Bacteria were evolved with the origin of the earth and it can be an inherent trait that renders it natural resistance, or it may be acquired by means of mutation in its own DNA or acquisition of resistance-conferring DNA from another source (Levin and Rosen, 2006). The intrinsic resistance of a bacterial species to a particular antibiotic is the ability to resist the action of that antibiotic as a result of inherent structural or functional characteristics (Aminov and Mackie, 2007).
In addition to intrinsic resistance, bacteria can acquire or develop resistance to antibiotics (Jain et al., 2003). This can be mediated by several mechanisms such as, antibiotic efflux pumps, post transitional modifications by genetic mutation and inactivation of the antibiotic by hydrolysis (Blair et al., 2014).
Bacterial efflux pumps actively transport many antibiotics out of the cell. When overexpressed, efflux pumps can also confer high levels of resistance to previously clinically useful antibiotics (Webber and Piddock, 2003). Compared with Gram-positive species, Gram-negative bacteria are intrinsically less permeable to many antibiotics as their outer membrane forms a permeability barrier (Pages et al., 2008).
Post translational modification is another mechanism of modification of targets. For an instance, antibiotic resistance to aminoglycosides occurs through modification of the target ribosome by methylation (Mingeot-Leclercq et al., 1999). Inactivation of antibiotics by hydrolysis is a major mechanism of antibiotic resistance. Thousands of enzymes that can degrade and modify antibiotics of different classes, including ?-lactams, aminoglycosides and macrolides have been identified (Wright, 2005). Some enzymes modify the antibiotic molecule by adding chemical groups to its vulnerable sites and prevent the antibiotic from binding to its target protein, as a result of steric hindrance (Davies, 1994).
1.7. Factors affecting Antibiotic Resistance
When antibiotics are underused, overused or misused, antibiotic resistance will be increased (Odonkor and Addo, 2011). The indiscriminate use of antibiotics resulted from patients’ incompliance to recommended treatment and demand, prescribers’ errors, drug advertisements, poor quality antibiotics, inadequate surveillance and susceptibility testing, exert a huge impact on antibiotic resistance (Griffin, 2009). Irrational use of antibiotics by patients was identified as one of the major cause of microbial resistance to antibiotics (Mao et al., 2015).
Some patients also misuse the antibiotics due to the fact that it is readily available in the pharmacy and they can buy it over the counter without a prescription (self-medication) which may be unnecessary and/or inadequate dose (Sullivan et al., 2001).
As a result of poor infection control practices like hand washing, changing gloves, face masks, cannulas etc., hospitals and clinics also contribute to enhance antibiotic resistance (Kümmerer, 2001). Poor quality of antibiotics has been increased, as a result of the use of expired and counterfeit antibiotics, due to lack of quality control and monitoring in pharmaceutical industry.
Irrational use of antibiotics in animals for their growth and disease control causes to predispose antibiotic resistance and these antibiotics can indirectly enter into humans when consuming such animals (Xu et al., 2014).
1.8. Preventive measures for antibiotic resistance
Physicians, for their part, can take some immediate steps to minimize any resistance ensuing rational prescribing of antibiotics. When possible, they should try to identify the causative pathogen before beginning the therapy, so they can prescribe an antibiotic targeted specifically to that microbe instead of having to choose a broad-spectrum product (Bonhoeffer et al., 1997). When patients receive prescriptions for antibiotics, they should complete the full course of therapy (to ensure that all the pathogenic bacteria die) and should not “save” any pills for later use (Tomson and Angunawela,1990).
The farmers should be helped to find inexpensive alternatives for encouraging animal growth and protecting crops (Thornton, 2010).
World Health Assembly in 2015 adopted a global action plan on antimicrobial resistance, which outlines five objectives:
1. Improve awareness and understanding of antimicrobial resistance through effective communication, education and training;
2. Strengthen the knowledge and evidence base, through surveillance and research;
3. Reduce the incidence of infection through effective sanitation, hygiene and infection prevention measures;
4. Optimize the use of antimicrobial medicines in human and animal health;
5. Develop the economic case for sustainable investment that takes account of the needs of all countries and to increase investment in new medicines, diagnostic tools, vaccines and other interventions (World Health Organization, 2015).
These evidences clearly emphasize the necessity of researches to investigate novel methods in order to minimize the occurrence of antibiotics and antibiotic resistant organisms in the environment.
1.9 Antibiotic degradation
The physico-chemical properties of antibiotic (eg:- molecular structure, size and shape) will define their distribution in the environmental matrices (solids or water). Most conventional water treatment processes are not designed for the treatment of wastewater containing highly polar contaminants such as detergents and pharmaceuticals (Bergland et al., 2014). Therefore, practical and economical solutions must be achieved in order to reduce the daily amounts of antibiotics discharged into the environment.
A wide range of chemical and physical methodologies can be employed for the removal of antibiotics (Bergland et al., 2014) where methods such as absorption, incineration, oxidation-reduction, photolysis, hydrolysis, reverse osmosis and chemical degradation are available at present for removing antibiotics from waste water (Ding et al., 2016). However, the real applicability of such techniques are expensive and inaccessible in most part of the world, especially in developing countries.
Bioremediation is an economically visible, cost effective green technology which may lead to degradation of antibiotics that produce simple compounds such as carbon dioxide, water nitrogen and organic materials during the microbial degradation processes (Li and shang, 2010; Sturini et al., 2012). Therefore, many number of scientific reports showed promising environmental friendly antibiotic degradation methods using native aquatic bacteria (Li and shang, 2010; Sturini et al., 2012).