Antibiotics: therapeutic importance and perspectives for the discovery and development of new agents. There is a continuous need for antibiotics, mainly with new mechanisms of action, since infectious diseases represent the second major cause of death in the world and bacteria resistance levels are high. This review describes the contribution of microbial natural products for the development of the major antibiotic classes, the mechanisms of action of current antibiotics, some modern approaches involving genetic tools for the discovery and development of new antibiotics from microbial products and antibiotics in clinical trials. Keywords: microbial natural products; drug discovery; antibiotics. Figura 1.
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Email: violetadb hotmail. Evaluation of screening tests for antimicrobial residues in milk from individual cows treated with a combination of penicillins G and streptomycin. Background: in Colombia, the law Resolution , prohibits the sale of milk that contains any antimicrobial drug residues, although no specific official screening tests and detection limits have been specified. To avoid contaminating bulk tanks with milk from individually treated cows, producers would benefit from having on-farm screening tests to conduct their own quality controls.
In addition, on-site testing would allow farmers to check if the withdrawal times of commercially-available generic products are in accordance with labeled recommendations. Cows were individually milked and samples collected every 12 hours the day before, and for 3 days after the recommended withdrawal time of three days post-treatment. To inactivate the potential action of natural inhibitors of microbial growth that may be present in milk ie.
The Colombian legislation does not allow bulk tank milk to be sold with detectable levels of antimicrobials Resolution , and current policies with regard to food safety issues for milk and meat products are intended to comply with strict regulations and to allow competition in a free market 1.
There are two particular situations in which both the veterinarian and producer would benefit from using on-farm screening tests: a extra-label use of a veterinary drug ie. On-site screening for antimicrobials has been long implemented in most NorthAmerican and British dairy operations 3, 4 and is recommended by the "Pasteurization Milk Ordinance" to certify grade A milk 5. The majority of them detect levels of antimicrobials that are usually below permissible levels established by the Codex Alimentarius, which makes them highly sensitive 6.
In countries like Colombia, where the law stipulates that milk should be devoid of any residues, using very sensitive assays guarantees that milk will not yield false-negative results.
By contrast, these assays, and particularly those based on microbial growth inhibition, tend to be less specific and it has been proved that many will yield false positive results 7 , which would unfairly result in milk being discarded and a financial loss to the producer. As such, it detects numerous antimicrobials and at thresholds generally close or below minimum permissible residue levels established by the Codex Alimentarius. This makes it very sensitive but of very low specificity.
The objective of this study was to compare the results of these two commonly used rapid detection assays in individual milk from cows treated for mastitis with a commercially available combination of procaine penicillin-G and streptomycin. The study was conducted in a dairy farm that has been officially accredited by the Colombian Institute of Agriculture with the equivalent to the grade A certification by the Pasteurized Milk Ordinance.
The farm is comprised of about Holstein cows and located on the high tropical plains of Antioquia, with pastures consisting of kikuyu grass Penissetum clandestinum. Animals that need to be treated for mastitis or other pathologies ie.
Throughout a 3 month period in , a total of 39 animals at different stages of lactation and parity that received a similar treatment for subclinical mastitis were included in the study. Isolation of pathogens was not routinely conducted at the farm, and so, the etiology of the mastitis was unknown. For a kg cow, this was approximately equivalent to daily dosages of The product was reconstituted immediately before use in 20 mL sterile saline, and injected deep intramuscularly in two equal volumes of 10 mL at each buttock.
Typically, cows that recovered after the withdrawal period were returned to their original lots with the rest of the herd and joined the milking regular schedule in pallor rooms; however, those used in the experiment were kept for an additional 3 days beyond the recommended withdrawal time to facilitate the collection of milk samples.
The experimental design is graphically shown in figure 1. On day 1, administrations were started for each cow and continued until day 4. On day 7, the withdrawal time for milk ended according to the product labeled instructions of 3 days following the last administration. On day 10, and for the purpose of the study, cows that were manually milked for an additional 3 days past the usual time of withdrawal were returned to their original destination lot.
All four quarters were manually milked by farm operators in stainless steel buckets from which a composite sample was directly collected into 10 mL sterile plastic vials. To avoid potential contamination between samples, buckets and hands were washed with an iodine solution and thoroughly rinsed between cows. This treatment has been shown to be a fast, simple, and inexpensive way to remove false-positive results due to natural inhibitors and has no effect on positive samples containing most antimicrobials 9.
To assess the extent of agreement between assays, the results presence or absence of residues were represented on 2 X 2 contingency tables and the Kappa Coefficient k was calculated as described by Landis and Koch 10 using the formula:.
Table 1 shows the numbers of positive and negative milk samples for each one of the screen assays in all milk samples that were taken throughout the study. Of the 15 4. The percentage of cows that were positive at each time point tested is shown in figure 2. The objective of this study was to assess the performance of common screening tests used for the detection of antimicrobial drug residues in individual milk from cows treated for subclinical mastitis with one of several dozen commercially available procaine penicillin-G products PPG in the Colombian market.
