Many plant species of the genus Dorstenia (Moraceae) are used for medicinal purposes in Africa, Middle East, Central and South America. African Dorstenia species has yielded a variety of mono-, di-, and triprenylated and also mono- and digeranylated flavonoids with interesting pharmacological properties 1234. Dorstenia mannii Hook f. (Moraceae) is a perennial herb growing in the tropical rain forest of West Africa 5. A decoction of the leaves is used for the treatment of many diseases, but mainly for rheumatism and stomach disorders 6. There are few pharmacological studies reported on D. mannii. However, prenylated flavonoids isolated from D. mannii such as 6,8-diprenyleriodictyol (5), dorsmanin C (3) and dorsmanin F (7) were found to be potent scavengers of the stable free radical 1,1-diphenyl-2-picrylhydrazyl 7. Compounds 3, 5 and 7 also inhibited Cu²⁺-mediated oxidation of human low density lipoprotein 7. In our continuous search of bioactive compounds from the genus Dorstenia, the present work was designed to evaluate the antimicrobial potency of the methanol extract and compounds isolated from D. mannii.

The tested compounds were isolated from DMT and identified as previously described as dorsmanin A (1), B (2), C (3), D (4) and 6,8-diprenyleriodictyol (5) 10, dorsmanin E (6), F (7), G (8) 11 and dorsmanin I (9) 12. These compounds together with the crude methanol extract were tested for their antimicrobial activities against bacteria and yeasts and the results are reported in Tables 1 and 2.

^aTested samples [DMT: methanol extract from the twigs of Dorstenia mannii; dorsmanins A(1), B(2), C(3), D(4) E(6), F(7), G(8), I (9) and 6,8 diprenyleriodictyol (5); ^bRA : reference antibiotics were chloramphenicol for bacteria, nystatin for C. albicans, ciprofloxacin for M. smegmatis, isoniazid for M.tuberculosis; (−): MIC > 1024 μg/ml

^aTested samples [DMT: methanol extract from the twigs of Dorstenia mannii; dorsmanins A(1), B(2), C(3), D(4) E(6), F(7), G(8), I (9) and 6,8 diprenyleriodictyol (5); ^bRA : reference antibiotics were chloramphenicol for bacteria, nystatin for C. albicans, ciprofloxacin for M. smegmatis, isoniazid for M.tuberculosis; (−): MIC > 1024 μg/ml; (ND): not determined as MIC was >1024 μg/ml

The results of the MIC determinations (Table 1) demonstrated that the methanol extract as well as compounds 3 and 8 were able to prevent the growth of all the fourteen studied microorganisms, including mycobacteria, yeast and Gram-negative bacteria, within the concentration range of 4 to 1024 μg/ml. Other compounds showed selective activities, their inhibitory effects being noted on 12/14 (85.7%) studied pathogens for compound 7, 7/14 (50%) for compound 6, 6/14 (42.9%) for compounds 9, 5/14 (35.7%) for compounds 2 and 5, 3/14 (21.4%) for compounds 1 and 4. The lowest MIC value for the methanol extract (64 μg/ml) was obtained on C. albicans. The lowest value for individual compounds (4 μg/ml) was recorded with compounds 3 on P. aeruginosa PA01 and 7 on E. coli ATCC strain. The corresponding values for the RA ranged from 0.5 to 64 μg/ml, P. aeruginosa PA01 and M. tubercolosis MTCS1 being the least sensitive. Results of MMC determinations (Table 2) also showed good activities for some of the tested samples such as compounds 3 and 8. MMC values not greater than 1024 μg/ml were recorded on all studied microorganisms with compounds 3 and 8, on 12/14 (85.7%) studied organisms for compound 7, 9/14 (64.3%) for the crude extract, 4/14 (28.6%) for compound 9, 3/14 (21.4%) for compounds 5 and 6, 2/14 (14.3%) for compounds 1 and 2, 1/14 (7.1%) for compound 4.

