Which organism does not require susceptibility testing to the indicated antimicrobial when isolated from a clinically significant source?

Which organism does not require susceptibility testing to the indicated antimicrobial when isolated from a clinically significant source?

• A. S. aureus: clindamycin
• B. P. mirabilis: gentamicin
• C. S. pyogenes: penicillin
• D. E. coli: levofloxacin

Answer: (C)

Penicillin is reliably effective against Streptococcus pyogenes, with virtually all clinical isolates remaining susceptible. Because resistance to penicillin in S. pyogenes has not emerged, susceptibility testing for penicillin is not routinely needed when this organism is recovered from a clinically significant source. This predictability makes penicillin the preferred, go-to therapy for streptococcal infections without the added step of confirming susceptibility in the lab. In contrast, susceptibility testing is important for the other scenarios because resistance patterns can vary: Staphylococcus aureus may show inducible or constitutive clindamycin resistance depending on the strain; Proteus mirabilis can exhibit variable gentamicin susceptibility with potential for resistance; and Escherichia coli often carries fluoroquinolone resistance affecting levofloxacin effectiveness. Testing helps ensure the chosen antibiotic will actually work in a real infection.

Penicillin is reliably effective against Streptococcus pyogenes, with virtually all clinical isolates remaining susceptible. Because resistance to penicillin in S. pyogenes has not emerged, susceptibility testing for penicillin is not routinely needed when this organism is recovered from a clinically significant source. This predictability makes penicillin the preferred, go-to therapy for streptococcal infections without the added step of confirming susceptibility in the lab.

In contrast, susceptibility testing is important for the other scenarios because resistance patterns can vary: Staphylococcus aureus may show inducible or constitutive clindamycin resistance depending on the strain; Proteus mirabilis can exhibit variable gentamicin susceptibility with potential for resistance; and Escherichia coli often carries fluoroquinolone resistance affecting levofloxacin effectiveness. Testing helps ensure the chosen antibiotic will actually work in a real infection.