LSTM researchers have uncovered a molecular mechanism of an emerging type of antimicrobial resistance in E.coli

News article 1 Oct 2020
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LSTM researchers have uncovered a molecular mechanism behind an unusual and emerging type of antimicrobial resistance in Escherichia coli (E. coli) in Liverpool. The results of the work, led by Dr Alasdair Hubbard and Dr Thomas Edwards in collaboration with Liverpool University Hospital Foundation Trust, is published today in Nature Communications

Antimicrobial resistance (AMR) in E. coli is increasingly prevalent leading to reduced treatment options, treatment failure, and the overuse of antibiotics usually only used as a last resort such as carbapenems. One of the most problematic resistance mechanisms in E. coli is the production of a particular enzyme, which breaks down certain antibiotics including pencicillins. One commonly used treatment for E. coli blood stream infections is piperacillin/tazobactam (TZP), a combination drug which includes an antibiotic with an enzyme inhibitor that protects the antibiotic from degradation. 

The team identified a collection of clinical isolates of E. coli from the Royal Liverpool University hospital with an unusual characteristic; resistance to TZP but susceptibility to 3rd generation cephalosporins/carbapenems. Dr Alasdair Hubbard said “Resistance to TZP is usually conferred by a group of β-lactamase called carbapenemases. However, these enzymes would also result in resistance to 3rd generation cephalosporins and carbapenems and we were not able to detect the presence of carbapenemases in the isolate collection. The only detectable β-lactamases were those that are usually inhibited by tazobactam. Therefore, there was a mismatch between the common mechanism of resistance and the observed TZP-resistant phenotype”.

Following a thorough search of the isolate collection, the team were able to find a pair of E. coli isolates which were isolated from the same patient but across two different infection episodes. The first isolate was TZP-susceptible but the second was TZP-resistant, suggesting the isolate had evolved to become TZP-resistant within the patient. Following confirmation that the paired isolates were identical, using a combination of molecular biology and next generation sequencing, the team were able to characterise a novel mechanism of gene amplification. The β-lactamase enzyme was hyperproduced by the cell via amplification of the gene which encodes the enzyme, overwhelming tazobactam by the sheer volume of the β-lactamase enzyme being produced, overcoming the inhibitory effect of tazobactam.

Dr Thomas Edwards said “Our study not only describes a novel mechanism of gene amplification which results in resistance to a clinical important antibiotic, but also really highlights the importance of understanding new resistance mechanisms and investigating phenotype-genotype mismatch. This will inform improved diagnostics, and the monitoring of antimicrobial resistance using next generation sequencing”. 

Hubbard, A.T.M., Mason, J., Roberts, P. et al. Piperacillin/tazobactam resistance in a clinical isolate of Escherichia coli due to IS26-mediated amplification of blaTEM-1B. Nat Commun 11, 4915 (2020).