The environmental presence of antimicrobial compounds at sub-inhibitory
concentrations may lead to the evolution of antimicrobial-resistant bacteria, and chronic
exposure to antibiotics at trace levels in drinking water may contribute to increased allergies
against antibiotics (Kummerer 2001). Removal of the antimicrobial parent compound is readily
achieved by oxidation processes, but little is known about the antimicrobial activity of oxidation
intermediates that are formed in the process. Therefore,
the overall objective of this research is the systematic characterization of a
sequential photochemical/biological oxidation process for the removal of the antimicrobial
compounds sulfamethoxazole (SMX) and sulfamethazine (SMZ) from natural water. Specific
objectives for the phase of the research described here included: quantification of SMX and
SMZ photolysis and UV/H<sub>2</sub>O<sub>2</sub> oxidation rates; and, assessment of residual antimicrobial
activity of photooxidation intermediates.
The sulfonamides SMX and SMZ were selected as model antimicrobial
compounds. The effects of the following factors on SMX and SMZ oxidation and mineralization
rates were evaluated: pH; H<sub>2</sub>O<sub>2</sub> concentration; and, presence/absence of natural organic
matter (NOM). Batch photolysis and UV/H<sub>2</sub>O<sub>2</sub> oxidation experiments were conducted by
recirculating water through an annular ultraviolet (UV) light reactor. The 25-W low pressure lamp had an
emission wavelength of 254 nm. SMX and SMZ concentrations were monitored as a function of
time and quantified by HPLC. Total organic carbon (TOC) analyses were performed to assess
the extent of SMX/SMZ mineralization. Experiments were conducted in ultrapure water (DI
water) buffered at pH values that were at least two pH units above and below the pKa of the
sulfonamide group; thus, the degradation of the neutral and anionic forms of SMX and SMZ
were evaluated. Tests conducted in the presence of NOM were performed by spiking SMX and
SMZ into Lake Wheeler water (Raleigh, North Carolina) that had been filtered through a 0.45-µm filter. The
TOC of filtered Lake Wheeler water was 5.3 mg/L, the pH was 7.6, and the total alkalinity was
20 mg/L as CaCO<sub>3</sub>.
To test whether photooxidation intermediates exhibit antimicrobial activity, a method based on
the 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 be
observed in the absence of antimicrobial compound addition. The antimicrobial activity was
determined by adapting the macrodilution method described by Andrews (2001) and the Clinical
and 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 to
Iso-Sensitest broth to yield 1x106 cells/mL. Then, 1 mL of this inoculum was added to 8-mL
tubes containing 1 mL of water spiked with different concentrations of antimicrobial agent or 1
mL of antimicrobial-spiked water following exposure to different photolysis or UV/H<sub>2</sub>O<sub>2</sub>
oxidation times. Also, positive controls were prepared without the addition of antimicrobial
agent. After incubating for 8 hours at 37ºC, the optical density of each sample was measured at
600 nm. To obtain an absorbance <0.6, samples were diluted (2 mL sample + 5 mL broth). Includes 4 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
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| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 5 |
| Published : | 06/01/2006 |
