ineffective to inhibit its growth and thus if given may lead to increased mortility and morbildity.