Bactrim (Trimethoprim-Sulfamethoxazole) is NOT Included in the Sensitivity Panel and Should NOT Be Used for This Enterococcus faecalis UTI
Critical Finding: Absence of Bactrim Testing
The urine culture sensitivity panel does not include trimethoprim-sulfamethoxazole (Bactrim), and this organism should be assumed resistant to this agent. The provided antibiogram lists ampicillin, daptomycin, levofloxacin, linezolid, nitrofurantoin, penicillin, streptomycin synergy screen, tetracycline, and vancomycin—but notably excludes trimethoprim-sulfamethoxazole 1.
Why Bactrim Should Never Be Used for Enterococcus faecalis
Even if the laboratory had tested and reported susceptibility to trimethoprim-sulfamethoxazole, this drug is clinically ineffective against enterococci and should not be used. This represents a critical disconnect between in vitro susceptibility testing and clinical efficacy 1, 2.
The Folate Bypass Mechanism
- Enterococci possess a unique ability to incorporate preformed exogenous folates from the environment (including urine), completely bypassing the antifolate activity of trimethoprim-sulfamethoxazole 1, 2
- Standard susceptibility testing uses folate-free media, which artificially shows susceptibility that does not exist in vivo 2
- When testing is performed in media containing fresh urine, the MIC increases 360-fold, revealing the true resistance 2
Clinical Failures and Dangers
- Two documented cases of patients with enterococcal UTIs treated with trimethoprim-sulfamethoxazole based on in vitro susceptibility developed bacteremia and required rescue therapy with penicillin or vancomycin plus streptomycin 1
- The eradication rate in the limited literature (38 evaluable cases) was only 82%, which is unacceptably low for a susceptible organism 2
- The European Committee on Antimicrobial Susceptibility Testing now categorizes wild-type enterococci as intermediate to trimethoprim/sulfamethoxazole, specifically to prevent inappropriate use 2
Appropriate Treatment Options for This E. faecalis UTI
Ampicillin is the drug of choice for this enterococcal urinary tract infection, as the organism is reported susceptible (S) on the sensitivity panel 3.
First-Line Therapy
- Ampicillin remains the preferred agent for enterococcal infections, including UTIs 3, 4
- The organism shows susceptibility to ampicillin on your sensitivity panel 3
- High urinary concentrations of ampicillin achieve necessary bactericidal activity even against some ampicillin-resistant strains 3
Alternative Oral Options
- Nitrofurantoin is FDA-approved for UTI caused by E. faecalis and shows susceptibility on your panel 3
- Nitrofurantoin has good in vitro activity against enterococci and achieves high urinary concentrations 3
- However, one study suggests nitrofurantoin exposure may increase enterococcal virulence properties, making it less ideal for recurrent infections 5
Other Susceptible Agents
- Levofloxacin shows susceptibility and can be used for uncomplicated UTI 3
- Penicillin shows susceptibility and is an acceptable alternative to ampicillin 3
- Vancomycin shows susceptibility but should be reserved for serious infections or when oral options fail 3
Clinical Context: Colony Count Interpretation
The colony count of 50,000-100,000 CFU/mL from a catheterized specimen meets diagnostic criteria for UTI:
- For catheterized specimens, colony counts as low as 10,000-50,000 CFU/mL are clinically significant when combined with symptoms and pyuria 6, 7
- The presence of a single organism (E. faecalis) rather than mixed flora supports true infection rather than contamination 6, 8
- Treatment is warranted if the patient has symptoms consistent with UTI (fever, dysuria, urgency, suprapubic pain) 8, 7
Critical Pitfall to Avoid
Never rely on trimethoprim-sulfamethoxazole for enterococcal infections, even if a laboratory reports susceptibility—this represents a dangerous disconnect between laboratory testing and clinical reality 1, 2. The practice of reporting enterococcal susceptibilities to drugs other than penicillins or vancomycin is misleading and potentially dangerous 1.