Degradation of Antibiotics in the Environment

1 Quinolone Compounds More recent generation quinolones have dehber-ately been designed to have a higher retention and bioavaUabihty in the body (Jjemba and Robertson, 2006) but substantial amounts are still excreted as the parent compound. For example, more than 90% of trovafloxacin is excreted as parent compound (Stass and Kubitza, 1999). Furthermore, because of their more complex structure, these newer 4-quinolone antimicrobial agents are likely to be more persistent in the environment. The increased complexity of this group of antibiotics comes at an additional risk, that is, increased toxicity. Thus, some of them, after a brief period of use, have aheady been suspended, put under even more restricted use, or 

TABLE 3.3 Sorption of Enrofloxacin on Eive Divergent Soils 

3 Degradation of Tetracyclines Tetracyclines are relatively stable under acidic conditions but not under alkaline conditions. They form complexes with chelating agents such as divalent metals, proteins, and silanol groups (Oka et al., 2000  

All samples contained the metabolite 4-epi-tetracycline, and this was even more pronounced in the liquid manure. Under strong acidic conditions (pH 2), tetracycline is metabolized to anhydro-tetracyclines, which are bioactive, whereas alkaline pH conditions (pH 7.5) can favor the formation of isotetracyclines, which show almost no in vitro activity. Tetracycline can also undergo extensive photodecomposition, forming a variety of products (Oka et al., 1989 Peterson et al., 1993). 

Erythromycin, another macrolide, is rapidly transformed to anhydro-erythromycin in the environment (Hirsch et ah, 1999). A recent study by Chander et al. (2005) compared the sorption of tetracycline and tylosin in two soils that differ in clay and organic matter content. The adsorption of each of the antibiotics was higher in the soil that has a higher clay and organic matter content as these soil traits also provide greater exchange capacity (Fig. 3.8). Those workers speculated that the larger tylosin molecules (MW = 916.1) are adsorbed on the surface of the clay 

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