Effect of Chloramphenicol on a Bioassay Response for the Detection of Tetracycline Residues in Milk
ORLANDO NAGEL1, MARÍA DE LA LUZ ZAPATA2, JUAN CARLOS BASÍLICO2, JORGE BERTERO1, MARÍA PILAR MOLINA3 AND RAFAEL ALTHAUS1*
1. Cátedra de Biofísica, Departamento de Ciencias Básicas, Universidad Nacional del Litoral, R.P.L. Kreder 2805, 3080 Esperanza, República Argentina.
2. Cátedra de Microbiología, Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2826, 3000 Santa Fe, República Argentina.
3. Departamento de Ciencia Animal, Universidad Politécnica de Valencia, Camino de Vera, Valencia, España.
(Received: Febuary 4, 2008; Accepted: October 9, 2008)
ABSTRACT
The tetracyclines are commonly used in veterinary medicine, and yet the residues are not always detected by microbiological inhibitor tests using Geobacillus stearothermophilus subsp. calidolactis C-953 at their Maximum Residue Limit levels. In order to improve the sensitivity of these methods, a bioassay was evaluated to study the effect produced by the incorporation of different chloramphenicol (CAP) concentrations in the culture medium. The specificity and detection limits of six tetracyclines in milk were determined. As the levels of CAP increased, a decrease in specificity, from 97.9% (for 0, 50, 200 and 400 μg/kg of CAP) to 88.0% (for 600 μg/kg of CAP) was observed. The logistic regression model indicates a significant effect of the CAP concentration. However, the tetracycline-CAP interaction was not significant, and thus, a synergetic effect can not be considered between the two antimicrobials, with only a simple sum of their antimicrobial effects. When the CAP concentration is increased from 0 to 400 μg/kg, the detection limits of chlortetracycline (590 μg/kg-316 μg/kg), doxycycline (115 μg/kg-62 μg/kg), meclocycline (105 μg/kg-52 μg/kg), oxytetracycline (446 μg/kg-273 μg/kg), rolitetracycline (191 μg/kg-134 μg/kg) and tetracycline (302 μg/kg-158 μg/kg) are observed to decrease.
Key words: tetracycline, chloramphenicol, milk, bioassay, microbiological inhibitor test