Antibiotics are frequently used in the United States as feed efficiency promoters and medicines for livestock that is destined for human consumption. These antibiotics are released into the environment through the runoff and wastewater streams from animal feedlots and land applications of manure. The exposure of microorganisms to these antibiotics has reportedly resulted in the development of resistant species of microorganisms, which in turn can lead to human health hazards. Phytoremediation of these antibiotics can be a useful tool for countering this problem. Aquatic plants, Myriophyllum aquaticum (parrot feather) and Pistia stratiotes (water lettuce), were used for studying phytoremediation of tetracycline (TC) and oxytetracycline (OTC) from aqueous media. TC and OTC are two of the most commonly used tetracyclines in veterinary medicine. M. aquaticum and P. stratiotes gave high antibiotic modification rates of both antibiotics. Kinetic analyses dismiss direct enzyme catalysis; the modification rates decreased with increasing OTC concentrations. Sterile, cell-free root exudates (filtered through 0.2 microm membranes) from both species also exhibited comparable antibiotic modification rates. The involvement of root-secreted metabolites in antibiotic modification is suggested. The changes in the UV absorbance spectra of OTC during treatment with the root exudates confirmed the modification.

When subjected to stress, plants produce reactive oxygen species (ROS) as a part of the defense response. The oxidative response is also used to degrade organic pollutants. Hairy roots of Helianthus annuus (sunflower) are shown to oxidize oxytetracycline (OTC) through the action of the ROS released to the nutrient medium by the hairy root cultures. Methyl jasmonate (MeJA) elicits ROS formation in the hairy root cultures. The activities of the antioxidant enzymes, ascorbate peroxidase (APX), catalase (CAT), and guaiacol peroxidase (GPX), are reported for hairy root cultures treated with increasing concentrations of MeJA. A bioassay using Enterococcus hirae as the test microorganism demonstrates the root-catalyzed oxidation process results in conversion of OTC into product(s) devoid of antibiotic activity. Direct evidence for putative ROS oxidation of OTC is obtained by mass spectrometry (MS) and HPLC/MS showing first quinone formation followed possibly by ring cleavage, which disrupts UV absorption and destroys antibiotic activity. ß

Though numerous models have been developed to describe the growth of microbial cell cultures, far fewer models are available to describe the growth of hairy root cultures. Here a population balance model is proposed to simulate the growth of hairy roots. The model accounts for the increase in biomass due to elongation of a branch by cell division as well as the formation of new branches. The model incorporates the fact that although the likelihood of the formation of a new lateral branch is a maximum at a specific age of the parent branch, lateral branches can form over a distribution of ages of the parent branch. Model parameters are estimated using the genetic algorithm based on experimental data for batch and continuous bioreactors. The model proposed here may provide a better understanding of the increase in biomass of hairy root cultures.

The release of antibiotics to the environment has to be controlled because of serious threats to human health. Hairy root cultures of Helianthus annuus (sunflower), along with their inherent rhizospheric activity, provide a fast growing, microbe-free environment for understanding plant-pollutant interactions. The root system catalyzes rapid disappearance of tetracycline (TC) and oxytetracycline (OTC) from aqueous media, which suggests roots have potential for phytoremediation of the two antibiotics in vivo. In addition, in vitro modifications of the two antibiotics by filtered, cell- and microbe-free root exudates suggest involvement of root-secreted compounds. The modification is confirmed from changes observed in UV spectra of exudate-treated OTC. Modification appears to be more dominant at the BCD chromophore of the antibiotic molecule. Kinetic analyses dismiss direct enzyme catalysis; the modification rates decrease with increasing OTC concentrations. The rates increase with increasing age of cultures from which root exudates are prepared. The decrease in modification rates upon addition of the antioxidant ascorbic acid (AA) suggests involvement of reactive oxygen species (ROS) in the antibiotic modification process.