Antimicrobial activity of selected natural products against Gram-positive, Gram-negative and Acid-fast bacterial pathogens

Recurring epidemics of drug resistant bacterial diseases such as those caused by mycobacteria (tuberculosis and non-tuberculous infections), staphylococci (methicillin-resistant Staphylococcus aureus or MRSA infections) and various Gram-negative enterobacteria (enterobacterial infections) have reinforced the need to search for alternative antimicrobials. In this context, we investigated the anti-bacterial potential of nine different natural products and compared them with the antibiotic controls, using three test bacterial species, representing the Gram-negative (Escherichia coli), Gram-positive (Staphylococcus epidermidis), and Acid-fast (Mycobacterium smegmatis) pathogen groups. Six of the nine products showed detectable but variable zones of inhibition (mm2). The anti-bacterial activity (mm2 per 100 mg) of the extracts from the four solid natural products was in the following order for all three pathogen groups: Mint (Mentha arvensis) leaf extract, 264-930>Mushroom (Agaricus bisporus) cap extract, 112-241>Turmeric (Curcuma longa) root extract, 4-10>Ginger (Zingiber officinale) root extract, 3-9. For the liquid products, the activity measured on 100 μL aliquots was in the following order: Eucalyptus (Eucalyptus globules) oil, 264-1044>Mustard (Brassica campestris L. var. brown sarson) oil, 45-96. Taken together, these results indicated the highest activity in Mint extract and Eucalyptus oil against all three test organisms. However, the individual test strains showed the following variable order of susceptibility: Mint extract (M. smegmatis>E. coli>S. epidermidis); Eucalyptus oil (M. smegmatis>S. epidermidis>E. coli). Based on these results it can be concluded that Mint leaves and Eucalyptus oil have an unusually broad spectrum activity and may, therefore, be promising sources of new broad spectrum antimicrobials.