Abstract
In pigs, Chlamydia suis has been associated with respiratory disease, diarrhea and conjunctivitis, but there is a high rate of inapparent C. suis infection found in the gastrointestinal tract of pigs. Tetracycline resistance in C. suis 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 C. suis samples in cell culture and to evaluate their antibiotic susceptibility in vitro under consideration of antibiotic treatment on herd level.Swab samples (n = 158) identified as C. suis 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 [tet(C)] by conventional PCR and isolates were further investigated for their antibiotic susceptibility in vitro. 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.
Highlights
We show that the number of tetracycline resistant C. suis isolates in pigs treated with tetracycline derivatives tends to increase between the beginning and end of the fattening period, whereas farms where no antibiotic treatment was applied only yielded tetracycline sensitive or intermediate C. suis isolates, providing evidence for selective pressure
Growing numbers of antibiotic resistant pathogens have caused the Swiss Federal Office of Public Health to target and monitor the consumption of antibiotics in human and veterinary medicine [21]. This strategy was developed in addition to the total ban on the use of antibiotic growth promoters in livestock initialized by Sweden in 1986, implemented by Switzerland in 1999 and followed by the European Union (EU) in 2004 [22, 23]
We aimed to compare the tetracycline susceptibility in vitro of porcine clinical samples with the presence or absence of the tet(C) gene, which required isolation of various C. suis samples previously identified by real-time PCR and subsequent species identification by Arraymate microarray [9]
Summary
Since the 1940s, antibiotics have been prescribed in human and veterinary medicine to treat bacterial infection, but only a few years after discovery of these antimicrobial drugs, the first. Cases of acquired antibiotic resistance emerged in the form of penicillin-resistant Staphylococcus aureus [1]. The use of antibiotics exerts selective pressure against the microbial community promoting the emergence of therapy-resistant bacteria [3]. In particular the microbiota of the gastrointestinal tract, have been reported to regularly acquire and transfer antibiotic resistance genes, often promoted by the use of oral antimicrobial drugs. With high bacterial loads of 1011 to 1012 bacteria/ml from several phyla, the colon offers plenty of opportunity for horizontal gene transfer and the selection for commensal bacteria resistant to antibiotics [4, 5]
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