The compounds isolated from D. mannii and tested herein were all flavonoids. This class of compounds is very common in the genus Dorstenia101112 and their antimicrobial activities were also reported 234. In the present work, broad spectrum of antimicrobial activities was recorded with the crude extract and compounds from D. mannii. Phytochemicals are routinely classified as antimicrobials on the basis of susceptibility tests that produce MIC in the range of 100 to 1000 μg/ml 18. Activity is considered to be significant if MIC values are below 100 μg/ml for crude extract and moderate when the MIC values vary from 100 to 625 μg/ml 1920. Therefore, the activity recorded with the crude extract on C. albicans can be considered as important. Also, compounds with significant activities (MIC < 10 μg/ml) on at least one of the studied organisms include 3, 6, 7 and 8. The MIC values recorded with compounds 3 on P. aeruginosa PA01, 6 on C. albicans,7 on P. aeruginosa PA01 and K. pneumoniae ATCC strain, and C. albicans,8 on P. aeruginosa PA01, PA124, P. stuartiiM. tuberculosis MTCS1 were lower than or equal to those of the reference drugs, highlighting their interesting activities. This observation is in consistence with previous work on flavonoids isolated from the genus Dorstenia. In fact, isobavachalcone, 4-hydroxylolonchocarpin, kanzonol C, stipulin, and many other flavonoids isolated from this genus were reported for their good antimicrobial potencies, with MIC values below 10 μg/ml on several tested microorganisms 23421. A Keen look at the MMC values indicates that most of them are not more than fourfold their corresponding MICs. This proves that the killing effects of many tested samples could be expected on the sensitive strains 22. The continuous emergence of multidrug-resistant (MDR) bacteria drastically reduces the efficacy of our antibiotic armory and, consequently, increases the frequency of therapeutic failure 23. MDR Enterobacteriaceae, including K. pneumoniae, E. aerogenes and E. coli have also been classified as antimicrobial-resistant organisms of concern in healthcare facilities 24. Besides, K. pneumoniae KP55 tested herein was reported to be resistant to most of the commonly used antibiotics, showing high levels of resistance to ampicillin, ceftazidime, and aztreonam with MIC values up to 512 μg/ml 25. In addition Pseudomonas aeruginosa has emerged as one of the most problematic Gram-negative pathogens, with the alarmingly high antibiotics resistance rates 26. The good activities of compounds 3 and 8 on most of the tested strains belonging to MDR phenotypes such as E. coli AG100, P. aeruginosa PA124, E. aerogenes CM64, K. pneumoniae KP55 as observed herein reinforce the hypothesis that these compounds are natural products with interesting antimicrobial potencies.

Tuberculosis (TB) is widely expanded in poor countries with the highest incidence (more than 80% of cases) occurring in Asia and Africa 27. Annual incidence of TB (over 600 cases per 100 000) has been reported in many sub-Saharan African countries 28. In this work, only compounds with inhibitory activity on M. smegmatis were tested on M. tuberculosis. However, it has been demonstrated that the sensitivity of M. tuberculosis is closer to that of M. smegmatis, a non pathogenic microorganism 29. Therefore, this microorganism can be used for a preliminary study to select samples with potential activity against M. tuberculosis29. Hence, the results obtained herein are in accordance with such recommendation.

To the best of our knowledge, the antimicrobial activity of D. mannii as well as that of the isolated compounds is being reported for the first time. However 6,8-diprenyleridictyol (5) an,dorsmanin F (7) were reported for their antitrichomonal activities 30.

The data reported herein are very important, taking into account the medical importance of the studied microorganisms. Hence, the overall results of the present investigation provide evidence that the crude extract of D. mannii as well as some of its compounds such as compounds 3 and 8 could be considered as interesting natural antimicrobial products.

The authors declare that they have no competing interests.

ATM, VK and BN carried out the study; ATM and VK wrote the manuscript; VK, BTN, VPB, JJMM and NL supervised the work. All authors read and approved the final manuscript.