Enhancement of bacterial competitive fitness by apramycin resistance plasmids from non-pathogenicEscherichia coli

Abstract

The study of antibiotic resistance has in the past focused on organisms that are pathogenic to humans or animals. However, the development of resistance in commensal organisms is of concern because of possible transfer of resistance genes to zoonotic pathogens. Conjugative plasmids are genetic elements capable of such transfer and are traditionally thought to engender a fitness burden on host bacteria. In this study, conjugative apramycin resistance plasmids isolated from newborn calves were characterized. Calves were raised on a farm that had not used apramycin or related aminoglycoside antibiotics for at least 20 months prior to sampling. Of three apramycin resistance plasmids, one was capable of transfer at very high rates and two were found to confer fitness advantages on new Escherichia coli hosts. This is the first identification of natural plasmids isolated from commensal organisms that are able to confer a fitness advantage on a new host. This work indicates that reservoirs of antibiotic resistance genes in commensal organisms might not decrease if antibiotic usage is halted.

What traits are carried on mobile genetic elements, and why?

“…They have thus been described as 'agents of open source evolution' (Frost et al, 2005), suggesting that they afford their host access to a vast genetic resource that can then be improved upon and made available to other organisms. Many traits that are beneficial to the host are carried by plasmids, including virulence factors, antibiotic resistance, detoxifying agents and enzymes for secondary metabolism (Philppon et al, 2002;Yates et al, 2006;Martínez, 2008;Martinez, 2009). As well as having an effect on the bacterial host, many horizontally transferred genes also code for traits that can affect the fitness of a host's neighbours.…”

Genotypic-Phenotypic Discrepancies between Antibiotic Resistance Characteristics of Escherichia coli Isolates from Calves in Management Settings with High and Low Antibiotic Use

“…For example, multidrug resistance genes may be linked on a common element, in which case selection for one resistance trait would lead to the propagation of all of the linked resistance traits (1,9,25,28). In addition, there is evidence that linkage to fitness traits unrelated to antimicrobial resistance may also be a mechanism by which resistance genes are maintained in bacterial populations in the absence of a resistance advantage (29)(30)(31)(32)52). If this is correct, we can predict that bacterial populations growing in the absence of antibiotic selection pressure will accumulate and retain more deleterious mutations in resistance genes than bacterial populations under intense antimicrobial selection pressure.…”

Insights into the Environmental Resistance Gene Pool from the Genome Sequence of the Multidrug-Resistant Environmental Isolate Escherichia coli SMS-3-5

“…While commensal bacteria are thought to function as reservoirs of antimicrobial resistance genes for pathogens in host-associated communities (74,77,95), further research is needed to address questions about the origin, presence, and persistence of antimicrobial resistance genes in the environment and their potential transfer from environmental to clinical settings.…”