Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 10/27/1998
Publication Date: N/A
Technical Abstract: Chlamydia trachomatis is widespread in swine. Most pigs become infected with one or more strains by 6 weeks of age. Isolates have been made from cases of conjunctivitis, enteritis, and pneumonia. Because sulfadiazine and tetracycline were commonly used as feed additives, it was decided to compare the eight recently-isolated swine strains along with a swine strain n(S45) from Austria with other chlamydial isolates for resistance to sulfadiazine and tetracycline. The isolates were tested for resistance by growing them in Vero cell monolayers in 96-multiwell plates. Sulfadiazine and tetracycline sensitivity tests were performed using a block titration procedure. Three 10-fold dilutions of chlamydiae were used to infect three columns of wells. Each row received a different 2-fold concentration of tetracycline and sulfadiazine. Six out of eight of the recently isolated trachomatis isolates were resistant to sulfadiazine. These isolates all formed inclusions at the 40 ug/ml concentration of sulfadiazine. Two recently isolated swine strains and the S45 strain from Austria were resistant to sulfadiazine and did not form inclusions at the 10 ug/ml level. Tetracycline results showed that all the recently isolated swine strains had MICs of 5 to 10 ug/ml of medium. This compared with MICs of 0.625 or lower for the S45 swine isolate and other strains tested. In summary, it was shown that a number of swine C. trachomatis strains are highly resistant to tetracycline and sulfadiazine. The resistance was likely acquired while these drugs were used as feed additives. Two of the tetracycline-resistant strains were not resistant to sulfadiazine, indicating that the drug resistance is due to different mechanisms.