The results showed that the labeled withdrawal time of 3 days after the last administration was adequate in 37 of 39 cows, and only 2 animals yielded milk positive for residues to both tests for an additional day. It has long been known that cows with mastitis yield milk with natural microbial growth inhibitors, such as lysozyme and lactoferrin 11, In general, drugs with milk withdrawal periods of more than 4 days are not approved for dairy cows in lactation. In Colombia, there are numerous procaine penicillin-G products that are registered for use in lactating cows, and the times of milk withdrawal for the majority are days, and exceptionally 4 days For the dosage and duration of treatment used in this study, the withdrawal time was not expected to exceed 3 days.
In other studies that evaluated the presence of milk residues beyond the recommended time of withdrawal to multiple types of antimicrobial drugs, penicillin was observed to account for the majority of positive cases Potential reasons that can explain a prolonged presence of drug residues in milk include different forms of extralabelled administrations.
For procaine-penicillin-G, all commercial products are aqueous suspensions recommended for intramuscular administration but contraindicated by the subcutaneous route. It has been shown that subcutaneous injections of procaine-penicillin-G will cause local hematomas, inflammation, and fibrosis, all of which can result in delayed absorption from the injection site and consequent persistent excretion in milk 17, Delayed absorption was also shown to occur when large volumes of more than 20 mL were injected in the same location.
In this study, the maximum volume injected per site did not exceed 10 mL, as recommended by FDA With respect to the presence of streptomycin sulfate in the commercial formulation, it is known that at the dosage given, this other antimicrobial drug is more rapidly eliminated than PPG Commercial products that are only based on streptomycin sulfate at the dosage given in this study have a milk withdrawal period of only 2 days, for that reason no specific assay was considered necessary to detect this antimicrobial.
Because some cows continued excreting penicillin past the point of withdrawal time, conducting these tests by producers, as a common tool of good husbandry practices, could avoid contaminating entire bulk tanks. Primer on estimating withdrawal times after extralabel drug use. J Am Vet Med Assoc ; Reliability of three bulk-tank antimicrobial residue detection assays used to test individual milk simples from cows with mild clinical mastitis.
Detection of antimicrobial substances in individual cow and quarter milk samples using Delvotest microbial inhibitor tests. J Dairy Sci ; Drug Residue Testing and Farm Surveillance. Grade "A" Pasteurized Milk Ordinance; Toldra F, Reig M. Methods for rapid detection of chemical and veterinary drug residues in animal foods.
Evaluation of selected antiobiotic residue screening tests for milk from individual cows and examination of factors that affect the probability of false-positive outcome. Alternatives for validation of diagnostic assays used to detect antibiotic residues in milk. False-positive outcome and drug residue in milk samples over withdrawal times.
Landis J, Koch G. The measurement of observer agreement for categorical data. Biometrics ; Lactoferrin and lysozyme in milk during acute mastitis and their inhibitory effect in Delvotest P. Determination of oxytetracycline residues in milk with the use of HPLC method and two microbial inhibition assays. Bull Vet Inst Pulawy ; Thomson PLM.
Extralabel use of penicillin in food animals. Depletion of intramuscularly and subcutaneously injected procaine penicillin G from tissues and plasma of yearling beef steers. Can J Vet Res ; Penicillin concentrations in serum, milk, and urine following intramuscular and subcutaneous administration of increasing doses of procaine penicillin in lactating dairy cows.
Effect of formulation and route of administration on tissue residues and withdrawal times. Absorption and excretion of penicillin G from the mastitic bovine udder. Excretion of penicillin G and dihydrostreptomycin from quarters of cows with experimental induced staphylococcal mastitis. Am J Vet Res ; Services on Demand Article. English pdf Article in xml format Article references How to cite this article Automatic translation Send this article by e-mail. Abstract Background: in Colombia, the law Resolution , prohibits the sale of milk that contains any antimicrobial drug residues, although no specific official screening tests and detection limits have been specified.
Introduction The Colombian legislation does not allow bulk tank milk to be sold with detectable levels of antimicrobials Resolution , and current policies with regard to food safety issues for milk and meat products are intended to comply with strict regulations and to allow competition in a free market 1. Materials and methods Experimental cows and treatment The study was conducted in a dairy farm that has been officially accredited by the Colombian Institute of Agriculture with the equivalent to the grade A certification by the Pasteurized Milk Ordinance.
Experimental design The experimental design is graphically shown in figure 1. Milk collection and residue analysis All four quarters were manually milked by farm operators in stainless steel buckets from which a composite sample was directly collected into 10 mL sterile plastic vials.
Results Table 1 shows the numbers of positive and negative milk samples for each one of the screen assays in all milk samples that were taken throughout the study. Discussion The objective of this study was to assess the performance of common screening tests used for the detection of antimicrobial drug residues in individual milk from cows treated for subclinical mastitis with one of several dozen commercially available procaine penicillin-G products PPG in the Colombian market. Referencias 1.
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