Aquaculture Can Promote the Presence and Spread of Antibiotic-Resistant Enterococci in Marine Sediments

Abstract

Aquaculture is an expanding activity worldwide. However its rapid growth can affect the aquatic environment through release of large amounts of chemicals, including antibiotics. Moreover, the presence of organic matter and bacteria of different origin can favor gene transfer and recombination. Whereas the consequences of such activities on environmental microbiota are well explored, little is known of their effects on allochthonous and potentially pathogenic bacteria, such as enterococci. Sediments from three sampling stations (two inside and one outside) collected in a fish farm in the Adriatic Sea were examined for enterococcal abundance and antibiotic resistance traits using the membrane filter technique and an improved quantitative PCR. Strains were tested for susceptibility to tetracycline, erythromycin, ampicillin and gentamicin; samples were directly screened for selected tetracycline [tet(M), tet(L), tet(O)] and macrolide [erm(A), erm(B) and mef] resistance genes by newly-developed multiplex PCRs. The abundance of benthic enterococci was higher inside than outside the farm. All isolates were susceptible to the four antimicrobials tested, although direct PCR evidenced tet(M) and tet(L) in sediment samples from all stations. Direct multiplex PCR of sediment samples cultured in rich broth supplemented with antibiotic (tetracycline, erythromycin, ampicillin or gentamicin) highlighted changes in resistance gene profiles, with amplification of previously undetected tet(O), erm(B) and mef genes and an increase in benthic enterococcal abundance after incubation in the presence of ampicillin and gentamicin. Despite being limited to a single farm, these data indicate that aquaculture may influence the abundance and spread of benthic enterococci and that farm sediments can be reservoirs of dormant antibiotic-resistant bacteria, including enterococci, which can rapidly revive in presence of new inputs of organic matter. This reservoir may constitute an underestimated health risk and deserves further investigation.

Erythromycin- and copper-resistant Enterococcus hirae from marine sediment and co-transfer of erm(B) and tcrB to human Enterococcus faecalis

“…Co-transfer of erm(B) and tcrB, possibly mediated by the conjugative plasmid pRE25, was also detected. These findings, while corroborating previous data on the involvement of marine sediment in the spread of antibiotic resistance Di Cesare et al, 2013;Vignaroli et al, 2012), show that E. hirae is involved in their transfer to the enterococcal species that are most commonly involved in human infections. This role is supported by early data about the transfer of vanA from an E. hirae to an E. faecium strain in chickens fed avoparcinsupplemented food (Robredo et al, 1999).…”

Intensive aquaculture selects for increased virulence and interference competition in bacteria

“…This may lead to more frequent or virulent outbreaks of opportunistic diseases if the use of antibiotics increases the intensity of the within-species competition [44] or selects for faster transmission rate [45]. The increasing amount of antibiotics used in food production [46] may thus affect microbial communities beyond the traditionally expected environmental effects [47][48][49]. Therefore, to secure the global food production it is vital to understand the factors that select for virulent pathogen strains.…”

Constitutive presence of antibiotic resistance genes within the bacterial community of a large subalpine lake

“…The qPCR results expressed as ng per reaction were converted in copy number of the single gene as described in details by Di Cesare et al . (). The average of the concentrations of the two replicates of each gene was calculated and used for subsequent analyses.…”