Callistemon citrinus bioactive metabolites as new inhibitors of methicillin-resistant Staphylococcus aureus biofilm formation.

The development of new anti-virulence drugs from natural origin has recently received significant attention. Callistemon citrinus Skeels is an important plant of great medicinal value. Its antimicrobial activity is well documented. Although several bioactive compounds were isolated from this plant, the actual bioactive compounds responsible for its antimicrobial activity still unrevealed.To evaluate the effect of C. citrinus crude extract and isolates on methicillin-resistant and sensitive Staphylococcus aureus.C. citrinus leaves methylene chloride-methanol extract (MME) was prepared by Soxhlet apparatus. Biologically guided fractionation of MME was accomplished using several normal and reversed phase silica gel columns. The potency of MME and isolates against methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) was evaluated. The mechanism of resistance was studied using three virulence factors; antibiofilm activity, inhibition of staphyloxanthin biosynthesis and effect on acid tolerance.Pulverulentone A (C1), 8- desmethyl eucalyptin (C2) and eucalyptin (C3) were isolated from the most bioactive fraction of MME. Pulverulentone (C1) showed the most potent anti-biofilm activity up to 71% and 62.3% against MRSA and MSSA, respectively. It also exhibited the highest inhibition of staphyloxanthin biosynthesis of MRSA and MSSA by 55.6% and 54.5%, respectively.C. citrinus phenolics and acylphloroglucinols may serve as potential source of plant-based antibacterials.