• sensitivity
• resistance
• selective medium
• bifidobacteria
• clostridia
• detection
• qualification

Bifidobacteria are commensal bacteria found mainly in the digestive tract of humans and animals, helping maintain the balance of the intestinal microbiota. In contrast, clostridia include common commensals as well as highly pathogenic representatives of this taxon. Both groups seem to be resistant to mupirocin. Thus, mupirocin-based media would enable the determination and quantification of the main represented group, but not their selective differentiation. However, the combination of selected antibiotics with other factors, such as acidity, could improve the selectivity of the media towards specific bacterial groups.
In total, 38 bifidobacterial and 29 clostridial strains belonging to different common species (Bifidobacterium spp.: n = 13, clostridia; n = 12) were screened for their sensitivity to 65 different antibiotics using Kirby-Bauer disk diffusion method. The same strains were screened for their sensitivity to acetic, formic, lactic, and propionic acids (0.5, 1, 2% v/v) using diffusion method. Selected strains and antibiotics were tested using E-test method and their sensitivity to acids were evaluated also using quantification on media supplemented with mentioned acids (0.25 and/or 0.5% v/v).
Bifidobacterial and clostridial resistance to mupirocin was confirmed. Therefore, mupirocin-based media are suitable for their common detection and isolation. Despite the large number of tested antibiotics, no suitable antibiotics were found that would allow supplementing the mupirocin medium for selective quantification of either bifidobacteria or clostridia. However, increasing the concentration of lactic or acetic acid seems to be a suitable solution for increasing the selectivity of the bifidobacterial mupirocin medium and inhibition of clostridia. Further researches are needed in order to find a suitable agent for the selective detection of clostridia.