Abstract
Gum arabic (GA) is a traditional herbal medicine from Acacia Senegal (L.) Willdenow trees, which consist of a complex mixture of polysaccharides and glycoproteins. It is used in daily applications for several diseases and is considered to protect against bacterial infections. The detailed mechanisms behind these observations are still unclear. In this study, we investigated the direct antibacterial activity of GA water and ethanol extracts against Staphylococcus (S.) aureus or Escherichia (E.) coli and the immunomodulating properties of those extracts on granulocytes as a first line of defense against bacteria. Firstly, the direct antimicrobial effect of GA was tested on three different S. aureus strains and two E. coli strains. The growth of bacteria was analyzed in the presence of different GA concentrations over time. GA water as well as ethanol extracts showed a significant growth inhibition in a concentration-dependent manner in the case of S. aureus Newman, S. aureus Rd5, and E. coli 25922, but not in the case of S. aureus USA300 and E. coli K1. Transmission electron microscopic analysis confirmed an antibacterial effect of GA on the bacteria. Secondly, the immunomodulatory effect of GA on the antimicrobial activity of bovine or human blood-derived granulocytes was evaluated. Interestingly, water and ethanol extracts enhanced antimicrobial activity of granulocytes by the induction of intracellular ROS production. In line with these data, GA increased the phagocytosis rate of E. coli. No effect was seen on neutrophil extracellular trap (NET) formation that mediates killing of extracellular bacteria such as S. aureus. In conclusion, we show that GA exhibits a direct antibacterial effect against some S. aureus and E. coli strains. Furthermore, GA boosts the antimicrobial activities of granulocytes and increases intracellular ROS production, which may lead to more phagocytosis and intracellular killing. These data might explain the described putative antimicrobial activity of GA used in traditional medicine.
Highlights
Gum arabic (GA) is a dried gummy exudate from stems and branches of Acacia Senegal (L.) Willdenow trees or closely related species
To investigate in more detail to what extent GA has antibacterial effects, the growth of bacterial S. aureus and E. coli strains was monitored over time in the presence of GA in a concentration range in liquid cultures in our presented study
The pathogenic bacterium S. aureus, for example, can rapidly adapt to the selective pressure of antibiotics and this resulted in the emergence and spread of methicillin-resistant S. aureus (MRSA) during the recent years [35]
Summary
Gum arabic (GA) is a dried gummy exudate from stems and branches of Acacia Senegal (L.) Willdenow trees or closely related species. GA is a water-soluble polysaccharide with sugars including rhamnose, arabinose, and galactose and contains highly branched complex arabinogalactan proteins (AGP). It contains glucuronic acid and minerals such as calcium, magnesium, and potassium [1,2,3]. In the Middle East and North Africa, GA is used as an oral hygiene substance that has anti-bacterial effects against periodontal pathogens [8]. It has been tested for treatment of patients with chronic renal failure [9]. It has been shown that GA influences the outcome of several different metabolism-related diseases or other non-infectious diseases [reviewed [13, 14]]
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