How can we define the minimum bactericidal concentration (MBC)?
Is there a difference between the definition of MBC for planktonic cells and biofilm?
According to EUCAST DEFINITIVE DOCUMENT E.Def 1.2
https://www.clinicalmicrobiologyandinfection.com/article/S1198-743X(14)63157-8/fulltext
MBC is defined as "This is the lowest concentration of an antibiotic, expressed in mg/L, that under defined in vitro conditions reduces by 99.9% (3 logarithms) the number of organisms in a medium containing a defined inoculum of bacteria, within a defined period of time".
So, if we have inoculum size of 10^7 CFU/ml so the threshold of the MBC (99.9%) will be 10^4 CFU/ml and if the inoculum size of 10^10 CFU/ml so the threshold of the MBC (99.9%) will be 10^7 CFU/ml, the question is how can we define the remaining bacteria
Similarly, if we need to target the persister cells by reducing 99.99%, how can we define the remaing bacteria
So the MBC is defined as the lowest concentration of the antimicrobial which reduces the initial inoculum of bacteria by at least 3 logs (>=99.9%).
Normally, in the tests performed for determination of MBC you would have an initial concentration of bacteria ca. 105 cfu/ml that would be reduced to below 10 cfu/ml, thus you would have a reduction of 4 logs (99.99%).
Therefore, the MBC would be calculated from the lowest concentration of the substance at which no colonies would be formed on the surface of a general purpose agar (i.e., TSA). For this to happen, you should remove a volume of 0.1 ml from the lowest concentration of antimicrobial inhibiting visible bacterial growth (MIC) and then spreading onto the surface of the agar. Following incubation for a defined optimum time-temperature combination depending on the medium used (e.g., 37oC x 48 h for TSA), MBC is calculated from the plates with no visible colonies, as previously mentioned.
Hope this helps a bit Mona Mahmoud!
- Badges
- Science method
- Similar topics
- Biological Science
- Histology
- Staining