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Pharmaceutics 2020, 12, 1044 7 of 19 3. Bacterial Resistance and Mutations Antimicrobial resistance is a natural, intrinsic phenomenon, which can also be acquired or transferred in an effort to escape the actions of antimicrobial agents. Bacterial species have capabilities to resist or reduce the effect of antibiotic due to their natural inherent functional or structural features. [64,65]. The evolution of drug resistance occurs in a minimum of three phases namely, acquisition, expression and selection for microbes expressing those resistance genes. Foremost, bacteria gain resistance to one or more drugs by transduction, transformation and conjugation, which happens via horizontal gene transfer (HGT). Such antimicrobial agents threatened by HGT are β-lactams, fluoroquinolones, etc. [65–68]. Another way in which bacteria acquire a resistance gene is through spontaneous mutation of existing genes [69,70]. Multiple drug resistance (MDR) happens when bacteria with an existing drug resistance gene acquires resistance to another drug [65]. Secondly, in defense against exposure to antimicrobials, bacteria express the resistance gene [67]. Thirdly, resistance becomes prevalent when there is a suitable environment of growth for microorganisms that express resistance genes against the antibiotic. This conditional/selective pressure happens when the microorganisms are exposed to the antibiotic without elimination either by bactericidal or bacteriostatic effects of the antibiotic itself [64,66]. Use of a time-dependent antibiotic with long half-life and poor patient compliance can create the selective pressure that aids drug resistance, and the likelihood of occurrence is increased by prolonged use of the antibiotics. The likelihood of developing resistance increases when antimicrobial drugs are used for a longer duration [67,71]. Bacteriostatic drugs, which do not kill bacteria but inhibits them, gives an opportunity for the regrowth of some bacterial cells and thus they develop resistance when exposed to the drug. An insufficient number of doses or missed scheduled doses (as a result of poor patient compliance) gives ample time for the development/acquisition of resistance genes [72]. Bacteria utilize several mechanisms for resisting antimicrobials. Of such, the mechanism is the decreased uptake and increased efflux of the drug from the bacterial cell. This happens by the transmembrane efflux pump that prevents the antimicrobial agent from attaining the toxicity level within the bacterial cell [7,71]. The low sensitivity of P. aeruginosa and E. coli to antibiotics is due to their drug efflux system. Both are Gram negative bacteria having a distinct outer membrane enclosing a periplasmic space. This periplasmic space contains a peptidoglycan cell wall that envelopes an inner membrane (Figure 4). It was reported that the drug efflux pump of P. aeruginosa contains an inner membrane H+/drug antiporter protein bound to a linker protein in the periplasmic space, which itself is bound to an outer membrane channel protein [73]. An over expression of these efflux proteins was seen in P. aeruginosa. This is usually seen when there is a mutation of the regulatory protein that is required to suppress genes coding for efflux proteins [73]. E. coli also uses the mechanism of drug efflux. It expresses a minimum of at least nine pumps whose energy source to expel different types of antibiotics is the transmembrane proton gradient. Thus, conferring multidrug resistance to E. coli. This drug efflux system is commonly seen in Gram −ve bacteria because their additional outer membrane consists of a lipopolysaccharide compared with Gram +ve bacteria with a peptidoglycan cell wall surrounding only a single plasma membrane (Figure 4). This explains why Gram −ve bacteria are less susceptible to many antibiotics compared with Gram +ve bacteria [20,67,73]. Other mechanisms of antibiotic resistance are by enzymatic inactivation of the antibiotic, covalent modification of the drug, mutation of antibiotic targets, protection of targets, etc., as seen in the case of methicillin resistant Staphylococcus aureus (MRSA) and Klebsiella pneumoniae [67,69,73].PDF Image | Bactericidal Antibacterial Mechanism of Plant Nanoparticles
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