In vitro antibacterial and antibiotic-potentiation activities of the methanol extracts from Beilschmiedia acuta, Clausena anisata, Newbouldia laevis and Polyscias fulva against multidrug-resistant Gram-negative bacteria

Phytochemicals are routinely classified as antimicrobials on the basis of susceptibility
tests that produce MICs in the range of 100 to 1000 ?g/mL 27]. Moreover, for crude extracts, the antimicrobial activity is considered to be significant
if MIC values are below 100 ?g/mL and moderate when 100?MIC?625 ?g/mL 28], 29]. Therefore, the activity recorded with B. acuta bark extract against the 26 tested bacterial strains can be considered as very important.
If we consider the alternative criteria described by Fabry et al.30], where extracts having MIC values less than 8000 ?g/mL have noteworthy antimicrobial
activity, the overall activity recorded with the leaves and fruit extracts of B. acuta, P. fulva and N. laevis leaves extracts can also be considered promising. A keen look of the results of MIC and MBC determinations (Table 3, Additional file 1: Tables S1 and S2) indicates that MBC/MIC ratios were mostly above four, suggesting
that studied extracts, including the most active ones, generally displayed bacteriostatic
effects (MBC/MIC??4) 31]–33]. Various classes of phytochemicals (Table 2) were previously detected in the extracts of the four tested plants 10] and this may explain their antibacterial activity.

The results obtained in this study, and mostly those obtained with the bark of B. acuta are very important when taking in consideration the fact that most of the bacterial
strains used were MDR phenotypes expressing active efflux pumps 7]–9], 34], 35]. In fact, the activity of antibiotics against the studied MDR bacteria was previously
found to increase in the presence of phenylalanine arginine ?-naphthylamide (PAßN),
a potent inhibitor of RND efflux systems, particularly AcrAB–TolC (of Enterobaceriaceae)
and MexAB–OprM (of Pseudomonas species) 7]–9], 34], 35]. In the present study, we demonstrated that beneficial effects when combining four
of the tested plant extracts [namely those from B. acuta (leaves and bark), N. leavis (leaves) and P. fulva (leaves)] with the first line antibiotics could be achieved. High percentages of
synergistic effects (100 %) obtained with B. acuta bark extract and TET as well as P. fulva leaves extract in combination with TET and KAN, clearly suggest that such associations
could improve the fight against MDR bacterial infections. This also suggests that
some of the constituents of the corresponding plants can act as efflux pump inhibitors,
as more than 70 % synergistic cases were observed with many combinations 26].

The antimicrobial potential of the genus Beilschmiedia has previously been documented. Chouna et al. 36] demonstrated that compounds such as beilschmiedic acid C isolated from B. anacardioides were significantly active against Bacillus subtilis, Micrococcus luteus and Streptococcus faecalis. Beilschmiedia cinnamomea was previously reported to have significant to moderate activities (64–1024 ?g/mL)
against the MDRGN tested in this work 7]. Beilschmedia obscura was also found to show a good and large spectrum of antibacterial activity against
MDRGN 37]. Some compounds previously isolated from the genus Beilschmiedia and belonging to alkaloids, phenols, saponines, sterols and triterpenoids 36], 38] were shown to possess antimicrobial activities 7]. The genus Beilschmiedia is also known traditionally to possess antimicrobial activities 7]. Beilschmedia acuta tested in this study is also used in Cameroon to treat gastrointestinal infections
10]. The obtained data highlight the importance of this plant in the control of microbial
infections and mostly those involving MDR phenotypes. The antimicrobial activities
of extracts and compounds from Newbouldia laevis towards sensitive bacteria and fungi were also reported 39], 40], and the present study provides additional data on the potential of this plant to
fight MDR bacteria. Also, the antimicrobial activity of essential oil from Clausena anisata was reported against Staphylococcus aureus, Streptococcus pneumoniae, Enterococcus species, Salmonella typhimurium and Pseudomonas aeruginosa41], 42]. The present report provides more evidence of the antimicrobial potential of this
plant.