Antibiotics as chemotherapeutic agents are released in large quantities in natural
ecosystems and exist at low concentrations in the environment due to the stability of
antibiotic precursors or their metabolic compounds which readily pass through water
treatment systems and leak into the ecosystem (Singer et al. 2002; Ferber 2003;
Jjemba 2006; Sarmah et al. 2006; Kümmerer 2009). Antibiotics are administered in
the treatment of human-infectious diseases as well as prophylactically to enhance
yield in the livestock farming, or non-target organisms, such as degradation of
pollutants and nutrient cycling (Flaherty and Dodson 2005). The native organisms
can interact with the antibiotics that entered the aquatic ecosystems and can then alter
and modify the microbial community genetically and structurally (Singer et al.
2016). The extensive use of chemotherapeutic drugs and the continuous exposure
of environment to these drugs can boost the selection of potentially resistant
pathogens, in other words, some species that are not directly affected by antibiotics
can be affected by antibiotic-indirect cascade effect, when feed on microorganisms
(Kümmerer 2004; Cabello 2006). Most notably, the environmental microorganisms
produce several antibiotics, whereas antibiotic resistance genes come from pathogenic bacteria via horizontal gene transfer (HGT) from environmental microorganisms as well; however, treatment of infections may generate a strong pressure for
antibiotic selectivity (Kümmerer 2004). The prevalence and persistence of bacteria
in the presence of antibiotics is due to the selective pressure that modifies their
genetic requirements and generates antibiotic-resistant bacteria (ARB), which act as
an ultimate threat for public health. ARBe not only have the action for proliferation,
but also have the ability to transfer resistance genes to sensitive bacteria (Martinez
et al. 2007). Antimicrobial-resistant bacteria possess several mechanisms to get rid
of the effect of these drugs such as secretion of enzymes that are able to degrade the
antibiotics, efflux pumps which are able to repel the drugs out of the bacterial cells,
modify bacteria cell wall to prevent the electrostatic interaction of antibiotics with
host cell wall, and downregulation of porins which in turn prevent antibiotics from
cross the cell, etc. (Bird et al. 2019).
Generally, antibiotics can be classified according to many different aspects such
as the mode of action, chemical structure, and the way of entrance (by injection, oral
and/or topical); however, the mechanism of action and activity spectrum are considered to be the most common ways to classify antibiotics to β-lactams,
carbapenem, aminoglycoside, macrolides, sulfonamides, quinolones, tetracycline,
glycopeptides, and fluoroquinolone (Calderon and Sabundayo 2007; Vila et al.
2007; van Hoek et al. 2011; Frank and Tacconelli 2012). According to WHO
classification (2019), antibiotics are further classified based on the importance of
their appropriate use, and to be used as a tool to better support antibiotic monitoring
and activities management in countries. Table 3.1 represents the WHO classification
of antibiotics and their mode of action in some details (Adzitey 2015).
The β-lactam antibiotics activity is restricted in their 3-carbon and 1-nitrogen ring
in the chemical structure, by which are able to destroy the bacterial cell wall by
interfering with proteins during peptidoglycan synthesis. The modification of betalactam drugs side chain increases the ability to evade the degradative capacity of
enzymes produced by some bacterial species and make the movement of antibiotics
42
H. H. Al-Haideri et al.
ecosystems and exist at low concentrations in the environment due to the stability of
antibiotic precursors or their metabolic compounds which readily pass through water
treatment systems and leak into the ecosystem (Singer et al. 2002; Ferber 2003;
Jjemba 2006; Sarmah et al. 2006; Kümmerer 2009). Antibiotics are administered in
the treatment of human-infectious diseases as well as prophylactically to enhance
yield in the livestock farming, or non-target organisms, such as degradation of
pollutants and nutrient cycling (Flaherty and Dodson 2005). The native organisms
can interact with the antibiotics that entered the aquatic ecosystems and can then alter
and modify the microbial community genetically and structurally (Singer et al.
2016). The extensive use of chemotherapeutic drugs and the continuous exposure
of environment to these drugs can boost the selection of potentially resistant
pathogens, in other words, some species that are not directly affected by antibiotics
can be affected by antibiotic-indirect cascade effect, when feed on microorganisms
(Kümmerer 2004; Cabello 2006). Most notably, the environmental microorganisms
produce several antibiotics, whereas antibiotic resistance genes come from pathogenic bacteria via horizontal gene transfer (HGT) from environmental microorganisms as well; however, treatment of infections may generate a strong pressure for
antibiotic selectivity (Kümmerer 2004). The prevalence and persistence of bacteria
in the presence of antibiotics is due to the selective pressure that modifies their
genetic requirements and generates antibiotic-resistant bacteria (ARB), which act as
an ultimate threat for public health. ARBe not only have the action for proliferation,
but also have the ability to transfer resistance genes to sensitive bacteria (Martinez
et al. 2007). Antimicrobial-resistant bacteria possess several mechanisms to get rid
of the effect of these drugs such as secretion of enzymes that are able to degrade the
antibiotics, efflux pumps which are able to repel the drugs out of the bacterial cells,
modify bacteria cell wall to prevent the electrostatic interaction of antibiotics with
host cell wall, and downregulation of porins which in turn prevent antibiotics from
cross the cell, etc. (Bird et al. 2019).
Generally, antibiotics can be classified according to many different aspects such
as the mode of action, chemical structure, and the way of entrance (by injection, oral
and/or topical); however, the mechanism of action and activity spectrum are considered to be the most common ways to classify antibiotics to β-lactams,
carbapenem, aminoglycoside, macrolides, sulfonamides, quinolones, tetracycline,
glycopeptides, and fluoroquinolone (Calderon and Sabundayo 2007; Vila et al.
2007; van Hoek et al. 2011; Frank and Tacconelli 2012). According to WHO
classification (2019), antibiotics are further classified based on the importance of
their appropriate use, and to be used as a tool to better support antibiotic monitoring
and activities management in countries. Table 3.1 represents the WHO classification
of antibiotics and their mode of action in some details (Adzitey 2015).
The β-lactam antibiotics activity is restricted in their 3-carbon and 1-nitrogen ring
in the chemical structure, by which are able to destroy the bacterial cell wall by
interfering with proteins during peptidoglycan synthesis. The modification of betalactam drugs side chain increases the ability to evade the degradative capacity of
enzymes produced by some bacterial species and make the movement of antibiotics
42
H. H. Al-Haideri et al.