Cefazolin as a predictor of urinary cephalosporin activity in indicated Enterobacterales

Traditionally, cephalothin susceptibility results were used to predict the susceptibility of additional cephalosporins; however, in 2013-2014, the Clinical and Laboratory Standards Institute (CLSI) revisited this practice and determined that cefazolin is a more accurate proxy than cephalothin for uncomplicated urinary tract infections (uUTIs). Therefore, a cefazolin surrogacy breakpoint was established to predict the susceptibility of seven oral cephalosporins for Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis in the context of uUTIs. Clinical microbiology laboratories face several operational challenges when implementing the cefazolin surrogacy breakpoint, which may lead to confusion for the best path forward. Here, we review the historical context and data behind the surrogacy breakpoints, review PK/PD profiles for oral cephalosporins, discuss challenges in deploying the breakpoint, and highlight the limited clinical outcome data in this space.

In vitro activity of oral third-generation cephalosporins plus clavulanate against ESBL-producing Enterobacterales isolates from the MERINO trial

Extended-spectrum-beta-lactamase (ESBL)-producing Enterobacterales as a cause of community-acquired uncomplicated urinary tract infection (UTI) is on the rise. Currently, minimal oral treatment options exist. New combinations of existing oral third generation cephalosporins paired with clavulanate may overcome resistance mechanisms seen in these emerging uropathogens. Ceftriaxone-resistant E. coli and Klebsiella pneumoniae containing CTX-M-type ESBLs or AmpC, in addition to narrow-spectrum OXA and SHV enzymes, were selected from blood culture isolates obtained from the MERINO trial. Minimum inhibitory concentration (MIC) values of third generation cephalosporins (cefpodoxime, ceftibuten, cefixime, cefdinir) both with and without clavulanate were determined. One hundred and one isolates were used with ESBL, AmpC and narrow-spectrum OXA genes (e.g. OXA-1, OXA-10) present in 84, 15 and 35 isolates, respectively. Susceptibility to oral third generation cephalosporins alone was very poor. Addition of 2 mg/L clavulanate lowered the MIC₅₀ values (cefpodoxime MIC₅₀ 2 mg/L, ceftibuten MIC₅₀ 2 mg/L, cefixime MIC₅₀ 2 mg/L, cefdinir MIC₅₀ 4 mg/L) and restored susceptibility (33%, 49%, 40%, and 21% susceptible, respectively) in a substantial number of isolates. This finding was less pronounced in isolates co-harbouring AmpC. In vitro activity of these new combinations may be limited in real world Enterobacterales isolates co-harbouring multiple antimicrobial resistance genes. Pharmacokinetic/pharmacodynamic data would be useful in further evaluating their activity.