A Comparative Assessment of the Antibacterial Activity in Fruit Juice of Sri Lankan Sweet Orange Cultivars vis a vis Sour Orange

Abstract

Sweet orange (Citrus sinensis), a key fruit species, is considered as a primary ingredient in herbal medical formulations against ailments such as food borne diseases. Sour orange (C. aurantium) is also very famous as a medicinal plant. There are six commonly grown sweet orange cultivars in Sri Lanka (Arogya, Bibila sweet, MKD, Sisila, BAN and MT) but the antibacterial activity present in their fruit juice is not well documented. Therefore, the present study was conducted to characterize the antibacterial activity of the fruit juice of these sweet oranges in comparison to sour orange and also to establish DNA barcodes for the tested cultivars for precise identification. Fruit juice was collected from sweet orange cultivars and sour orange and antibacterial activity was measured against three model pathogenic bacterial species, Escherichia coli, Staphylococcus aureus and methicillin-resistant S. aureus. After employing filter paper disc method, the diameter of zone of bacterial inhibition (DZI) was measured as the parameter of antibacterial activity. The genomic DNA was extracted from all the tested plants and PCR amplified using trnH–psbA primer pair and subjected to DNA sequencing, followed by alignment analysis and dendrogram construction. Arogya and MKD did not show any antibacterial activity (DZI = 0.0 mm), whereas Sisila, BAN and MT showed antibacterial activity only against E. coli and S. aureus (mean DZI of 8.2 mm and 8.4 mm respectively). Bibila sweet and sour orange showed significantly higher antibacterial activity against all E. coli, S. aureus and methicillin-resistant S. aureus (mean DZI of 10.2 mm, 10.5 mm and 7.8 mm respectively). DNA barcoding provided unique sequence identifiers for each cultivar. These antibacterial activity data in combination with DNA barcodes could help to develop new cultivars through breeding to promote the sweet orange industry in Sri Lanka.