DISCUSSIONTheantibacterial effect of C.
macrostachyus stem bark was tested using theagar well diffusion and broth dilution methods.Each of the extracts tested in the present study displayed antibacterialactivity on all bacterial strains tested. Though, differences were observedbetween antibacterial activities of the extracts. These differences could bedue to the variations in the chemical composition of these extracts.
Inthe present study, chloroform, methanol, and water extracts of C. macrostachyusstem bark were evaluated for their antibacterial activity against Gram-negative(E. coli) and Gram-positive (S. aureus) bacteria, which are the majorimportant human pathogenic microorganisms. Antibacterial activity of each plantextract was tested by agar well diffusion and broth dilution (MIC) methods. The extracts from C. macrostachyus stem bark persuaded growthinhibition against all the studied bacterial pathogens.
Our results illustratedthat between the bacterial strains there was variation in susceptibility toextracts. This may be due to the antibacterial effect of the extract depends onthe bacterial strain and the extraction solvent used to extract thephytochemicals which contain antibacterial effect from the medicinal plant. Inthis study, C. macrostachyus stembark extracted by methanol has shown the highest inhibition zone (17+1)against S. aureus (standard)and the lowest zone of inhibition was recorded in E. coli (clinical). Itis reported that Gram-positive bacteria are more susceptible to antibiotics sincethey have only an outer peptidoglycan layer which, is an ineffective barrier (Lulekalet al.
, 2014; Karou etal., 2005). But Gram-negativebacteria have an outer phospholipidic membrane that makes the cell wallimpermeable to lipophilic solutes, whereas the porines contain a selectivebarrier to hydrophilic solutes with an elimination limit of about 600 Da (Karouetal., 2005).
In addition to this periplasmic space ofGram-negative bacteria contains enzymes, which are able to break strangemolecules and become to be less susceptible to plant extracts than the Gram-positiveones. Several research findings supported this justification,therefore extracts from some medicinal plants were found to be more effectiveagainst Gram-positive bacteria than Gram-negatives (Kelmanson et al., 2000; Masika and Afolayane,2002). The lowest inhibition zone was recorded against E.
coli which is the clinical isolate; this may be due todevelopment of resistance in the clinical isolated. Chloroformextract of the C. macrostachyus stembark was the second strong extract for its antibacterial activity and this isin agreement with Taye et al. (2011).But C. macrostachyus water extracthad lower activity against all bacteria tested. This indicates, in comparisonto water, the active ingredient which inhibits the growth of bacteria maydissolve better in methanol. However, Sendeku et al.
(2015) reported chloroform extract from C. macrostachyus leaves shows significant antimicrobial activity.Furthermore, water extract from leaves of P.
acerifolium had been reportedto have strong antimicrobial activity against several gram positive and gramnegative human pathogenic bacteria (Thatoi etal., 2008) and as stated by Dabur et al., 2007, the water extracts of A.nilotica, J. zeylanica, L. camera and S.
asoca, were found tobe the most active against different bacteria as well as fungal pathogens. Itis clear that the effectiveness of the extracts largely depends on the type ofsolvent used to extract the phenolic compound from plants. The organic extractsprovided more powerful antimicrobial activity as compared to the water extracts.This observation clearly indicates that the existence of non-polar residues inthe extracts which have higher both bactericidal and bacteriostatic abilities. Thatoiet al., 2008, mentioned that most ofthe antibiotic compounds already identified in plants are reportedly aromaticor saturated organic molecules which can easily solubilized in organicsolvents. Similar results showing that the alcoholic extract having the bestantimicrobial activity is also reported by Antarasen and AmlaBatra (2012) in Melia azedarach leafextracts Theantimicrobial analysis using the MIC value is been used by many researchers.In the present study, the MIC value of the active C.
macrostachyus stem bark extracts obtained was lower than the MBCvalues suggesting that the extracts were bacteriostatic at lower concentrationbut bactericidal at higher (Maji et al., 2010; Antarasen and Amlabatra,2012). Minimum inhibitory concentration values of 62.5–500 mg/ml. However, Jackieet al.
(2016) reported MIC value rangefrom 125-500mg/m of C. macrostachyusethanol extract against selected human pathogens.When testing methanol extractsof C. macrostachyus leaves and rootsWagate and colleagues found MICs from 15.6 to 250 mg/ml against three bacteria, E.
coli, Bacillus cereus, and Pseudomonas aeruginosa.