What is the recommended treatment for a 17-year-old female with a urinary tract infection (UTI) caused by Morganella morganii, presenting with frequency and a sensation of incomplete voiding, with a urine culture showing >100,000 Colony-Forming Units per milliliter (CFU/ml)?

Treatment of Morganella morganii UTI in a 17-Year-Old Female

This patient should be treated with antimicrobial therapy guided by susceptibility testing, with trimethoprim-sulfamethoxazole (TMP-SMX) 160/800 mg twice daily for 3 days as a first-line option if susceptible, or alternatively nitrofurantoin 100 mg twice daily for 5 days, based on culture sensitivities. 1, 2

Clinical Context and Classification

This 17-year-old female presents with uncomplicated cystitis based on the following criteria 1:

• Lower urinary tract symptoms (frequency, incomplete voiding sensation) without systemic features
• No anatomic or functional urinary tract abnormalities mentioned
• Non-pregnant status
• No immunosuppression or comorbidities described

The presence of >100,000 CFU/ml of Morganella morganii confirms a true UTI requiring treatment 1.

Antimicrobial Selection Strategy

First-Line Considerations

The choice of antibiotic must be guided by the susceptibility profile of the isolated Morganella morganii 1. While standard first-line agents for uncomplicated cystitis include fosfomycin, nitrofurantoin, and pivmecillinam 1, Morganella morganii has specific susceptibility patterns that require attention:

• TMP-SMX is FDA-approved specifically for UTIs caused by Morganella morganii and should be used if susceptibility is confirmed 2
• Nitrofurantoin remains an option if the organism is susceptible, though resistance patterns should be verified 1
• Fosfomycin is recommended only for E. coli-predominant uncomplicated cystitis in the guidelines 1

Treatment Duration

Treat for 3 days if using TMP-SMX (160/800 mg twice daily), or 5 days if using nitrofurantoin (100 mg twice daily) 1. The guideline emphasizes treating with "as short a duration of antibiotics as reasonable, generally no longer than seven days" 1.

Alternative Agents Based on Susceptibility

If first-line agents show resistance, consider 1, 3:

• Cephalosporins (e.g., cefadroxil 500 mg twice daily for 3 days) if local E. coli resistance is <20% and organism is susceptible 1
• Fluoroquinolones (ciprofloxacin) based on susceptibility testing, though these should be reserved as second-line due to collateral damage concerns 1, 3
• Aminoglycosides (gentamicin) in combination therapy for resistant isolates, though typically reserved for more severe infections 3, 4

Critical Pitfalls with Morganella morganii

Morganella morganii possesses inducible AmpC β-lactamases, making it potentially resistant to multiple antibiotics including some cephalosporins 3, 4. Key considerations:

• Do not assume susceptibility to standard agents - await culture sensitivities before finalizing therapy 1
• Most isolates show susceptibility to ceftazidime, imipenem, and amikacin in invasive infections, but UTI isolates may differ 3
• Resistance to ciprofloxacin, TMP-SMX, gentamicin, and nitrofurantoin has been documented in some M. morganii strains 5

Follow-Up Management

Do not obtain routine post-treatment urine cultures if the patient becomes asymptomatic 1. However, specific situations require reassessment 1:

• If symptoms do not resolve by end of treatment, obtain repeat culture and assume resistance to the initial agent 1
• If symptoms recur within 2 weeks, obtain culture and retreat with a 7-day regimen using a different agent 1
• If symptoms recur after 2 weeks or with a different organism, this represents reinfection rather than treatment failure 1

When to Consider Complicated UTI

This patient should NOT be classified as having a complicated UTI unless specific risk factors emerge 1. Avoid this classification as it leads to unnecessarily broad-spectrum antibiotics and longer treatment durations 1.

Reassess for complicated UTI only if 1:

• Symptoms persist despite appropriate therapy
• Rapid recurrence within 2 weeks occurs repeatedly
• Urea-splitting bacteria are identified (though M. morganii is not typically urea-splitting) 1
• Structural abnormalities are suspected

Imaging Considerations

No imaging is indicated for this first episode of UTI in a young woman without risk factors 1. Imaging should be reserved for patients with 1:

• Three or more infections within 12 months (recurrent UTI)
• Rapid recurrence within 2 weeks suggesting bacterial persistence
• Suspected anatomic abnormalities
• Failure to respond to appropriate therapy