Selective Pressure Promotes Tetracycline Resistance of <i>Chlamydia Suis</i> in Fattening Pigs

<div><p>In pigs, <i>Chlamydia suis</i> has been associated with respiratory disease, diarrhea and conjunctivitis, but there is a high rate of inapparent <i>C</i>. <i>suis</i> infection found in the gastrointestinal tract of pigs. Tetracycline resistance in <i>C</i>. <i>suis</i> has been described in the USA, Italy, Switzerland, Belgium, Cyprus and Israel. Tetracyclines are commonly used in pig production due to their broad-spectrum activity and relatively low cost. The aim of this study was to isolate clinical <i>C</i>. <i>suis</i> samples in cell culture and to evaluate their antibiotic susceptibility <i>in vitro</i> under consideration of antibiotic treatment on herd level.</p><p>Swab samples (n = 158) identified as <i>C</i>. <i>suis</i> originating from 24 farms were further processed for isolation, which was successful in 71% of attempts with a significantly higher success rate from fecal swabs compared to conjunctival swabs. The farms were divided into three treatment groups: A) farms without antibiotic treatment, B) farms with prophylactic oral antibiotic treatment of the whole herd consisting of trimethoprime, sulfadimidin and sulfathiazole (TSS), or C) farms giving herd treatment with chlortetracycline with or without tylosin and sulfadimidin (CTS). 59 isolates and their corresponding clinical samples were selected and tested for the presence or absence of the tetracycline resistance class C gene [<i>tet</i>(C)] by conventional PCR and isolates were further investigated for their antibiotic susceptibility <i>in vitro</i>. The phenotype of the investigated isolates was either classified as tetracycline sensitive (Minimum inhibitory concentration [MIC] < 2 μg/ml), intermediate (2 μg/ml ≤ MIC < 4 μg/ml) or resistant (MIC ≥ 4 μg/ml). Results of groups and individual pigs were correlated with antibiotic treatment and time of sampling (beginning/end of the fattening period). We found clear evidence for selective pressure as absence of antibiotics led to isolation of only tetracycline sensitive or intermediate strains whereas tetracycline treatment resulted in a greater number of tetracycline resistant isolates.</p>