Antibiotics for today might not be effective tomorrow as microbes are tirelessly developing mechanisms of resisting antibiotics (Cashel & Raxlen, 2015; White, Duncan & Baumle, 2012, p. 75). The evolution of antimicrobial resistance in microganisms can be intrisic or acquired. Natural resistance is attained by gene mutation (Hemaiswarya et al., 2008) while the acquired resistance occurs through acquistion of a new DNA fragement such
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as transposons, plasmids, integrons from another microorganism (Van Hoek, Mevius, Guerra, Mullany, Roberts & Aarts, 2011) The DNA fragements can be transferred within the same or different species of microorganism.
Microorganisms become resistant to antimicrobial agents through several mechanisms such as receptor or active site modification, enzymatic degradation and modification of the drug, decreased outermembrane permeability, active efflux (Van Hoek et al., 2011; Hemaiswarya et al., 2008) acquisition of alternative metabolic pathways to those inhibited by the drug, increased production of the target enzyme (Van Hoek et al., 2011).
2.8.1 Receptor or active site modification
For an antimicrobial agent to be effective, it should be made in a such way that it can interfere with its critical targets in the microbe. Introduction of mutation at the molecular target lessens the activity of the drug, rendering the drug ineffective (Hemaiswarya et al., 2009). Hemaiswarya et al. (2008) believe that mutations in RNA polymerase and DNA gyrase have contributed to unfavourable therapeutic results with rifamycin and quinolones. Another worrisome problem is the structural modulation of the penicillin- binding proteins (PBPs) by the penicillin resistant microbes (Hemaiswarya et al., 2008). Nevertheless, studies have shown that some SM such as Corilagin when combined with oxacillin and cefmetazole inhibits PBP2a production in methicillin resistant S. aureus (MRSA) (Hemaiswarya et al., 2008).
2.8.2 Enzymatic degradation and modification of the drug
Microbes continue to pose threat by synthesising enzymes that modulate the antimicrobial agent. The modulation involves hydrolysis, redox reaction and group transfer of the active group (Hemaiswarya et al., 2008).
Chemical bonds of the antimicrobial agent such as ester bonds or amide bonds are cleaved by the enzymes synthesised by the resistant microbe leading to formation of an inactive compound. Redox reaction involves oxidation and reduction of the antimicrobial agent rendering the drug impotent. Modulation of the active group include acylation,
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phosphorylation, glycosylation, nucleotidylation, rebosylation by the microbe rendering the drug ineffective (Hemaiswarya et al., 2008).
However numerous studies have indicated SM as resistance modulating compounds (Upadhyay et al., 2014; Wagner & Ulrich-Merzenich, 2009). For example, a natural product Epigallocatechin gallate (EGCg) when combined with penicillin inhibits penicillinase in S. aureus rendering penicillin effective (Hemaiswarya et al., 2008). 2.8.3 Decreased outer membrane permeability
All bacteria possess cell wall except mycoplasma. The cell wall is made up of peptidoglycan which provide rigidity to the bacteria. All Gram-negative bacteria have an additional layer consisting of lipopolysaccharide (LPS) molecules which act as a permeability barrier for many hydrophobic substances such as detergents, hydrophobic dyes and antimicrobial agents (Hemaiswarya et al., 2008). In addition, the LPS prevent the entry of toxic host defence factors such as lysozyme, β-lysin and various leukocyte proteins. Certain Gram-negative bacteria have been reported to possess glycosphingolipids instead of LPS (Hemaiswarya et al., 2008).
Hemaiswarya et al. (2008) and Helander, Alakomi, Latva-kala, Mattila-Sandholm, Pol, Smid, Gorris and Von Wright (1998) highlight that plant SM such as thymol and carvacrol act as membrane permeabilisers enhancing the uptake of drugs and other hydrophobic antimicrobial agents.
2.8.4 Efflux of the antimicrobial agent from the cell wall
Microbes have developed multidrug resistance pumping systems (MDRPs) that inhibit antimicrobial agents from permeating the bacteria through the cell membrane or extruding the antimicrobial agents that have already penetrated into the microbe. The efflux pumping systems utilise ATP hydrolysis or ion gradient to extrude the antimicrobial agents out of the microbial cell (Hemaiswarya et al., 2008).
Hemaiswarya et al., (2008) highlights five major classes of MDRPs namely, the adenosine triphosphate (ATP)-binding cassette (ABC) superfamily, the major facilitator superfamily (MFS), the small multidrug resistance family (SMR), the resistance-
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nodualtion-cell division (RND) superfamily and the multidrug and toxic compound extrusion (MATE). Typical classes of MDRPs in prokaryotic cells are RND, SMR and MATE (Hemaiswarya et al., 2008).
Plant SM have been reported to block the MDRPs and potentiate the activity of certain antibiotics. For instance, two diterpenes from Lycopus europaeus have been reported to potentiate the activities of tetracycline and erythromycin against two strains of S. aureus by blocking MDRPs Tek (K) and Msr (A) (Hemaiswarya et al., 2008).
2.8.5 Use of alternative pathway to bypass the sequence inhibited by the agent
Resistant microbes may either utilise an alternative metabolic pathway to bypass the sequence inhibited by the agent or increase the production of the target metabolite (Akpan, Odeomena, Nwachukwu & Danladi, 2012). For example, in fungi ergosterol is the main sterol of fungi and it maintains the integrity and fluidity of cell membranes. Azole compounds inhibit cytochrome P450 enzyme which is involved in 14α-demethylation of lanosterol in Saccharomyces cerevisiae and 24-methylenedihydrolanosterol in majority of the fungus and thus blocking ergosterol biosynthesis (Kelly, Lamb, Kelly, Manning, Loeffler, Hebart, Schumacher & Einsele, 1997).
Some fungal strains have been reported to resist azole compounds. Inhibition of ergosterol biosynthesis leads to depletion of ergosterol and increased levels of substrates and synthesis of abnormal sterols without removal of 14α-methyl group such as 14α- methyl-3,6-diol (Kelly et al., 1997). In a normal fungal cell 14α-methyl-3,6-diol is toxic and its accummulation can lead to growth arrest of a fungal cell (Kanafani & Perfect, 2008; Kelly et al., 1997). The compound, 14α-methyl-3,6-diol is synthesised from14α-methyl- fecosterol in a chemical reaction catalysed by the enzyme sterol ∆5,6 desaturase (Kelly et
al., 1997). Mutation of ERG3 gene negates the function of sterol ∆ 5,6 desaturase leading
to accumulation of 14α-methyl-fecosterol. In ergosterol deficiency, 14α-methyl-fecosterol maintains the fungal membrane functional (Kanafani & Perfect, 2008; Kelly et al.,1997). Given that other major antifungals such as amphotericin B interrupt cell membrane through binding to ergosterol, cross resistance may occur rendering amphotericin B ineffective (Kelly et al., 1997).
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Unlike azole compounds which has ergosterol as a molecular target (Kanafani & Perfect, 2008; Kelly et al., 1997), plant SM possess multitarget activities which can be used in fungal therapy (Dhamgaye, Devaux, Vandeputte, Khandelwal, Sanglard, Mukhapadhyay & Prasad, 2014).