Molecular epidemiology of β-lactamases in ceftriaxone-resistant Enterobacterales bloodstream infections in the mid-Atlantic United States

Ceftriaxone-resistant Enterobacterales remain a public health threat; contemporary data investigating their molecular epidemiology are limited. Five hundred consecutive ceftriaxone-resistant (MIC ≥ 4 µg/mL) Enterobacterales bloodstream isolates were collected between 2018 and 2022 from three Maryland hospitals. Broth microdilution confirmed antibiotic susceptibilities. Whole-genome sequencing identified extended-spectrum β-lactamase (ESBL) and <i>ampC</i> genes both in bacterial chromosomes (c-<i>ampC</i>) and on plasmids (p-<i>ampC</i>). Mutations in promoter or attenuator regions of the <i>Escherichia coli</i> c-<i>ampC</i> gene (i.e., <i>bla</i>_EC gene) with the potential to result in <i>ampC</i> derepression were investigated. The presence of ESBL or <i>ampC</i> genes was confirmed in 497 (99.4%) isolates. Two hundred seventy-nine (55.8%) isolates had both ESBL and <i>ampC</i> genes. ESBL families were identified among 398 (80%) patients: <i>bla</i>_CTX-M (<i>n</i> = 370), <i>bla</i>_SHV (<i>n</i> = 17), <i>bla</i>_OXY (<i>n</i> = 14), and <i>bla</i>_VEB (<i>n</i> = 5). Ceftriaxone-resistant Enterobacterales species carrying ESBL genes included the following: <i>E. coli</i> (67%), <i>Klebsiella pneumoniae</i> (24%), <i>Klebsiella oxytoca</i> (4%), <i>Proteus mirabilis</i> (2%), <i>Enterobacter cloacae</i> complex (2%), <i>Klebsiella aerogenes</i> (1%), <i>Providencia stuartii</i> (<1%), and <i>Serratia marcescens</i> (<1%). c-<i>ampC</i> genes were identified in 374 (75%) of the 500 isolates. Only 7% of <i>E. coli</i> isolates with mutations in the promoter or attenuator region of the c-<i>ampC</i> gene exhibited resistance to cefoxitin, a proxy for increased AmpC production. Two p-<i>ampC</i> genes were confirmed in 25 (5%) of the 500 isolates: <i>bla</i>_CMY-59 (72%) and <i>bla</i>_DHA-1 (28%; confined to <i>E. coli</i> [92%] and <i>K. pneumoniae</i> [8%]). Until comprehensive β-lactamase molecular testing is available, the species-specific prevalence of ESBL and <i>ampC</i> genes in ceftriaxone-resistant Enterobacterales should be considered to promote effective albeit judicious antibiotic prescribing. Mutations in promoter or attenuator regions of the <i>E. coli</i> c-<i>ampC</i> gene do not appear to contribute significantly to increased AmpC production in this species.