Which organism would not routinely undergo antimicrobial susceptibility testing?

Which organism would not routinely undergo antimicrobial susceptibility testing?

• A. Streptococcus pyogenes
• B. Staphylococcus aureus
• C. Proteus mirabilis
• D. Escherichia coli

Answer: (A)

Routine antimicrobial susceptibility testing is done when knowing an organism’s exact susceptibility helps choose effective therapy, especially when resistance patterns vary or have major clinical implications. Streptococcus pyogenes, a Group A Streptococcus, is nearly universally susceptible to penicillin and related beta-lactams, with no clinically significant penicillin resistance in routine practice. Because a reliable first-line treatment exists for GAS, labs do not routinely run susceptibility testing on these isolates unless unusual circumstances arise. In contrast, Staphylococcus aureus, Proteus mirabilis, and Escherichia coli can exhibit a range of resistance patterns that affect treatment decisions. Staphylococcus aureus includes MRSA strains that resist many beta-lactams, making susceptibility testing essential. Proteus mirabilis and Escherichia coli commonly produce various beta-lactamases or other resistance mechanisms, so determining susceptibility guides appropriate antibiotic choice and helps curb resistant infections.

Routine antimicrobial susceptibility testing is done when knowing an organism’s exact susceptibility helps choose effective therapy, especially when resistance patterns vary or have major clinical implications. Streptococcus pyogenes, a Group A Streptococcus, is nearly universally susceptible to penicillin and related beta-lactams, with no clinically significant penicillin resistance in routine practice. Because a reliable first-line treatment exists for GAS, labs do not routinely run susceptibility testing on these isolates unless unusual circumstances arise.

In contrast, Staphylococcus aureus, Proteus mirabilis, and Escherichia coli can exhibit a range of resistance patterns that affect treatment decisions. Staphylococcus aureus includes MRSA strains that resist many beta-lactams, making susceptibility testing essential. Proteus mirabilis and Escherichia coli commonly produce various beta-lactamases or other resistance mechanisms, so determining susceptibility guides appropriate antibiotic choice and helps curb resistant infections.