Mechanisms involved in the active secretion of CTX-M-15 β-lactamase by pathogenicE. coliST131

Abstract

Infections caused by antimicrobial resistant bacterial pathogens are fast becoming an important global health issue. Strains of Escherichia coli are common causal agents of urinary tract infection and can carry multiple resistance genes. This includes the gene blaCTX-M-15 that encodes for an extended spectrum β-lactamase (ESBL). While studying antimicrobial resistance (AMR) in the environment we isolated several strains of E. coli ST131 downstream of a WWTP in a local river. These isolates were surviving in the river sediment and characterisation proved that a multi-resistant phenotype was evident. Here, we show that E. coli strain 48 (river isolate ST131), provided a protective effect against a third-generation cephalosporin (cefotaxime) for a susceptible E. coli strain 33 (river isolate ST3576) through secretion of a functional ESBL into the growth medium. Furthermore, extracellular ESBL activity was stable for at least 24 h after secretion. Proteomic and molecular genetic analyses identified CTX-M-15 as the major secreted ESBL responsible for the observed protective effect. In contrast to previous studies, OMVs were not the sole route for CTX-M-15 secretion. Indeed, mutation of the Type I secretion system led to a significant reduction in the growth of the ESBL-producing strain as well as a significantly reduced ability to confer protective effect. We speculate that CTX-M-15 secretion, mediated through active secretion using molecular machinery provides a public goods service by facilitating the survival of otherwise susceptible bacteria in the presence of cefotaxime.