The microdilution broth method is very useful for determination of the MIC of an antimicrobial compound.
However, one of the major problems with the use of turbidimetric analysis to determine MIC values is the
minimal range of detection. A bacterial culture with a concentration of 106-107 CFU ml-1 is needed for the
spectrophotometer to record a meaningful reading. Thus, in many cases an actively growing culture may be
undetected with no increase in absorbance using a spectrophotometer (Kim, Marshall, & Wei, 1995).Therefore,
kill-time studies were performed to gain a better insight on bacteriostatic or bactericidal effects of the most
potent ethanol extracts. Ethanol extracts of oregano and clove used in the present study appeared to be
particularly effective, as bactericidal effects were observed at MICs already after 1 h of exposure, and were
maintained during 24 h for both Gram-positive and Gram-negative bacteria. In accordance with our results,
Moreira et al. (2005) reported that clove essential oil used at MIC level against E. coli exerted its maximum
bactericidal effect after 15 min and its action was maintained during 20 h of incubation. On the other hand, in
this study, extracts of sage or celery in most cases exerted bacteriostatic activity against Gram-positive bacteria,
while levels of Gram-negative bacteria slightly increased over a 24 h period, indicating that periodic sampling of
cultures exposed to extracts should be performed in addition to turbidimetric measurements to determine
bacterial survival.