Like other carnivorous plants, Nepenthes species grow on poor soil. They need to complement their mineral nutrients – primarily with nitrogen and phosphorus – from caught and digested prey. When visiting the pitfall traps, the attracted prey, mainly arthropods, falls into the trap, drowns and is digested by the enzyme cocktail of the pitcher fluid.
Due to the fact that closed Nepenthes pitchers have no direct contact with the environment, it has been widely claimed that their pitcher fluid is sterile and that all proteins and compounds identified in this pitcher fluid are solely plant-derived. But only two experiments had been conducted to demonstrate the sterility of pitcher liquid: fluid taken from a closed pitcher was plated either on plain nutrient agar (Hepburn, 1918) or on meat agar plates (Lüttge, 1964) and incubated for several days. In no case were any bacterial colonies detected and the authors concluded that the pitcher fluid is sterile. However, the presence of microbes cannot be excluded by such simple experiments because most micro-organisms cannot be grown in culture.
Researchers have now analysed the composition of Nepenthes digestive fluid from closed pitchers to reveal whether or not pitchers are really sterile inside and how these plants manage to keep microbial growth under control. Thecontent of proteins, inorganic ion compositions and secondary metabolites were studied. In addition, the effect of pitcher fluid on microbial growth was investigated. The results reveal that the fluid of closed Nepenthes pitchers is composed provides anti-microbial conditions. Thus these plants can avoid, at least to some extent, the growth of microbes that compete with the plant for the prey-derived nutrients available in the pitcher.