The microbiology lab was able to isolate one of the organisms from Tara’s blood. Using a Kirby-Bauer test (disk diffusion assay), they assessed the organism for antibiotic resistance. The first column of numbers shows the size of the zone of inhibition (ZOI) in millimeters (mm). The remaining columns show the values needed to define the organism as "resistant," "intermediate," or "susceptible" to that antibiotic. Based on these data, which antibiotic(s) would not be an appropriate choice for treatment? Select all that apply. (NCLEX-HESI-TEAS style)
Kirby-Bauer Assay
Measured
ZOI (mm) R I S
Amoxicillin 25 ≤28 - ≥29
Ciprofloxacin 25 ≤15 16-20 ≥21
Erythromycin 23 ≤13 14-22 ≥23
Gentamicin 16 ≤12 13-14 ≥15
Rifampin 26 ≤16 17-19 ≥20
Tetracycline 18 ≤14 15-18 ≥19
Rifampin
Tetracycline
Erythromycin
Ciprofloxacin
Amoxicillin
Gentamicin

Amoxicillin and Tetracycline

Explanation:

Kirby-Bauer test is used to determine the susceptibility of a bacteria to an antibiotic. The target bacteria is grown on an agar plate. Antibiotic wafers are places on these plates. After some time, the plate is checked. Zone of inhibition (ZOI) is the area around the wafer where no bacteria survived. A reference table is checked for determining the susceptibility of the bacteria to a particular antibiotic. If the value of ZOI is smaller than the value required for susceptibility (S), it means that the bacteria is resistant to that antibiotic.

From the given data, ZOI for Amoxicillin is 25 and S value is ≥29 so the organism in question is resistant to Amoxicillin and Amoxicillin is not an appropriate choice for treatment. Similarly, ZOI for Tetracycline is 18 and S value is ≥19 hence it is not the appropriate choice for treatment.