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Enhanced resistance to cefotaxime and imipenem associated with outer membrane protein alterations in Enterobacter
1Department of Microbiology, University Hospital, Nottingham, UK.
Abstract:
Mutants exhibiting enhanced resistance to cefotaxime and imipenem were selected by plating a strain of Enterobacter aerogenes, which already produced chromosomal beta-lactamase constitutively, on to varying concentrations of different beta-lactam antibiotics. Frequencies of mutation varied from 10(-5) to 10(-8), depending upon the particular antibiotic and concentration used for selection. Only minor variations in beta-lactamase specific activities were observed and these could not be directly correlated with changes in resistance when compared with the original strain. In the majority of mutants, the selection of an enhanced level of resistance to cefotaxime was associated with a significant increase in resistance to imipenem, but no increase in resistance to the non-beta-lactam antibiotics tested was observed. Examination of outer membrane protein profiles revealed a number of complex changes in the mutants when directly compared to the original strain. In one mutant imipenem/cefotaxime resistance was directly associated with almost total loss of a 42K protein which was non-covalently associated with peptidoglycan and therefore possibly a porin protein.
Insights
Mutant Enterobacter aerogenes strains developed resistance to cefotaxime and imipenem antibiotics. This resistance was linked to changes in outer membrane proteins, potentially porins, rather than beta-lactamase activity.
Area of Science:
- Microbiology
- Molecular Biology
- Antibiotic Resistance
Background:
- Enterobacter aerogenes possesses constitutive chromosomal beta-lactamase production.
- Beta-lactam antibiotics like cefotaxime and imipenem are crucial for treating bacterial infections.
- Understanding resistance mechanisms is vital for developing effective antimicrobial strategies.
Purpose of the Study:
- To select and characterize mutants of Enterobacter aerogenes with enhanced resistance to cefotaxime and imipenem.
- To investigate the correlation between beta-lactamase activity, outer membrane protein alterations, and antibiotic resistance.
- To identify specific genetic or protein changes responsible for increased resistance.
Main Methods:
- Selection of resistant mutants by plating Enterobacter aerogenes on media with increasing concentrations of beta-lactam antibiotics.
- Measurement of beta-lactamase specific activities in wild-type and mutant strains.
- Analysis of outer membrane protein profiles using gel electrophoresis.
- Determination of resistance levels to various beta-lactam and non-beta-lactam antibiotics.
Main Results:
- Mutants with significantly enhanced resistance to cefotaxime and imipenem were successfully isolated.
- Mutation frequencies ranged from 10^-5 to 10^-8, varying with antibiotic selection pressure.
- No direct correlation was found between minor changes in beta-lactamase activity and the observed resistance levels.
- Outer membrane protein profiles showed complex alterations in resistant mutants compared to the parent strain.
- One mutant displayed imipenem/cefotaxime resistance associated with the near-complete loss of a 42K outer membrane protein, potentially a porin.
Conclusions:
- Enhanced resistance to cefotaxime and imipenem in Enterobacter aerogenes is primarily mediated by alterations in outer membrane permeability, possibly through porin modification or loss.
- Beta-lactamase activity does not appear to be the primary driver of the observed increase in resistance to these specific antibiotics.
- The loss of a specific 42K outer membrane protein is strongly implicated in the dual resistance phenotype to cefotaxime and imipenem.