Antimicrobial Susceptibility Test of an Enteric Organism


Diagnostic Microbiology In Action
  Microbiology of the Intestinal Tract
    The Enterobacteriaceae (Enteric Bacilli)
      Identification of Pure Cultures of Enterobacteriaceae from the Normal Intestinal Flora
      Isolation Techniques for Enteric Pathogens
      Identification Techniques for Enteric Pathogens
      Serological Identification of Enteric Organisms
      Techniques to Distinguish Nonfermentative Gram-Negative Bacilli from Enterobacteriaceae
      Rapid Methods for Bacterial Identification
    Clinical Specimens from the Intestinal Tract
      Culturing a Fecal Sample
      Identification of an Unknown Enteric Organism
      Antimicrobial Susceptibility Test of an Enteric Organism

Purpose To determine the antimicrobial susceptibility pattern of a gram-negative enteric bacillus
Materials Nutrient agar plates (Mueller-Hinton if available)
Antimicrobial disks
Sterile swabs
Beaker containing 70% alcohol
Pure plate or slant culture of unknown from Experiment 25.2
Tube of nutrient broth (5.0 ml)

  1. Using a sterile swab or inoculating loop, take some of the growth of the pure culture of your unknown organism and emulsify it in 5.0 ml of nutrient broth to equal the turbidity of a McFarland 0.5 standard. (Discard the swab.)
  2. Take another sterile swab, dip it in the broth suspension, drain off excess fluid against the inner wall of the tube.
  3. Inoculate an agar plate as described in Experiment 15.1.
  4. Follow steps 4 through 7 of Experiment 15.1.
  5. Incubate the agar plate at 35°C for 24 hours.
  6. Examine plates and record results for each antimicrobial disk as S (susceptible), I (intermediate), or R (resistant).
  7. Compare results with those obtained for the organism you tested in Experiment 15.1 and Experiment 23.3.

Record your findings.

  1. Judging by the results of your tests, what group of antimicrobial agents appear to be indicated for the treatment of patients with gram-negative infections? Gram-positive infections?
  2. What conclusions can you draw as to the importance of testing each suspected bacterial pathogen for its antimicrobial susceptibility?