The agricultural antibiotic carbadox induces phage-mediated gene transfer in Salmonella - PubMed (original) (raw)

The agricultural antibiotic carbadox induces phage-mediated gene transfer in Salmonella

Bradley L Bearson et al. Front Microbiol. 2014.

Abstract

Antibiotics are used for disease therapeutic or preventative effects in humans and animals, as well as for enhanced feed conversion efficiency in livestock. Antibiotics can also cause undesirable effects in microbial populations, including selection for antibiotic resistance, enhanced pathogen invasion, and stimulation of horizontal gene transfer. Carbadox is a veterinary antibiotic used in the US during the starter phase of swine production for improved feed efficiency and control of swine dysentery and bacterial swine enteritis. Carbadox has been shown in vitro to induce phage-encoded Shiga toxin in Shiga toxin-producing Escherichia coli (STEC) and a phage-like element transferring antibiotic resistance genes in Brachyspira hyodysenteriae, but the effect of carbadox on prophages in other bacteria is unknown. This study examined carbadox exposure on prophage induction and genetic transfer in Salmonella enterica serovar Typhimurium, a human foodborne pathogen that frequently colonizes swine without causing disease. S. Typhimurium LT2 exposed to carbadox induced prophage production, resulting in bacterial cell lysis and release of virions that were visible by electron microscopy. Carbadox induction of phage-mediated gene transfer was confirmed by monitoring the transduction of a sodCIII::neo cassette in the Fels-1 prophage from LT2 to a recipient Salmonella strain. Furthermore, carbadox frequently induced generalized transducing phages in multidrug-resistant phage type DT104 and DT120 isolates, resulting in the transfer of chromosomal and plasmid DNA that included antibiotic resistance genes. Our research indicates that exposure of Salmonella to carbadox induces prophages that can transfer virulence and antibiotic resistance genes to susceptible bacterial hosts. Carbadox-induced, phage-mediated gene transfer could serve as a contributing factor in bacterial evolution during animal production, with prophages being a reservoir for bacterial fitness genes in the environment.

Keywords: Salmonella; antibiotic; bacteriophage; carbadox; gene transfer; prophage; transduction.

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Figures

Figure 1

Figure 1

Carbadox induction of Enterobacteriaeae prophages. Bacterial strains lysogenized with P22 (UB-1790), λ (UB-1703), HK97 (UB-1704), and Sf6 (UB-1496) were grown in LB broth at 37°C with shaking. At a density of 1 × 108 bacterial cells/ml, carbadox was added to a final concentration of 0.5 μg/ml. Phage lysates were obtained by shaking with chloroform at the indicated times after carbadox addition and titered on a permissive host. Open symbols indicate cultures with no added carbadox and closed symbols indicate cultures with carbadox added. The different induced lysogens are indicated in the figure insert.

Figure 2

Figure 2

Carbadox exposure of wild-type S. Typhimurium LT2 results in bacterial cell lysis. S. Typhimurium strains (LT2 and BBS 1008) were grown in LB glucose medium at 37°C with shaking. At OD600 = 0.2 (arrow), carbadox (2.5 μg/ml) was added to cultures indicated by the closed symbols. The open symbols indicate control cultures without carbadox.

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