14 terms

Microbiology - post-lab 5

Which bacterium used in this experiment was the most resistant to disinfectants? Why?
Pseudomonas aeruginosa was the most resistant. It possesses genes for resistance.
How was a control included in this experiment? Is this considered a positive control or a negative control? Why?
Water was the negative control, because all organisms grew in its presence (i.e., it was
ineffective against any of the bacteria).
What is the difference between antiseptics and disinfectants?
Disinfectants are chemical agents used on inanimate objects to decrease the number of
microbes on surfaces. Antiseptics are chemical agents used on living tissue to decrease
the number of microbes.
This test can be modified to evaluate chemicals for their ability to kill 106 to 108 dried Clostridium sporogenes or Bacillus subtilis endospores. What are three (3) reasons why this is considered a stringent test?
Chemicals do not readily penetrate bacterial endospores, and the number of endospores used is higher than would be expected in normal use of a disinfectant.
What is the name of the disinfectant used to clean surfaces in our laboratory? What is its mode of action?
Accel (hydrogen peroxide) = oxidizer
What is a surgeon trying to accomplish with a 10-minute scrub with soap and a brush, followed by washing with an antiseptic before surgical procedures?
Scrubbing is an attempt to dislodge as many microbes as possible, and an antiseptic is used to kill as many of the remaining microbes as possible.
If most of the normal and transient microbiota aren't harmful on skin, then why must hands be scrubbed before surgery?
Any microbe that enters a normally sterile location or a compromised host can be harmful.
How were results evaluated in this experiment?
Measure the zone of inhibition (in mm) around each antibiotic disk, and compare it with the chart
values, to determine whether the organism is sensitive (susceptible) or resistant to each
antibiotic, or whether the results are intermediate.
If an "intermediate" result is obtained for an antibiotic, what should the doctor (or vet) do to treat the patient? Why?
Another antibiotic must be chosen to treat the patient. It is impossible to tell whether the
antibiotic is effective against the bacterium or not.
Will all antibiotics be equally effective against Gram-positive and Gram-negative bacteria? Why or why not?
No. Cell wall structure and permeability and species-specific enzymes affect susceptibility.
Why isn't one antimicrobial agent equally effective against all three bacteria used in this experiment?
No. These bacteria have different virulence factors (determined genetically). Also, cell wall
structure and permeability and species-specific enzymes affect susceptibility. (This is
the same reasoning as for Question #10 above.)
Is the disk-diffusion method measuring bacteriostatic or bactericidal activity? Explain.
It is impossible to tell. A dilution method would have to be performed for each antibiotic used, so
that the exact amount required to kill the organism could be determined (i.e., to tell whether it is
bactericidal as opposed to bacteriostatic)
In which growth phase is an organism most sensitive to an antimicrobial agent? Why?
It is most sensitive during the logarithmic phase of growth, when it is the most metabolically
Why is the disk-diffusion method not a perfect indication of how the drug will perform in vivo (i.e., in the patient's body)? What are two (2) factors that could affect this?
The body's metabolism and excretion of the drug aren't addressed on an agar plate. Other variables include the effect of serum; the body's pH, ionic content, and oxygen level; and the side effects of the drug