Abstract
We evaluated the consequences of prolonging the time between initial bacterial inoculum suspension preparation and susceptibility test inoculation. Extending the current National Committee for Clinical Laboratory Standards-recommended time of 15 min between suspension preparation and test inoculation should allow laboratories more flexibility to optimize efficiency from the standpoint of workflow. We assessed the length of time for which viable-bacterium counts remain stable in three liquid media at room temperature. Fifty isolates were examined in water, saline, and cation-supplemented Mueller-Hinton broth (CSMHB). Disk diffusion and microdilution MIC tests were performed on nine of these. Our results suggest that directly prepared inoculum suspensions of members of the family Enterobacteriaceae, Pseudomonas aeruginosa, staphylococci, and enterococci can be held for up to 6 h in water or saline prior to inoculation of disk diffusion and MIC tests without compromising test accuracy. The same organisms can be held for at least 1 h in CSMHB. Viridans group streptococci can be held for up to 6 h in saline and CSMHB and for up to 3 h in water. Similarly, Haemophilus influenzae and Streptococcus pneumoniae isolates may be held in CSMHB for up to 3 h. Because of an early decrease in viable-bacterium counts in water and saline with some H. influenzae and S. pneumoniae isolates, we recommend that National Committee for Clinical Laboratory Standards recommendations be followed for these species.
Full text
PDF
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Baker C. N., Thornsberry C., Hawkinson R. W. Inoculum standardization in antimicrobial susceptibility testing: evaluation of overnight agar cultures and the Rapid Inoculum Standardization System. J Clin Microbiol. 1983 Mar;17(3):450–457. doi: 10.1128/jcm.17.3.450-457.1983. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Barry A. L., Badal R. E., Hawkinson R. W. Influence of inoculum growth phase on microdilution susceptibility tests. J Clin Microbiol. 1983 Sep;18(3):645–651. doi: 10.1128/jcm.18.3.645-651.1983. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Brook I. Inoculum effect. Rev Infect Dis. 1989 May-Jun;11(3):361–368. doi: 10.1093/clinids/11.3.361. [DOI] [PubMed] [Google Scholar]
- D'Amato R. F., Hochstein L. Evaluation of a rapid inoculum preparation method for agar disk diffusion susceptibility testing. J Clin Microbiol. 1982 Feb;15(2):282–285. doi: 10.1128/jcm.15.2.282-285.1982. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Lund M. E., Hawkinson R. W. Evaluation of the Prompt inoculation system for preparation of standardized bacterial inocula. J Clin Microbiol. 1983 Jul;18(1):84–91. doi: 10.1128/jcm.18.1.84-91.1983. [DOI] [PMC free article] [PubMed] [Google Scholar]