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The environmental presence of antimicrobial compounds at sub-inhibitoryconcentrations may lead to the evolution of antimicrobial-resistant bacteria, and chronicexposure to antibiotics at trace levels in drinking water may contribute to increased allergiesagainst antibiotics (Kummerer 2001). Removal of the antimicrobial parent compound is readilyachieved by oxidation processes, but little is known about the antimicrobial activity of oxidationintermediates that are formed in the process. Therefore,the overall objective of this research is the systematic characterization of asequential photochemical/biological oxidation process for the removal of the antimicrobialcompounds sulfamethoxazole (SMX) and sulfamethazine (SMZ) from natural water. Specificobjectives for the phase of the research described here included: quantification of SMX andSMZ photolysis and UV/Hsub2/subOsub2/sub oxidation rates; and, assessment of residual antimicrobialactivity of photooxidation intermediates.The sulfonamides SMX and SMZ were selected as model antimicrobialcompounds. The effects of the following factors on SMX and SMZ oxidation and mineralizationrates were evaluated: pH; Hsub2/subOsub2/sub concentration; and, presence/absence of natural organicmatter (NOM). Batch photolysis and UV/Hsub2/subOsub2/sub oxidation experiments were conducted byrecirculating water through an annular ultraviolet (UV) light reactor. The 25-W low pressure lamp had anemission wavelength of 254 nm. SMX and SMZ concentrations were monitored as a function oftime and quantified by HPLC. Total organic carbon (TOC) analyses were performed to assessthe extent of SMX/SMZ mineralization. Experiments were conducted in ultrapure water (DIwater) buffered at pH values that were at least two pH units above and below the pKa of thesulfonamide group; thus, the degradation of the neutral and anionic forms of SMX and SMZwere evaluated. Tests conducted in the presence of NOM were performed by spiking SMX andSMZ into Lake Wheeler water (Raleigh, North Carolina) that had been filtered through a 0.45-m filter. TheTOC of filtered Lake Wheeler water was 5.3 mg/L, the pH was 7.6, and the total alkalinity was20 mg/L as CaCOsub3/sub.To test whether photooxidation intermediates exhibit antimicrobial activity, a method based onthe concept of minimum inhibitory concentrations (MICs) was implemented. For sulfonamides,the MIC is recorded as the lowest concentration that inhibits 80 % of the growth that would beobserved in the absence of antimicrobial compound addition. The antimicrobial activity wasdetermined by adapting the macrodilution method described by Andrews (2001) and the Clinicaland Laboratory Standards Institute (CLSI) Standard M7-A6. The Enterobacteriaceae organism E.coli ATCC 25922 was acquired to conduct MIC tests. In MIC tests, E. coli cells were added toIso-Sensitest broth to yield 1x106 cells/mL. Then, 1 mL of this inoculum was added to 8-mLtubes containing 1 mL of water spiked with different concentrations of antimicrobial agent or 1mL of antimicrobial-spiked water following exposure to different photolysis or UV/Hsub2/subOsub2/suboxidation times. Also, positive controls were prepared without the addition of antimicrobialagent. After incubating for 8 hours at 37C, the optical density of each sample was measured at600 nm. To obtain an absorbance 0.6, samples were diluted (2 mL sample + 5 mL broth). Includes 4 references, tables, figures.

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Edition: Vol. - No. Published: 06/01/2006 Number of Pages: 5File Size: 1 file , 250 KB