Enterobacter spp. and
Citrobacter spp. are opportunistic human pathogens that primarily cause bloodstream infections (BSIs) and urinary tract infections (UTIs). Commonly, the β-lactam antibiotics ceftazidime (CAZ) and piperacillin/tazobactam (TZP) are used to treat such infections. However,
Enterobacter spp. and many
Citrobacter spp. contain chromosomal ampC β-lactamase genes which can become hyper-expressed following mutation, which confers CAZ resistance (CAZ-R). Here, we phenotypically and genotypically characterise antibiotic resistance (ABR) within 99 clinical isolates of
Enterobacter spp. and
Citrobacter spp. from the 1980s (19) and 2020 (63) in Bristol and compare ABR mechanisms. We conclude that the predominant mechanism of CAZ-R is AmpC hyper-production due to the inactivation of
ampD, encoding a protein involved in the
ampC regulatory pathway. The most common cause of
ampD inactivation was missense mutations. We characterise AmpD amino acid substitutions in CAZ-R isolates at three previously unreported residues: S73, I78 and S100. A handful of isolates, from 2020 only, contained mobile ABR genes, which accounted for their resistance to a broader range of β-lactam and non-β-lactam antibiotics. Proteomics analysis suggests that cefepime (FEP) resistance, which was seen predominantly in
Enterobacter spp., and did not differ between the time periods, is occurring through a yet unelucidated mechanism rather than being linked to specific porin changes as reported in other
Enterobacterales species. We show that there has been a significant increase in TZP resistance from the 1980s to 2020, likely associated with selection pressure from the increased use of TZP. Although likely linked to AmpC hyperproduction, the mechanism for TZP resistance is unclear. This work highlights how external factors, such as antibiotic use, can affect the prevalence of ABR as well as demonstrating the change, or lack thereof, in mechanisms of resistance to different antibiotics over time and between genera.
Comparison of antibiotic resistance mechanisms in
Enterobacter and
Citrobacter spp. bloodstream isolates from the 1980s and 2020 in Bristol
Syvret, E. A. (Author). 18 Jun 2024
Student thesis: Master's Thesis › Master of Science by Research (MScR)