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Background  The development of antibiotic resistant bacteria stems from a number of factors, including inappropriate use of antibiotics in human and animal health and their prolonged use as growth promoters at sub-clinical doses in poultry and livestock production. We were interested in investigating plants that could be useful in protecting humans or animals against diarrhoea. We decided to work on extracts of nine plant species with good activity against  Escherichia coli  based on earlier work in the Phytomedicine Programme. Leaves of nine medicinal plant species with high antibacterial activity against  Escherichia coli  were extracted with acetone and their minimal inhibitory concentration (MIC) values determined using a microplate serial dilution technique against Gram-positive ( Staphylococcus aureus ,  Enterococcus faecalis  and  Bacillus cereus ) and Gram-negative ( Escherichia coli ,  Salmonella  Typhimurium and  Pseudomonas aeruginosa ) bacteria. Bioautography was used to determine the number of bioactive compounds in each extract. In vitro safety of the extracts was determined using the 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide reduction assay on Vero cells.    Results  The extracts were active against all the pathogens with average MICs ranging from 0.02 to 0.52 mg/ml. As expected  E. coli  was relatively sensitive, while  E. faecalis  and  S.  Typhimurium were more resistant to the extracts (average MICs of 0.28 mg/ml and 0.22 mg/ml respectively).  Cremaspora triflora  and  Maesa lanceolata  leaf extracts had higher activity than the other extracts against Gram-positive and Gram-negative pathogens with mean MICs of 0.07 mg/ml and 0.09 mg/ml respectively. Extracts of  Maesa lanceolata  and  Hypericum roeperianum  had the highest total antibacterial activity (TAA) at 1417 and 963 ml/g respectively. All extracts with the exception of that of  Maesa lanceolata ,  Elaeodendron croceum  and  Calpurnia aurea  had relatively low cytotoxicity with LC 50  > 20 μg/ml.  Cremaspora triflora  had the best selectivity index (SI) against  S. aureus  and  E. coli  of 2.87 and 1.15 respectively.  Hypericum roeperianum  had a SI of 1.10 against  B. cereus . Bioautography revealed 1–6 visible antimicrobial compounds that were generally non-polar.    Conclusions  There was a weak positive, but statistically non-significant correlation between the potency of the extracts and their cytotoxicity ( R  = 0.45,  ρ  > 0.05). The activity of the extracts on the test bacteria was in some cases not correlated with cytotoxicity, as shown by selectivity indices >1. This means that cellular toxicity was probably not due to compounds with antibacterial activity. Some of the extracts had a good potential for therapeutic use against the bacterial pathogens or for application in treating diarhoea. It does not appear that activity against  E. coli  is a good predictor of activity against Gram-negative rather than Gram-positive bacteria. Further investigation is in progress on  C. triflora  and  H. roeperianum , both of which had promising activities and potential safety based on cytotoxicity.

bmc complementary and alternative medicine

The antibacterial activity of extracts of nine plant species with good activity against Escherichia coli against five other bacteria and cytotoxicity of extracts