Introduction

It is well understood that standard sensitivity testing for bacteria using agar diffusion tests (Kirby-Bauer or KB antibiotic testing)^1,2 or Vitek 23 requires up to 5–7 days for accurate results. If a bacterial infectious disease is suspected the clinician is forced to rely on empiric therapies based on antibiograms of bacteria from other patients or environmental cultures in the same location and/or previous cultures taken from the patient.

Although standard KB methodology assures precision and accuracy this long delay can mean life or death to the patient, so in the early stage of such infections a more rapid turnaround of antimicrobial susceptibility is vital for any chance of a successful outcome. For this reason, we have developed a rapid sensitivity test utilizing standard KB methodology with verification of results using Vitek 2.

Materials and Methods

The method requires placing the sample onto a standard bacterial isolation agar media and allowing the bacteria to grow for 12–18 hours. Once sufficient growth is observed colonies are removed using a inoculation loop and placed into 0.45% sodium chloride at a turbidity of 0.6–0.8 McF. The bacterial suspension is transferred to a TSA bacterial agar plate and KB discs are applied immediately in a standard fashion.

The inhibition zones can be analyzed as early as 6–8 hours but usually finalized in 12–24 hours. For mixed species specimens, it is possible to observe complete clearing or two or more inhibition zones. In practice, only those antibiotics that produce complete clearing are considered effective and suitable for treatment. Once analyzed by KB the individual species are isolated and processed in the standard fashion by Vitek 2 for identification and susceptibility.

Results

Twenty cultures were tested using the rapid KB method where bacteria were taken from cultures and then identified and MIC by Vitek. Ninety percent of bacterial isolates were run on Vitek 2 and sensitivities correlated favorably with those KB results.

Discussion

Although this methodology is not a standard Kirby-Bauer procedure, it can provide very useful results in as little as 24–36 hours or at least 3–5 days faster than standard testing with either KB or Vitek. Vitek 2 confirmed the results on over 90% of isolates when comparing the two methods. In the author’s opinion, this “Rapid” method of testing can provide results within the window of opportunity to successfully treat very sick individuals or at least provide a reason to make a change within 1–2 days of instituting empiric antibacterial therapy.

* Presenting author

Literature Cited

1.  Bauer AW. 1959. Single-disk antibiotic-sensitivity testing of staphylococci. AMA. Archives of Internal Medicine. 104(2):208–216.

2.  Bauer AW, Kirby WM, Sherris JC, Turck M. 1966. Antibiotic susceptibility testing by a standardized single disk method. American Journal of Clinical Pathology. 45(4):493–496.

3.  Ligozzi M, Bernini C, Bonora MG, de Fatima M, Zuliani J, Fontana R. 2002. Evaluation of the VITEK 2 System for identification and antimicrobial susceptibility testing of medically relevant gram-positive cocci. J Clin Microbiol. 40(5):1681–1686.