Final answer:
To determine the susceptibility of bacteria to tetracycline with a 19 mm zone of inhibition in a Kirby-Bauer disk diffusion test, you must compare the measurement to standardized charts specific to the antibiotic. However, without these charts, it's often assumed that such a size could indicate susceptibility.
Step-by-step explanation:
When interpreting the results of a Kirby-Bauer disk diffusion test to determine the susceptibility of a microorganism to an antimicrobial drug like tetracycline, the key step is comparing the measured zone of inhibition (ZOI) to a standardized chart. If you measure the ZOI around a tetracycline disk as 19 mm, you need to refer to the specific standard values for tetracycline to decide whether the bacteria exhibit resistance, intermediate susceptibility, or susceptibility. Without the particular standardized values for tetracycline, it is difficult to categorically state the susceptibility; however, common breakpoints may consider a 19 mm ZOI indicative of susceptibility for some bacteria. The zones of inhibition are areas where bacterial growth has been inhibited by the antibiotic, with larger zones typically indicating a more susceptible bacterium. It's important to note that the disk-diffusion method cannot be used to compare the potencies or efficacy of different antibiotics, nor can it differentiate between bactericidal and bacteriostatic activities.