Bioavailability of in-feed tetracyclines is influenced to a greater extent by crude protein rather than calcium

Feed is a common vehicle for tetracycline administration in animal farming. However, veterinarians and farmers often overlook that common feed components, such as proteins and calcium, may interact with these drugs and thereby alter their properties. In this study, we aimed to quantify the effect of crude protein and calcium on the bioavailability of 14 first-, second-, and third-generation tetracyclines, tetracycline epimers that may arise during storage, and other tetracycline derivatives in 84 feedstuffs for shrimp, swine, tilapia, and poultry marketed in Costa Rica. To this end, crude protein was quantified with a Kjeldahl method, calcium was determined by flame atomic spectroscopy (FAAS), and tetracyclines were detected by high-performance liquid chromatography (HPLC) with fluorescence detection (FLD) and with two different whole-cell biosensors to discriminate matrix-bound, inactive tetracyclines from free, active fractions. Tilapia feed contained the highest amounts of HPLC-detectable tetracyclines (119–8365 mg kg−1). Poultry (78–438 mg kg−1) and swine feed (41–1076 mg kg−1) were characterized by intermediate concentrations, while shrimp feed showed the lowest amounts of these drugs (21–50 mg kg−1). Ten samples contained unauthorized antibiotics, including doxycycline, and another three had 4-epimers of tetracyclines, indicating degradation during prolonged storage. Biological detection of tetracyclines was inversely related to both feed crude protein (r2 = 0.58–0.75) and calcium content (r2 = 0.54–0.63). Moreover, only 12–55% of the concentrations determined by HPLC were appraised by the biosensors. Shrimp feed, followed by tilapia feed, showed the highest crude protein (14.5–52.2g 100 g−1) and calcium (0.89–5.90 mg kg−1) concentrations and, as predicted, the lowest bioavailable fractions of tetracyclines (12–40%). Finally, we provide equations that predict bioavailability as a function of crude protein, calcium, and tetracycline concentrations in feed. Our data reveals medication abnormalities in commercial feeds and provide novel biological information for several tetracyclines. UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias de la Salud::Centro de Investigación en Enfermedades Tropicales (CIET) UCR::Vicerrectoría de Docencia::Salud::Facultad de Microbiología