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Resistance Development Potential of Antibiotic/Antimicrobial Residue Levels Designated as "Safe Levels"
Marietta Sue Brady1, Nicholas White1, Stanley E Katz1
1Department of Biochemistry and Microbiology, Cook College/New Jersey Agricultural Experiment Station, Rutgers University - the State University of New Jersey, New Brunswick, New Jersey 08903-0231.
Abstract:
Staphylococcus aureus ATCC 9144 was exposed to "safe levels" of ampicillin, dihydrostreptomycin, erythromycin, neomycin, oxytetracycline, and sulfamethazine, singly and in combinations of three drugs, for a 14-d interval, transferring daily. After the 14-d exposure interval, the minimum inhibitory concentration (MIC) of the organism for each treatment was determined for nine antibiotics/antimicrobials. There was one increase in the MIC out of 81 possibilities for the control or an incidence of 1.23%. There were four increases in the MIC in the single drug exposures for 54 possible increases in the MIC or an incidence of 7.40%. For the 18 combinations of three drugs, there were 55 increases in the MIC for 162 possible increases or 33.95%. This was an increase of 27.6 times over the control incidence and 4.6 times over the single drug incidence. The single drug incidence is 6.0 times the control incidence. These data indicate that the "safe levels" of antibiotics/antimicrobials that can appear in milk have the strong potential to select for antibiotic/antimicrobial-resistant bacterial populations.
Insights
Low-level antibiotic exposure, even at "safe levels," significantly increases the potential for antibiotic resistance in bacteria like Staphylococcus aureus. Combinations of antibiotics were particularly effective at driving resistance development.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Public Health
Background:
- Antibiotic residues in food products, such as milk, are a growing concern.
- Understanding the impact of sub-inhibitory antibiotic concentrations on bacterial resistance is crucial.
Purpose of the Study:
- To investigate the potential for antibiotic resistance development in Staphylococcus aureus ATCC 9144 when exposed to "safe levels" of various antibiotics.
- To compare the resistance-inducing effects of single-drug versus multi-drug (three-drug combinations) exposures.
Main Methods:
- Exposure of Staphylococcus aureus to "safe levels" of ampicillin, dihydrostreptomycin, erythromycin, neomycin, oxytetracycline, and sulfamethazine, individually and in combinations.
- Daily transfer of bacteria over a 14-day period.
- Determination of the minimum inhibitory concentration (MIC) for nine antibiotics/antimicrobials post-exposure.
Main Results:
- A significant increase in MIC was observed, with resistance incidence rising from 1.23% in controls to 7.40% for single-drug exposures and 33.95% for three-drug combinations.
- Three-drug combinations showed a 27.6-fold increase in resistance incidence compared to controls.
- Single-drug exposures demonstrated a 6.0-fold increase in resistance incidence compared to controls.
Conclusions:
- "Safe levels" of antibiotics, especially in combination, can strongly select for antibiotic-resistant bacterial populations.
- These findings highlight the potential public health risks associated with antibiotic residues in food.
- Further research is needed to establish safe limits and mitigate resistance development.