Date Published: February 28, 2019
Publisher: Public Library of Science
Author(s): Jong-Hoon Lee, Sojeong Heo, Miran Jeong, Do-Won Jeong, Kyoung Heon Kim.
The complete nucleotide sequence of a tetracycline-resistance gene (tetK)-carrying plasmid from a Staphylococcus saprophyticus isolate from jeotgal, a Korean high-salt-fermented seafood, was determined. The plasmid, designated pSSTET1, was 4439 bp in length and encoded typical elements found in plasmids that replicate via a rolling-circle mechanism, including the replication protein gene (rep), a double-stranded origin of replication, a single-stranded origin of replication, and a counter-transcribed RNA sequence. Additionally, the plasmid recombination enzyme gene (pre), which may be involved in inter-plasmid recombination and conjugation, was found. Each gene exhibited >94% sequence identity with those harbored in other Staphylococcus species. pSSTET1 was conditionally transferred to Staphylococcus species in a host-dependent manner and transferred to an Enterococcus faecalis strain in vitro. Antibiotic susceptibility of the transconjugants was host-dependent and transconjugants maintained a tetracycline-resistant phenotype in the absence of selective pressure over 100 generations.
For decades, antibiotic resistance studies of bacteria have mainly focused on clinically-important species that are directly exposed to antibiotics; however, antibiotic-resistant bacteria are found in diverse niches including soil, water, foods, and the gastrointestinal tract. Recently, many studies have speculated that commensal bacteria may act as reservoirs of antibiotic resistance genes that can be transferred to other resident intestinal bacteria or transient bacteria that pass through . The intestinal microflora is a potential source of antibiotic-resistant pathogens and the food chain is considered as one of the possible transfer routes of antibiotic resistance from animal and environment-associated antibiotic-resistant bacteria into the human gastrointestinal tract where these genes may be transferred to pathogenic and opportunistic bacteria [2, 3]. In this context, fermented and raw foods harboring large numbers of living bacteria have received increasing attention as potential vehicles of antibiotic resistance determinants , which can be transferred to the gut microbiota.
In the current study, plasmid pSSTET1 carrying the tetK gene was identified from S. saprophyticus isolated from fermented seafood. The plasmid harbors three genes, in the order tetK, pre, and rep, and the same gene organization is also identified in Staphylococcus spp. of human and animal origin (Fig 1). Before our identification of pSSTET1, the same gene organization has been identified from coagulase-positive S. aureus as well as CNS including S. epidermidis, Staphylococcus hemolyticus, and S. lentus. Although CNS are ubiquitously distributed in a vast array of natural origin, these CNS species have been mainly detected in skin and mucous membranes of mammals [37–40]. Among the three species, S. epidermidis and S. haemolyticus represent the major nosocomial pathogens as typical opportunists. While, S. saprophyticus has been frequently identified from fermented foods together with S. equorum, Staphylococcus succinus, and Staphylococcus xylosus . In this context, CNS species of human and animal origin have higher potential chances to be exposed to antibiotics than those from foods. The advent of a food-originated S. saprophyticus harboring pSSTET1 insinuates the plasmid can be transferred in the absence of tetracycline exposure and our conjugal transfer experiment proved it.