Spurious sulfamethoxazole-trimethoprim resistance of Salmonella typhi

Area of Science:

• Microbiology
• Clinical Laboratory Science
• Antimicrobial Resistance

Background:

• Sulfamethoxazole-trimethoprim is a critical antibiotic for treating typhoid and paratyphoid fevers.
• Previous research indicates thymidine excess in culture media can lead to inaccurate bacterial resistance testing.
• Spurious resistance results can compromise patient treatment strategies.

Purpose of the Study:

• To investigate the occurrence of thymidine-induced spurious resistance to sulfamethoxazole-trimethoprim in Salmonella typhi and Salmonella paratyphi-A.
• To determine the prevalence of this phenomenon in commonly used Mueller-Hinton agar batches in Lima, Peru.

Main Methods:

• Testing bacterial susceptibility to sulfamethoxazole-trimethoprim using Mueller-Hinton agar.
• Documenting instances of unexpected resistance patterns in Salmonella isolates.
• Analyzing multiple batches of Mueller-Hinton agar from clinical laboratories.

Main Results:

• The study confirmed thymidine excess in susceptibility test media causes spurious resistance in Salmonella typhi and Salmonella paratyphi-A.
• 18% (3 out of 17) of tested Mueller-Hinton agar lots exhibited this issue.
• This phenomenon was observed in major medical laboratories in Lima, Peru.

Conclusions:

• Thymidine excess in Mueller-Hinton agar is a significant factor causing false sulfamethoxazole-trimethoprim resistance in Salmonella.
• Laboratory practices and media quality control are crucial to ensure accurate antimicrobial susceptibility testing.
• Accurate susceptibility data is vital for effective treatment of Salmonella infections.