English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

The General Referee will report on the activities by the Associate Referees as well as focus on some of the basic steps that are important for successful determination of antibiotics in feeds by diffusion methods. (a) Sampling of Feeds .—The objective in any sample preparation is to provide the laboratory with a representative sample of the large manufactured lot. Inhomogeneity is a common source of error. Sampling of a large lot of feed is made using the slotted single or double tube technique as recommended by AOAC (1). For complete feeds, at least 500 g or preferably 1 kg is collected and transported quickly to the laboratory. Laboratory samples are ground to a powder fine enough to pass through a wire mesh screen 0.75–1.0 mm in diameter. The grinding process should not generate excessive heat. Test samples are riffled (divided) into equally representative portions. Premixes are usually finely ground and do not require further grinding or rifling. We prefer to refrigerate all samples until analysis. (b) Weighing and Extraction of Feed Samples .—The amount of test sample to be weighed depends on label potency. For example, when analyzing chlortetracycline premix purported to contain 10–50 g/lb, sample weight is 5–10 g; if the label is 100 g/2000 lb or lower, sample weight is 50–100 g. It is always best to weigh a larger than a smaller amount of sample and keep the extraction volume to weight ratio about 10×. For some antibiotics of low potency such as bacitracin (contains 42 units/ mg), the extractant to weight ratio could be between 2–5. The ideal extracting agent is one that solubilizes all the antibiotic present in the sample and none of the interfering substances. After extraction, filtration and centrifugation usually follows. The objective is to obtain an extract as clear as possible for further dilution. This will improve accuracy and precision of results. (c)Dilution.—Class A volumetric flasks and pipets should be used. The glassware must be clean as indicated by lack of droplets upon draining. Volumetric pipets should be drained slowly (preferably < 0.5 mL/min) and the analyst should carefully touch the pipet tip to the receiving flask. Serologic or blow-out pipets are not suitable for precise quantitative work. The pipet should be wetted with the antibiotic solution before actual pipetting. For some antibiotics (e.g., monensin), the extractant is 90% methanol which is difficult to dilute manually because of low surface tension and high vapor pressure. To minimize the problem, attempt to decrease the organic solvent content or if possible automatic pipets can be used but must first be calibrated with a dye solution. (d) Preparation of Plates and Inoculation .—Most assay media used for antibiotic analysis have similar constituents. The basic composition of many media is pancreatic digest of gelatin, yeast extract, beef extract, and agar. Agar Medium A (Difco Antibiotic Medium 1) is used for bacitracin assay; when casein hydrolysate is omitted, it is called Antibiotic Medium B (Difco Antibiotic Medium 4) for penicillin assay; when glucose is deleted, it is called Antibiotic Medium C (Difco Antibiotic Medium 2) for novobiocin assay. When agar Medium C is adjusted to pH 5.7–5.9 (instead of 6.5–6.6), it is called Antibiotic Medium D (Difco Antibiotic Medium 8) used for the tetracyclines. A simple pH change of Agar Medium A to pH 8.9–9.1 makes it appropriate for assay of spectinomycin; at pH 7.9–8.0, it is described as Difco Agar Medium J suitable for analysis of erythromycin and linomycin. Few antibiotics however require special media. Monensin assay medium contains low concentrations of phosphate and potassium salts. Assay media should be sterilized (avoid overheating which may cause destruction of the gelling property of the medium). Inoculate and pour in Petri plates or large dishes on perfectly level surface. The amount of medium in each plate and the rate of inoculation must be determined by trial plates. The objective is to obtain clear zones of inhibition of adequate size. For cylinder assays, 10–12 mL of medium is used for 15× 100 mm plates; 15–20 mL for the wells technique; and 240 mL for large plates. Microbiological assays are non-specific. Many organisms are common for assay of more than one antibiotic. M. luteus ATCC 10240 is used for the assay of bacitracin, penicillin, novobiocin, erythromycin, lincomycin and tylosin; B. cereus ATCC 11778 for chlortetracycline or oxytetracycline and S. epidermitisfor neomycin. (e)Dosing and Incubation .—After pouring the plates, they should be refrigerated or allowed to stand at room temperature 1–2 h. Dosing of cold plates will allow the antibiotic to diffuse in the medium before growth of the test organism begins. There are many reservoirs used in antibiotic assay. Filter paper pads result in lower precision ( ± 10–20%) and are rarely GENERAL REFEREEREPORTS: JOURNAL OF AOAC INTERNATIONAL VOL. 83, NO. 2, 2000 503

journal of aoac international

Committee on Feeds, Fertilizers, and Related Agricultural Products: Antibiotics in Feeds: Drugs in Feeds: Feeds: Fertilizers and Agricultural Liming Materials: Nutrients in Soils: Tobacco: Veterinary Analytical Toxicology