Farmed fish and shellfish accounted for more than 50% of the worldwide human consumption of these products in 2014. They are expected to provide two-thirds of global consumption by 2030, if not sooner. Hundreds of tons of antimicrobials and antibiotics are used each year to prevent and treat bacterial infections in these farmed animals, a usage that select for antimicrobial-resistant microorganisms in the aquatic environment and its animals.
Dr. Tomova and her colleagues previously found that bacteria isolated from patients with urinary tract infections in a Chilean coastal region adjacent to an area of intensive fish farming with heavy marine quinolone usage contained significantly more genes for quinolone-resistance than did isolates from comparable patients in New York. Even more important, the quinolone resistance genes in these Chilean marine bacteria were also DNA sequence identical to those present in patients with quinolone-resistant urinary tract infections in this same coastal region.
In order to begin to analyze potential networks of horizontal gene transfer between marine and terrestrial bacteria, these workers have now studied the genomic location of quinolone-resistance genes and Class 1 integrons in a group of randomly chosen, quinolone-resistant marine bacteria and clinical isolates from the previously studied adjacent Chilean coast. (Integrons are a genetic mechanism for bacterial acquisition and differential expression of new antimicrobial resistance genes).
In four quinolone-resistant marine bacterial isolates from this aquacultural area, quinolone-resistance genes qnrA, qnrB, and qnrS were chromosomally located. While qnrA was chromosomally located in two quinolone-resistant clinical isolates of Escherichia coli from patients with urinary tract infections in the adjacent coastal region, qnrB and qnrS in two other isolates were located in small molecular weight plasmids, a location that renders them more easily transferable to other bacteria.
In two isolates with chromosomally-located qnrS genes (a Marinobacter sp. marine isolate and an E. coli clinical isolate), sequences immediately upstream of the qnrS gene were homologous to comparable sequences of numerous reported plasmid-located qnrS genes, while downstream sequences were different. Interestingly, in both marine bacteria and uropathogenic E. coli, class 1 integrons had similar co-linear structures, identical gene cassettes, and similarities in their flanking regions.
The observed commonality of quinolone resistance genes and integrons between these limited numbers of marine and clinical isolates suggests that heavy aquacultural use of antimicrobials might facilitate horizontal gene transfer between bacteria in marine and terrestrial locations.
It also suggests that identification of the genetic and molecular mechanisms involved in sharing of antimicrobial resistance genes between environmental bacteria and animal and human pathogens will constitute an important area of future research because of the potential negative impacts of such processes on animal and human health.
These findings are described in the article entitled Plasmid-Mediated Quinolone Resistance (PMQR) Genes and Class 1 Integrons in Quinolone-Resistant Marine Bacteria and Clinical Isolates of Escherichia coli from an Aquacultural Area, published in the journal Microbial Ecology. This work was led by Aleksandra Tomova, Larisa Ivanova, Alejandro Buschmann, Henry Godfrey, & Felipe Cabello from New York Medical College, Comenius University in Bratislava, and Universidad de Los Lagos.
Additional information about this study can be found here: Tomova, A., Ivanova, L., Buschmann, A.H. et al. Microb Ecol (2018) 75: 104. https://link.springer.com/article/10.1007%2Fs00248-017-1016-9
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One complicating factor is the local human usage of the same drugs. In aquaculture, even in the US where I was in aquaculture, the lower costs antibiotics we purchased for our aquatic animals were often diverted to human usage. This common usage would genetically speed the transfer between species while the humans were selecting for any gene transfer.
We knew that the big price difference between human and animal drugs was not a difference is chemistry or activity but in packaging, marketing and bureaucratic costs (prescriptions, etc.).