Antibacterial mushrooms

In an age where we are becoming increasingly concerned about bacterial evolution and the effectiveness of antibiotics, there is an on-going need for novel antimicrobial compounds to outwit bacteria and other pathogens.

A team of researchers from the University of Auckland and Dr Peter Buchanan from Manaaki Whenua – Landcare Research have recently shown that several varieties of edible mushrooms, five of which are native and were collected from New Zealand forests and parks, have antibacterial and antioxidant properties.

Oyster mushroom

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In recent research, the brown oyster mushroom (Pleurotus australis), has been shown to inhibit the growth of five common bacterial strains.

Rights: Morrie

Eight species of mushroom were selected for testing. Four of the mushroom species known to early Māori as edible mushrooms: Auricularia cornea Ehrenb.; Calvatia gigantea; Hericium coralloides; and Pleurotus australis.

Of the eight varieties tested for their ability to inhibit the growth of five common bacterial strains, one of the most effective proved to be the New Zealand native brown oyster mushroom (Pleurotus australis). The brown oyster mushroom also had the highest antioxidant activity.

Mushrooms, the fruiting body of a macrofungus, have been used throughout history for medicinal purposes. Indeed, in modern times, several antibiotics have already been isolated from various mushrooms and microfungi (including penicillin and griseofulvin, which are isolated from microfungi). However, there are tens of thousands of mushroom varieties as well as other fungi, only a handful of which have been studied, that can potentially have antibiotic, antifungal, antiviral and/or antiprotozoan properties, as well as other health benefits.

In this study, the researchers used paper disc diffusion and microdilution techniques to measure antibacterial activity. They also compared the two techniques to determine which method was superior and determined that the microdilution method was more reliable than the disc diffusion method in detecting antibacterial activity.

The extracts they removed from the test mushrooms are called polysaccharides.

“Two major groups of mushroom bioactives are the polysaccharides and triterpenes. Polysaccharides are responsible for the rigidity and morphological properties of the fungal cell wall. Many display potent activity against common strains of bacteria. Fungi produce polysaccharides, phenolics and various metabolites that represent potential sources of novel natural antioxidants,” write the researchers in their published paper.

The researchers found that the polysaccharides extracted from the mushroom Cordyceps sinensis inhibited the growth of the bacteria Bacillus subtilis and Streptococcus epidermidis, and the mushroom P. australis extract restricted the growth of S. epidermidis. “Further testing of those polysaccharides for antibiotic properties should be considered. The results confirmed that Gram-negative bacteria were more resistant to the growth inhibitory effects of fungal polysaccharides. Further work is required to understand the molecular basis of the antibacterial activity of the polysaccharides.”

The antioxidant activity of the mushroom extracts was assessed by measuring their radical scavenging activity. The polysaccharides for all eight mushroom species showed radical scavenging activities. However, P.australis exhibited the highest antioxidant activity.

The researchers explain that stress on the body due to ageing, obesity and detrimental lifestyle choices is a significant health issue, which often takes the form of oxidative damage to tissues by free radicals. Antioxidants are compounds that can protect biological systems against the potentially harmful effects of these free radicals.

The researchers concluded,

The antibacterial and antioxidant activities detected here warrant investigation for their potential to improve human health, and application as dietary supplements.

The research was published in 2014 in the scientific journal Bioactive Carbohydrates and Dietary Fibre.