Absence of association between Plasmodium falciparum small sub-unit ribosomal RNA gene mutations and in vitro decreased susceptibility to doxycycline.

Background: Doxycycline is an antibiotic used in combination with quinine or artesunate for malaria treatment or alone for malaria chemoprophylaxis. Recently, one prophylactic failure has been reported, and several studies have highlighted in vitro doxycycline decreased susceptibility in Plasmodium falciparum isolates from different areas. The genetic markers that contribute to detecting and monitoring the susceptibility of P. falciparum to doxycycline, the pfmdt and pftetQ genes, have recently been identified. However, these markers are not sufficient to explain in vitro decreased susceptibility of P. falciparum to doxycycline. In this paper, the association between polymorphism of the small sub-unit ribosomal RNA apicoplastic gene pfssrRNA (PFC10_API0057) and in vitro susceptibilities of P. falciparum isolates to doxycycline were investigated.

Methods: Doxycycline IC50 determinations using the hypoxanthine uptake inhibition assay were performed on 178 African and Thai P. falciparum isolates. The polymorphism of pfssrRNA was investigated in these samples by standard PCR followed by sequencing.

Results: No point mutations were found in pfssrRNA in the Thai or African isolates, regardless of the determined IC50 values.

Conclusions: The pfssrRNA gene is not associated with in vitro decreased susceptibility of P. falciparum to doxycycline. Identifying new in vitro molecular markers associated with reduced susceptibility is needed, to survey the emergence of doxycycline resistance.