Evidence of Antibiotic Resistance Gene Silencing in Escherichia coli

Abstract

The possibility that unexpressed antibiotic resistance genes are carried by bacterial genomes is seldom investigated. Potential silencing of the resistance genes bla OXA-2 , aadA1, sul1, and tetA carried on the plasmid pVE46 in a recent porcine isolate of Escherichia coli was investigated following oral inoculation of the strain into organic piglets. A small proportion of isolates recovered from feces did not express one or more resistance genes, despite retaining the pVE46 plasmid. Different combinations of unexpressed resistance genes were observed, and 12 representative isolates were selected for further study. Surprisingly, in most cases the resistance genes and their promoters, although not expressed, were intact, with fully wild-type sequences. Apart from four isolates exhibiting intermediate-level tetracycline resistance, no mRNA for the unexpressed genes was detected. Silencing of resistance genes was reversible at low frequencies between 10 ؊6 and 10 ؊10 . Introduction of the plasmid from silenced isolates to another strain restored expression, indicating that gene silencing was a property of the host chromosome rather than the plasmid itself. When the same recent porcine E. coli strain carrying the unrelated plasmid RP1 was inoculated into piglets, three isolates (of 9,492) that no longer expressed RP1-encoded resistance genes were recovered. As with pVE46, in most cases the coding sequences and promoter regions of these genes were found to be intact, but they were not transcribed. Such gene silencing indicates a previously unrecognized form of transcriptional control that overrides standard expression signals to shut down gene expression. These findings suggest that unexpressed resistance genes may occur in the wild and hence may have clinical implications.

Evaluation of an Expanded Microarray for Detecting Antibiotic Resistance Genes in a Broad Range of Gram-Negative Bacterial Pathogens

“…These represent pseudogenes or genes that are present but not expressed or poorly expressed. Silent (nonexpressing) resistance genes have been reported in numerous species; include several aminoglycoside resistance genes, such as strA (16), aadA1 (17), aadA2 (18), and both aac(6=)-Ib and aac(3=)-Ia (19); and probably account for the low correlation in this class. However, although these genes may currently be poorly expressed or silent, this may change if a selection pressure, e.g., administration of an aminoglycoside antibiotic, is applied, hence posing problems in treatment.…”

Dissemination of IncF plasmids carrying beta-lactamase genes in Gram-negative bacteria from Nigerian hospitals

“…There has been an increase in the isolation of CTX-Mproducing bacteria globally, with reports from Europe, North and South America, Africa and Asia [4]. [32] or contain multiple silenced antibiotic resistance genes [33] are of particular concern, as they may serve as reservoirs of antibiotic resistance determinants in bacteria that cannot be detected phenotypically. A previous report of CTX-M in Nigeria identified CTX-M-15 [2].…”

Chlamydia trachomatis clinical isolates identified as tetracycline resistant do not exhibit resistance in vitro: whole-genome sequencing reveals a mutation in porB but no evidence for tetracycline resistance genes

“…For example, in one study, 52 E. coli isolated from pigs were found to harbour the resistance plasmid pVE46 carrying bla (beta-lactam resistance), aadA1 (streptomycin resistance), sul1 (sulphonomide resistance) and tetA (tetracycline resistance), but reduced or complete loss of resistance to tetracycline was recorded for all isolates, despite retention of the resistance plasmid; this was suggested to be due to chromosomally located regulatory genes (Enne et al, 2006). The clinical significance is that isolates such as these may revert to a resistant phenotype upon activation of the gene by the presence of the antibiotic (Enne & Bennett, 2010).…”