Microbiology, 2021

548 13 • Control of Microbial Growth
CHECK YOUR UNDERSTANDING
• Is the use-dilution test performed in a clinical setting? Why?
In-Use Test
An in-use test can determine whether an actively used solution of disinfectant in a clinical setting is
microbially contaminated (Figure 13.32). A 1-mL sample of the used disinfectant is diluted into 9 mL of sterile
broth medium that also contains a compound to inactivate the disinfectant. Ten drops, totaling approximately
0.2 mL of this mixture, are then inoculated onto each of two agar plates. One plate is incubated at 37 °C for 3
days and the other is incubated at room temperature for 7 days. The plates are monitored for growth of
microbial colonies. Growth of five or more colonies on either plate suggests that viable microbial cells existed
in the disinfectant solution and that it is contaminated. Such in-use tests monitor the effectiveness of
disinfectants in the clinical setting.
Figure 13.32
Used disinfectant solutions in a clinical setting can be checked with the in-use test for contamination with microbes.
CHECK YOUR UNDERSTANDING
• What does a positive in-use test indicate?
Clinical Focus
Resolution
Despite antibiotic treatment, Roberta’s symptoms worsened. She developed pyelonephritis, a severe
kidney infection, and was rehospitalized in the intensive care unit (ICU). Her condition continued to
deteriorate, and she developed symptoms of septic shock. At this point, her physician ordered a culture
from her urine to determine the exact cause of her infection, as well as a drug sensitivity test to determine
what antibiotics would be effective against the causative bacterium. The results of this test indicated
resistance to a wide range of antibiotics, including the carbapenems, a class of antibiotics that are used as
the last resort for many types of bacterial infections. This was an alarming outcome, suggesting that
Roberta’s infection was caused by a so-called superbug: a bacterial strain that has developed resistance to
the majority of commonly used antibiotics. In this case, the causative agent belonged to the carbapenem-
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