Tetracycline-resistant Chlamydia suis isolates in Italy

Antimicrobials are often used for prophylaxis and therapy of chlamydial infections. The large use of tetracycline has encouraged the selection of resistant organisms. The aim of this study was to evaluate the sensitivity to tetracycline of some Chlamydia suis strains isolated in Italy. The sensitivity of 14 C. suis isolates to doxycycline was tested and compared with the susceptibility to the same drug of the urethral Italian isolate GO86 of C. trachomatis serovar D, the reference strain 6BC of C. psittaci and three isolates of C. abortus, C. pecorum and C. felis, respectively. C. suis isolates were tested for the presence of the tet(C) resistance gene by a PCR assay amplifying a 525 base pair product of the tet(C) gene-coding region. As far as sensitivity to doxycycline is concerned, in comparison to the five strains of C. trachomatis, C. psittaci, C. abortus, C. pecorum and C. felis which were sensitive to doxycycline with MIC and MBC ranging from 0×03 to 0×125 µg/ml, respectively, the MIC and MBC values of doxycycline ranged from 4 to 8 µg/ml, respectively, in most swine isolates tested. All the C. suis isolates carried an identical nucleotide sequence that showed 100 per cent homology with those of the structural gene tet(C) described by Dugan and others (Gene Bank Accession AY428551). None of the other five chlamydial tetracycline sensitive strains carried the resistance gene tet(C).