Abstract
Chlamydia suis is a swine pathogen that has also recently been found to cause zoonotic infections of the human eye, pharynx, and gastrointestinal tract. Many strains contain a tetracycline class C gene [tet(C)] cassette that confers tetracycline resistance. The cassette was likely originally acquired by horizontal gene transfer from a Gram-negative donor after the introduction of tetracycline into animal feed in the 1950s. Various research groups have described the capacity for different Chlamydia species to exchange DNA by homologous recombination. Since over 90% of C. suis strains are tetracycline resistant, they represent a potential source for antibiotic-resistance spread within and between Chlamydiaceae species. Here, we examined the genetics of tet(C)-transfer among C. suis strains. Tetracycline-sensitive C. suis strain S45 was simultaneously or sequentially co-infected with tetracycline-resistant C. suis strains in McCoy cells. Potential recombinants were clonally purified by a harvest assay derived from the classic plaque assay. C. suis strain Rogers132, lacking transposases IS200 and IS605, was the most efficient donor, producing two unique recombinants detected in three of the 56 (5.4%) clones screened. Recombinants were found to have a minimal inhibitory concentration (MIC) of 8-16 μg/mL for tetracycline. Resistance remained stable over 10 passages as long as recombinants were initially grown in tetracycline at twice the MIC of S45 (0.032 μg/mL). Genomic analysis revealed that tet(C) had integrated into the S45 genome by homologous recombination at two unique sites depending on the recombinant: a 55 kb exchange between nrqF and pckG, and a 175 kb exchange between kdsA and cysQ. Neither site was associated with inverted repeats or motifs associated with recombination hotspots. Our findings show that cassette transfer into S45 has low frequency, does not require IS200/IS605 transposases, is stable if initially grown in tetracycline, and results in multiple genomic configurations. We provide a model for stable cassette transfer to better understand the capability for cassette acquisition by Chlamydiaceae species that infect humans, a matter of public health importance.
Highlights
Bacteria develop resistance to antibiotics either as a result of mutation in their chromosomal genes or from acquisition of antibiotic resistance genes by horizontal gene transfer (HGT)
Previous studies have shown that co-infection of the tetracycline resistant (tetR) C. suis strain R19 with a tetracycline sensitive (tetS) C. trachomatis L2 strain results in tetR C. trachomatis recombinants (Suchland et al, 2009; Jeffrey et al, 2013)
We aimed to obtain tet(C)-positive C. suis S45 recombinants by co-infecting S45 with three tetR C. suis strains representing three of the four tet(C)-containing cassette Classes: I; II; and IV (Figure 1)
Summary
Bacteria develop resistance to antibiotics either as a result of mutation in their chromosomal genes or from acquisition of antibiotic resistance genes by horizontal gene transfer (HGT). Resistance through mutation or HGT is promoted by sub-inhibitory concentrations, broad-spectrum and high doses of antibiotics; patient noncompliance with treatment regimens; and antibiotic use in mammalian and avian species bred for human consumption (Andersson and Hughes, 2014). These latter practices have led to an alarming increase in microbial pathogen resistance such as colistin-resistant Escherichia coli and multidrug-resistant Staphylococcus aureus. It has recently been associated with zoonoses including trachoma (a chronic ocular disease) (Dean et al, 2013), ocular infection in abattoir workers (De Puysseleyr et al, 2014) and asymptomatic nasal, pharyngeal, and intestinal infections in farmers (De Puysseleyr et al, 2015)
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