According to the result of this study, antimicrobial activity was assessed by inhibition diameters which were found to range from 8 - 21.5 mm for the two extracts against all the bacterial strains tested. The M. germanica extract showed the more effective impact on the growth S. pyogene and L. innocua than E. aerogenes and K. pneumoniae. The results showed 2 mg/mL concentration of ethanolic extract, were quite effective on reduce of growth S. pyogenes, L. innocua, E. aerogenes and K. pneumonia and were had prevent growth over the medium. The aqueous extract, only had antimicrobial effect in 2 mg/mL concentration on growth of, S. pyogenes and L. innocua.
Many plant extracts owe their potency to the presence of substances such as tannins, phenolics, micro elements, essential oils, peptides, unsaturated long chain aldehydes, alkaloids and so on. These substances are usually found in various parts of the plants like roots, leaves, shoots and bark [
18]. Many plants have therefore become sources of important drugs and the pharmaceutical industries have come to consider them as a source of bioactive agents that can be used in the preparation of synthetic medicine [
19].
The results of the present study clearly showed that
M. germanica extracts showed antibacterial activity against tested pathogenic bacterial strains including antibiotic resistant strains. The effectiveness of the active compounds present in plant extracts cause the production of growth inhibition zones that appear as clear areas surrounding the disk. Antibacterial activity may be due to active components which are present in plant extracts. However, some plant extracts were unable to exhibit antibacterial activity against tested bacterial strains. These bacterial strains may have some kind of resistance mechanisms e.g. enzymatic inactivation, target sites modification and decrease intracellular drug accumulation [
20] or the concentration of the compound used may not be sufficient. No inhibition was observed with controls, which proves that solvents could not act as antibacterial agents. Further investigations were done using extracts of
M. germanica only, since they showed considerable antibacterial activity. In almost all tests, ethanol extract showed better inhibition against all tested bacterial strains, indicating that active ingredients in plant materials could be extracted into ethanol. However, highest antibacterial activity was observed against
S. pyogenes.
On the basis of the above results, it showed that ethanol extract of
M. germanica exhibited a greater inhibition compared with aqueous extract. Parekh et al. reported that most of the antimicrobial active compounds were soluble in polar solvent such as alcohol instead of water [
21]. This result is comparable to the study by de Souza et al. using alcohol extract of
L. sibiricus that showed effective antibacterial activity on
Bacillus subtilis [
22]. Bajwa and Shafique showed that methanol fraction of
A. rabiei exhibited more promising results in suppressing the fungal growth rather than aqueous extract [
23]. This was also reported by Zafar et al. where chloroform extract of
Melia azedarch leaves was active against
Fusarium chamdosporum while water extract of the leaves did not show any positive results [
24].
In this study, by increasing the amount of alcoholic extract on disc diffusion method, inhibition zone around the disc was increased. Various sizes of inhibition zones were measured by a ruler in millimeters. The maximum size of inhibition zone for M. germanica ethanol extract in the disc method was 21.5 mm (40 mg/mL) and M. germanica aqueous extract was 20 mm (40 mg/mL), respectively. The results show that ethanolic and aqueous extract of M. germanica in all concentrations (10, 20, 30 and 40 mg/mL) had the inhibitory effect on S. pyogenes, L. innocua, E. aerogenes and K. pneumonia.
The results of MIC and MBC assay revealed that
M. germanica have more antibacterial effect on Gram-positive bacteria compared to Gram-negative bacteria. So that Gram-positive bacteria such as
S. pyogenes and
L. innocua and Gram-negative bacteria such as
E. aerogenes and
K. pneumonia showed lowest and highest MIC, respectively. The MIC of ethanolic extract of
M. germanica for
S. pyogenes,
L. innocua,
E. aerogenes and
K. pneumonia were 2, 4, 32 and 32 mg/mL, respectively. But MIC of the aqueous extract of
M. germanica for
S. pyogenes,
L. innocua,
E. aerogenes and
K. pneumonia were 4, 8, 64 and 32 mg/mL, respectively. The MBC of ethanolic extract of
M. germanica for
S. pyogenes,
L. innocua,
E. aerogenes and
K. pneumonia were 4, 8, 64 and 64 mg/mL, respectively. But MBC of the aqueous extract of
M. germanica for
S. pyogenes,
L. innocua,
E. aerogenes and
K. pneumonia were 8, 16, 128 and 64 mg/mL, respectively. This is probably due to presence of lipopolysaccharide in cell wall of Gram-negative bacteria. Lipopolysaccharides of cell wall can prevent influx of active compounds to cytoplasmic membrane of these bacteria [
25]. In study of Khalil and Li reported the sage essential oil was effective against both Gram positive (
Streptococcus) and Gram negative bacteria (
E. coli,
S. typhi) and the antibacterial effect against Gram positive bacteria was more than Gram negative bacteria which had similar result with this study [
26]. Arias et al. investigated the antibacterial effects of aqueous and ethanolic extract of Acacia aroma They reported that Gram positive bacteria (
Bacillus cereus and
Bacillus Subtlis) are more sensitive than Gram negative bacteria (
Escherichia coli) to plant extracts. This result is consistent with the findings of this study [
27]. Also, they reported that ethanolic extract compared to the aqueous extract was more effective and has a greater inhibitory effect [
27]. These results are consistent with reported by Karsha and Laskhmi, that
Piper nigrum L. extract is more susceptible to Gram positive compared against Gram negative. It might because
P. nigrum L. extract altered the membrane permeability results in the leakage of nucleic acid and protein into the extracellular medium [
28]. The results of this study provide informative data for the use of the extracts from
M. germanica against bacterial microbial infections. However, the clinical application of this plant are needed more and larger studies and if successful and standardization of results. These plants can use as an alternative instead of inert and ineffective anti microbial drugs currently used.