# P-795. Comparison of Clinical Outcomes after Treatment of Urinary Tract Infections Caused by Serratia marcescens, Morganella morganii, and Providencia spp. with Antibiotics Susceptible to versus Stable against AmpC Hydrolysis

## TL;DR

The study compared antibiotic treatment outcomes for urinary tract infections caused by specific bacteria, finding similar recurrence rates between antibiotics that are susceptible or stable against AmpC hydrolysis.

## Contribution

This is the first study to compare clinical outcomes of UTIs caused by Serratia marcescens, Morganella morganii, and Providencia spp. using AmpC-susceptible versus AmpC-stable antibiotics.

## Key findings

- UTI recurrence rates were similar between patients treated with AmpC-susceptible and AmpC-stable antibiotics.
- Only 2.53% in Group 1 and 3.8% in Group 2 experienced infection recurrence within 28 days.
- All recurrent infections had unchanged susceptibility patterns.

## Abstract

The 2024 IDSA Guidance on the Treatment of Antimicrobial-Resistant Gram-Negative Infections suggests selecting antibiotics according to susceptibility results to treat infections caused by S. marcescens, M. morganii, and Providencia spp. Although these organisms are thought to be at moderate to low risk of developing clinically significant AmpC expression, use of broad-spectrum beta-lactam antibiotics remains common even for urinary tract infections (UTIs). No study to date has compared the clinical outcomes for UTIs caused by these organisms in patients who initially received antibiotics susceptible to AmpC hydrolysis to those treated with AmpC-stable antibiotics.Table 1.Baseline CharacteristicsTable 2.Primary and Secondary Outcomes

Baseline Characteristics

Primary and Secondary Outcomes

This was a single-center retrospective study of non-pregnant patients > 18 years old with S. marcescens, M. morganii, or Providencia spp. isolated from a urine culture who received treatment with active antibiotics for at least 48 hours from October 2019 to June 2024. Patients with antibiotic use for other indications, polymicrobial urine cultures warranting broader antibiotic coverage, or isolates resistant to ceftriaxone were excluded. The primary outcome was recurrence of infection – defined as re-isolation of the same organism in a urine culture within 28 days of treatment.

112 unique patients met the inclusion criteria, Patients were divided into two cohorts—Group 1 consisted of 79 patients (70.5%) who received ceftriaxone (CRO), cefpodoxime (CPD), or piperacillin/tazobactam (TZP), and Group 2 had 33 patients (29.5%) receiving cefepime (FEP), meropenem (MEM), trimethoprim/sulfamethoxazole (SXT), or a fluoroquinolone (FQ). 90 patients (80.36%) met IDSA criteria for complicated UTI (Table 1). The primary outcome occurred in 2 patients (2.53%) in Group 1 and 3 patients (3.8%) in Group 2. The same organism was reisolated within a year of treatment in 5 patients from Group 1 and 4 patients from Group 2 – all of which had unchanged susceptibility patterns (Table 2).

In this population of patients with UTI caused by organisms with low risk of AmpC expression, the observed rate of UTI recurrence was similar after initial treatment with antibiotics susceptible to AmpC hydrolysis compared to antibiotics stable against AmpC hydrolysis.

All Authors: No reported disclosures