title: An improved SYBR Green-1-based fluorescence method for the routine monitoring of Plasmodium falciparum resistance to anti-malarial drugs.
abstract: BACKGROUND
  The recently introduced SYBR Green1 (SG) assay for testing parasites susceptibility to anti-malarial drugs needs further improvement. This has been necessitated by various setbacks, the major one being the low fluorescence intensity associated with it use. This shortcoming diminishes the anticipated hope that this novel method was going to replace the more traditional ones, such as the isotopic and microscopy. In order to restore confidence in its use, series of experiments to determine conditions that give the best fluorescence intensity were conducted.
  
  
  METHODS
  Conditions that yield the maximum fluorescent signal were ascertained by measuring the fluorescence after incubation of Plasmodium falciparum culture at different parasites concentration with lysis buffer containing SYBR Green (LBS) at different time period. In order to ascertain the effect of freeze-thaw on fluorescence intensity, P. falciparum culture was frozen for 1 h, thawed, incubated with LBS and the fluorescence measured. The optimized conditions determined in this study were then used to assess the susceptibility of clinical isolates of P. falciparum to artesunate, chloroquine and mefloquine. The concentration of anti-malarial drug inhibiting parasite growth by 50 % (IC50) for each drug was estimated using the online ICEstimator. The IC50 generated using the optimized SG method determined in this study was compared with that obtained using microscopic method and the previously reported standard SG method.
  
  
  RESULTS
  Over all, the SG method was found to be easy to perform and sensitive. Freeze-thaw of parasite culture followed by incubation with lysis buffer containing the dye for 3 h was consistently observed to give the highest fluorescence signal. The IC50 values for chloroquine, mefloquine and artesunate determined were consistent and comparable with that determined with the previously reported standard SG method and the microscopic method.
  
  
  CONCLUSIONS
  The authors conclude that freezing and thawing of parasite culture, followed by incubation with LBS in the dark for 3 h provided a significant improvement in fluorescence signal. The IC50 generated using the improved SG method is comparable with that from microscopy and the standard method.
authors.affiliation: Department of Animal Biology and Conservation Sciences, University of Ghana, Accra, Ghana. dery.victor@yahoo.com.
authors.forename: Victor
authors.initials: V
authors.lastname: Dery
authors.affiliation: Epidemiology Department, Noguchi Memorial Institute for Medical Research, University of Ghana, Accra, Ghana. nduah@noguchi.ug.edu.gh.
authors.forename: Nancy O
authors.initials: NO
authors.lastname: Duah
authors.affiliation: Epidemiology Department, Noguchi Memorial Institute for Medical Research, University of Ghana, Accra, Ghana. rayanful@noguchi.ug.edu.gh.
authors.forename: Ruth
authors.initials: R
authors.lastname: Ayanful-Torgby
authors.affiliation: Epidemiology Department, Noguchi Memorial Institute for Medical Research, University of Ghana, Accra, Ghana. smatrevi@noguchi.ug.edu.gh.
authors.forename: Sena A
authors.initials: SA
authors.lastname: Matrevi
authors.affiliation: School of Public Health, University of Ghana, Accra, Ghana. fanto@ug.edu.gh.
authors.forename: Francis
authors.initials: F
authors.lastname: Anto
authors.affiliation: Epidemiology Department, Noguchi Memorial Institute for Medical Research, University of Ghana, Accra, Ghana. nquashie@noguchi.mimcom.org.|Centre for Tropical Clinical Pharmacology and Therapeutics, School of Medicine and Dentistry, University of Ghana, Accra, Ghana. nquashie@noguchi.mimcom.org.
authors.forename: Neils B
authors.initials: NB
authors.lastname: Quashie
mesh_terms: D000962:Antimalarials / Q000494:pharmacology*; D037621:Artemisinins / Q000494:pharmacology; D000077332:Artesunate; D052160:Benzothiazoles; D002648:Child; D002675:Child, Preschool; D002738:Chloroquine / Q000494:pharmacology; D003959:Diamines; D004351:Drug Resistance; D005470:Fluorometry / Q000379:methods*; D006801:Humans; D020128:Inhibitory Concentration 50; D015767:Mefloquine / Q000494:pharmacology; D009930:Organic Chemicals / Q000378:metabolism*; D021261:Parasitic Sensitivity Tests / Q000379:methods*; D010963:Plasmodium falciparum / Q000187:drug effects* / Q000235:genetics; D011804:Quinolines; D013194:Staining and Labeling / Q000379:methods*
chemical_list: D000962:Antimalarials; D037621:Artemisinins; D052160:Benzothiazoles; D003959:Diamines; D009930:Organic Chemicals; D011804:Quinolines; C098022:SYBR Green I; D000077332:Artesunate; D002738:Chloroquine; D015767:Mefloquine