A killer concentration

Another measure is to define an antibiotic's minimal bactericidal concentration (MBC), which defines the lowest concentration that's needed to kill the bacteria. This is especially important for patients with inflammatory diseases or who are immunocompromised - as their immune system may not be working well enough to fully eliminate the pathogens should an antibiotic only inhibit its growth. Determining the MBC involves a test to work out the lowest concentration of an antibiotic that reduces the number of bacterial colonies on subculture by 99.9%. But this test attracts criticism, due to its poor reproducibility between laboratories. Involving broth dilution and re-growth of bacteria in antibiotic-free media, potential variables include the size of the inoculum, sample mixing and growth phase of the bacteria.

Combining MBC and MIC approaches

Assessing both the MBC and MIC may be a more useful strategy for determining the clinical relevance of an antibiotic. A new study, published in the Journal of Microbiological Methods, carried out an objective, detailed comparison of a range of different methodologies for determining the MIC and MBC (1). The researchers tested two bacterial strains, P.aeruginosa PA01 and E.coli ATCC 25922 against four antibiotics: gentamicin, tobramycin, ciprofloxacin, and colistin - as well as S.aureus, which they additionally tested with vancomycin. They used ultrapure water purified using PURELAB Flex apparatus from ELGA LabWater for the preparation and analysis of all samples. The team worked out the MIC using broth microdilution, a calorimetric method and a microscopy-based screening system (MBSS). For determining the MBC, they carried out flurophore staining, comparing this with the broth regrowth method.