Reversion of High-level Mecillinam Resistance to Susceptibility in Escherichia coli During Growth in Urine

Abstract

Mecillinam (amdinocillin) is a β-lactam antibiotic used to treat uncomplicated urinary tract infections (UTIs). We have previously shown that inactivation of the Escherichia coli cysB gene is the major cause of mecillinam resistance (Mec ^R) in clinical isolates. In this study, we used different E. coli strains (laboratory and clinical isolates) that were Mec ^R due to cysB mutations to determine how mecillinam susceptibility was affected during growth in urine compared to growth in the commonly used growth medium Mueller Hinton (MHB). We also examined mecillinam susceptibility when bacteria were grown in urine obtained from 48 different healthy volunteers. Metabolome analysis was done on the urine samples and the association between the mecillinam susceptibility patterns of the bacteria and urine metabolite levels was studied.

Two major findings with clinical significance are reported. First, Mec ^R E. coli cysB mutant strains (both laboratory and clinical isolates) were always more susceptible to mecillinam when grown in urine as compared to laboratory medium, with many strains showing complete phenotypic susceptibility in urine. Second, the degree of reversion to susceptibility varied between urine samples obtained from different individuals. This difference was correlated with osmolality such that in urine with low osmolality the Mec ^R mutants were more susceptible to mecillinam than in urine with high osmolality.

This is the first example describing conditional resistance where a genetically stable antibiotic resistance can be phenotypically reverted to susceptibility by metabolites present in urine. These findings have several important clinical implications regarding the use of mecillinam to treat UTIs. First, they suggest that mecillinam can be used to treat also those clinical strains that are identified as Mec ^R in standard laboratory tests. Second, the results suggest that testing of mecillinam susceptibility in the laboratory ought to be performed in media that mimics urine to obtain clinically relevant susceptibility testing results. Third, these findings imply that changes in patient behavior, such as increased water intake or use of diuretics to reduce urine osmolality and increased intake of cysteine, might induce antibiotic susceptibility in an infecting Mec ^R E. coli strain and thereby increase treatment efficiency.

Graphical Abstract

Highlights

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  Resistance to the antibiotic mecillinam can be phenotypically reverted to susceptibility by metabolites present in urine.

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  The degree of reversion to susceptibility varies between urine samples obtained from different individuals.

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  High cysteine levels and low osmolality in urine confer mecillinam susceptibility.

Antimicrobial Susceptibility Testing (AST) is necessary in order to decide on a suitable antibiotic treatment for bacterial infections. This study demonstrates that for the bacterium Escherichia coli, the most common cause of common uncomplicated urinary tract infections, resistance to the antibiotic mecillinam is strongly influenced by the conditions under which the bacteria grow. Thus, the bacteria are resistant when grown in the standard media used for AST but become susceptible when grown in urine. This finding shows that for ASTs to give correct results they need to be performed under conditions that mimic those in the human body.