Characterization of a P1-like bacteriophage carrying CTX-M-27 in Salmonella spp. resistant to third generation cephalosporins isolated from pork in China

Abstract

The aim of this study was to elucidate the epidemiology of third generation cephalosporin resistant Samonella isolates from pork of a slaughterhouse in China and the features of transferable elements carrying bla_CTX-M genes. One hundred and twenty-six (7.3%) Salmonella isolates were identified; S. Derby and S. Rissen were the most two prevalent serotypes. Among these isolates 20 (15.8%) were resistant to third generation cephalosporins and nine of them carried bla_CTX-M-27. S1-PFGE and replicon typing of bla_CTX-M-27-carrying plasmids showed that seven were untypeable plasmids of about 104 Kb and two were IncP plasmids of about 300 Kb. Complete sequence analysis of one PBRT-untypeable plasmid showed it was a P1-like bateriophage, named SJ46, which contained a non-phage-associated region with several mobile elements, including Tn1721, ISEcp1B and IS903D. The other six 104 Kb PBRT-untypeable bla_CTX-M-27-carrying plasmids also harboured the same phage-insertion region of SJ46 suggesting that they were the same P1-like bacteriophage. PFGE profiles of the parental strains revealed both potential vertical and horizontal spread of this P1-like bla_CTX-M-27-containing element. Additionally, the representative gene of the P1 family bacteriophage, repL, was detected in 19.0% (24/126) of the isolates. This study indicated a potential role of P1-family bacteriophage in capture and spread of antimicrobial resistance in pathogens.

Figure 1. PFGE profiles of twenty 3GC-resistant Salmonella isolated from pork in slaughterhouse in China.

MIC, minimal inhibitory concentration; CTX, cefotaxime; CTF, ceftiofur; CTR, ceftriazone; CTZ, ceftazidime; ESBL, extended-spectrum β-lactamase; PFGE, pulsed-field gel electrophoresis. “-”, not detected; “R”, resistant; “S”, susceptible.

Figure 2. Circular map of SJ46.

The coloured arrows represent ORFs and their direction of transcription.

Figure 3

(a) Sequence synteny comparisons between SJ46, bacteriophage P1 (AF234172), and bacteriophage RCS47 (FO818745) as determined with the Artemis comparison tool. The green rectangles are specific genomic regions. Strand conservations are showed in red and the inserted region is indicated in orange. (b) Gene environment representation of bla_CTX-M-27 in SJ46 and gene structure comparison of the bla_CTX-M-containing region of SJ46 and pKP96. Black arrows represent ORFs and their direction. IRs of the respective mobile elements are shown as black triangles. Nucleotide letters with underlining represent a direct duplication (DR).