Unculturable soil bacteria is a misleading term, as these bacteria hypothetically can be cultured. More accurately, ‘unculturable’ bacteria cannot be cultured using traditional techniques, such as traditional agars. These unknown bacteria are a potential untouched pool of medically useful secondary metabolites, such as antibiotics.
‘Unculturable’ soil bacteria are of particular interest because of the genus Streptomyces, a group of bacteria growing in soil which has provided around 70 to 80% of all existing antibiotics in use. Seeing as less than 1% of soil bacteria are currently culturable with present techniques, the possibility of novel antibiotics from ‘unculturable’ soil bacteria could help to alleviate the rapidly evolving issue of antibiotic resistance.
Traditional culture techniques have failed to facilitate growth for most soil bacteria as these techniques do not mimic natural conditions. These techniques favour fast-growing species who can survive in nutrient-rich environments. However, slow-growing species, species who grow in nutrient-deprived conditions, or those which rely on other helper species and helper agents are penalised by these methods and fail to grow. New techniques, such as diffusion bioreactors, attempt to more accurately mimic the conditions found in nature. A diffusion bioreactor, commonly known as a diffusion chamber, allows for the passage of necessary elements from the soil into the media being used for growth. This also has the added advantage of allowing an interaction of biotic and abiotic factors which more easily enables co-dependent interactions.
How successful have these new techniques been? In 2015, a novel antibiotic named teixobactin was discovered using a diffusion chamber device known as the iChip (isolating chip), from a soil sample taken from Maine, United States. The new antibiotic was said to have no detectable resistance, killing gram-positive bacteria by binding to lipids in the bacterial cell wall. In a study carried out in 2019, 35 previously uncultured taxa were isolated. Any one of these 35 previously uncultured taxa could contain a species which produces an undiscovered antibiotic. Further research into contemporary culturing techniques could yet yield many more novel antibiotics.
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