Novobiocin is produced by the bacteria streptomyces

Novobiocin is produced by the bacteria, Streptomyces .

Figure 3. DNA replication.
Explore Gentamicin
Gentamicin is a narrow spectrum, bacteriostatic antibiotic that targets
aerobic, Gram-negative bacteria and Gram-positive Staphylococcus .
Gentamicin binds with bacterial ribosomes, cellular structures
responsible for protein synthesis. See Figure 4. When ribosomes are
bound with gentamicin they are no longer able to synthesize protein
and the bacterial cell ceases to grow and divide. Gentamicin is
synthesized by the Gram-positive bacteria, Micromonospora .

Figure 4. Protein synthesis.
Explore Kirby-Bauer Diffusion Test
The most common method used in hospital laboratories to assess
bacterial antibiotic resistance is the Kirby-Bauer Diffusion test . The
Kirby-Bauer Diffusion test is performed using commercially prepared
filter paper disks that have been infused with a specified concentration
of a particular antibiotic. When placed on an agar containing newly
plated bacteria, the antibiotic diffuses away from the disk into the
agar, creating a concentration gradient that decreases logarithmically
with the distance from the disk. As the bacteria grow, they will be
inhibited in regions where the antibiotic concentration is above what
the microorganism can handle. Areas where the antibiotic
concentration is low will allow for bacterial growth; this growth will lead
to the creation of a lawn across the plate, creating a zone around each
disk. See Figure 5.
Figure 5. Kirby-Bauer Diffusion test plate with zones of inhibition.
The diameter of this zone of inhibition is measured in millimeters
and compared to a chart that links the size of the zone of inhibition to
the dosage that is often administered to a patient. The diameter of the
zone is dependent on the sensitivity of a microorganism to a particular
antibiotic and the physical properties of the antibiotic itself. Some
antibiotics are not as soluble as others and as such will have a different
diffusion rate through the agar. Therefore, a very large zone of
inhibition does not necessarily mean that the antibiotic is extremely
effective. The effective size for a zone of inhibition is calculated
through hundreds of assays on many different strains of bacterial
isolates correlating this information with many other susceptibility
tests. In this method, an organism may be categorized as resistant,
susceptible, or intermediate to a panel of antimicrobial agents. An
intermediate result is based on when the results can be influenced by
other factors. For example, the zone of inhibition may indicate the
possibility that an agent would be effective, but the site of infection

and the ability of the antibiotic to reach this site is suspect, and may
not fully reach inhibitory concentrations. Under this circumstance, it
would be recommended that a different antibiotic be selected.