First Detection of a Fosfomycin Resistance Gene, fosA7, in Salmonella enterica Serovar Heidelberg Isolated from Broiler Chickens

Abstract

We previously described Salmonella enterica serovar Heidelberg isolates harboring a chromosomal gene cluster similar to the glutathione S-transferase gene, a putative fosA gene conferring resistance to fosfomycin. Here, we show that this new gene, named fosA7, confers resistance to fosfomycin. The introduction of fosA7 into the fosfomycin-susceptible Salmonella enterica serovar Enteritidis resulted in a substantial increase in the fosfomycin MIC. This finding increases the awareness of antibiotic resistance in Salmonella Heidelberg from broilers as related to the food safety and public health.

Keywords: Salmonella Heidelberg; broiler chicken; fosA7 gene; fosfomycin resistance.

FIG 1

(a) Genetic environment of a chromosomal fosfomycin resistance gene, fosA7, and its neighboring region (∼19.7 kb) in Salmonella Heidelberg ABB07-SB3031. The gene cassettes are not drawn to scale. (b) Alignment generated in T-Coffee of amino acid sequences of the newly identified FosA7 (in bold) with 13 known fosfomycin resistance proteins found in different bacterial species. Asterisk, amino acid residues conserved among the 14 fosfomycin resistance determinants; colon and dots, amino acid substitutions that result in homologous amino acid residues. The protein GenBank accession numbers are FosA7, KKE03230; FosA2, ACC85616; FosA3, AB522970; FosA4, AB908992; FosA5, AJE60855; FosA6, NG051497; FosA^PA, AAT49669; FosA^TN, AAA98399; FosB, ABS73480; FosC, AAZ14834; FosC2, AB522969; FosK, AB917040; FosX, CWV56762; and open reading frame 1 (ORF1), AAP50248. (c) Phylogenetic relationships between 14 fosfomycin determinants, including the newly described FosA7 (boxed) calculated at http://www.phylogeny.fr.

FIG 2

Inhibition zone sizes by fosfomycin (50 μg/disk) in the Kirby-Bauer drug susceptibility test (KBDST) on MH-G6p agar plates (n = 3). (a and b) Salmonella Enteritidis ABB07-SB3071 containing the empty vector (a) and containing the fosA7 gene from Salmonella Heidelberg ABB07-SB3031 (b). (c) Salmonella Enteritidis ABB07-SB3071 transformed with fosA7 on Mueller-Hinton (MH) agar plate; the fosfomycin disk on the left contained 50 μg glucose-6-phosphate (G6P), and the disk on the right contained 50 μg of G6P in addition to 1 mg of phosphonoformate (PPF).