Antimicrobial use and resistance in swine waste treatment systems.

Chlortetracycline and the macrolide tylosin were identified as commonly used antimicrobials for growth promotion and prophylaxis in swine production. Resistance to these antimicrobials was measured throughout the waste treatment processes at five swine farms by culture-based and molecular methods. Conventional farm samples had the highest levels of resistance with both culture-based and molecular methods and had similar levels of resistance despite differences in antimicrobial usage. The levels of resistance in organic farm samples, where no antimicrobials were used, were very low by a culture-based method targeting fecal streptococci. However, when the same samples were analyzed with a molecular method detecting methylation of a specific nucleotide in the 23S rRNA that results in resistance to macrolides, lincosamides, and streptogramin B (MLSB), an unexpectedly high level of resistant rRNA (approximately 50%) was observed, suggesting that the fecal streptococci were not an appropriate target group to evaluate resistance in the overall microbial community and that background levels of MLSB resistance may be substantial. All of the feed samples tested, including those from the organic farm, contained tetracycline resistance genes. Generally, the same tetracycline resistance genes and frequency of detection were found in the manure and lagoon samples for each commercial farm. The levels of tetracycline and MLSB resistance remained high throughout the waste treatment systems, suggesting that the potential impact of land application of treated wastes and waste treatment by-products on environmental levels of resistance should be investigated further.

Detection of tetracycline resistance genes by PCR methods.

Rapid, accurate, and sensitive determination of antibiotic resistance profiles of various human and animal pathogens becomes a vital prerequisite for successful therapeutic intervention in the face of the increased occurrences of drug-resistant bacterial infections. The current methods, which are dependent on cultivation of pathogens and phenotypic expression of antibiotic resistance, usually require excessive time, special microbiological equipment, and qualified personnel. However, even with all these requisites, for example, no bacteria can be grown from more than 80% of all clinical samples sent to clinical microbiology laboratories. Besides the cultivation limitations, the cultivation-based determination of an antibiotic resistance profile lacks the genotypic information, which is essential for understanding the epidemiology and routes of transmission of antibiotic resistance genes. These genes often reside on mobile genetic elements and can move freely between commensal and pathogenic microbiota, occurring even between taxonomically distant clinical and environmental microbiota. Therefore, development of genotyping methods for detection of antibiotic resistance genes is highly desirable for fast, accurate, and sensitive detection of antibiotic resistance genes in a broad range of pathogenic and commensal bacteria in both clinical and environmental samples. As a model for our studies we have chosen the genes conferring resistance to tetracyclines. Tetracyclines belong to a family of broad-spectrum antibiotics that include tetracycline, chlortetracycline, oxytetracycline, demeclocycline, methacycline, doxycycline, minocycline, and a number of other semisynthetic derivatives. These antibiotics inhibit protein synthesis in Gram-positive and Gram-negative bacteria by preventing the binding of aminoacyl-tRNA molecules to the 30S ribosomal subunit. The antibiotics of this group were introduced in the late 1950s and since then have been widely used in clinical and veterinary medicine, as well as for prophylaxis and growth promotion in food animals. Because of the possible misuse and overuse of these drugs, resistance to this class of antibiotics is widespread among many clinical isolates, thus limiting the utility of tetracyclines in treating infections. Despite this shortcoming, antibiotics of this class still remain in the active arsenal for dermatologists to treat skin infections such as acne and rosacea.