Influence of the culture medium on antibiotic susceptibility testing of food-associated lactic acid bacteria with the agar overlay disc diffusion method.
OBJECTIVE
To investigate the influence of the culture medium on antibiotic susceptibility testing of food-associated lactic acid bacteria (LAB) with the agar overlay disc diffusion (DD) method.


METHODS
The antibiotic resistance profile of 39 food-associated lactobacilli and enterococci was determined with the agar overlay DD method using a defined medium (i.e. Iso-sensitest agar; ISA) or an undefined medium (i.e. de Man, Rogosa, Sharpe or MRS agar).


RESULTS
The study revealed that ampicillin discs and, although to a lesser extent, also tetracycline discs consistently produced larger zones on MRS medium compared to ISA medium. For the antibiotics gentamicin, bacitracin and erythromycin, the radius of the inhibition zones produced on MRS medium was significantly smaller in relation to ISA. For categorizing LAB isolates into resistant, intermediate and susceptible groups, it was demonstrated that major errors can occur in determining bacitracin and gentamicin resistance if MRS medium instead of ISA medium is used. On the other hand, the performance of both media was found to be equivalent for testing tetracycline resistance.


CONCLUSIONS
Despite the fact that MRS medium generally supports the growth of lactic acid bacteria much better than the nutrient-poor ISA medium, the present study clearly demonstrates that both media are not compatible in susceptibility testing against various classes of antibiotics. These results may stimulate future discussions on a generally recommended DD method for susceptibility testing of food LAB strains.
Huys
G
G
Laboratory of Microbiology, Ghent University, Ghent, Belgium. geert.huys@rug.ac.be
D'Haene
K
K
Swings
J
J
D000362:Agar; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003470:Culture Media; D004058:Diffusion; D016983:Enterococcus / Q000187:drug effects*; D007778:Lactobacillus / Q000187:drug effects*; D008826:Microbial Sensitivity Tests
D000900:Anti-Bacterial Agents; D003470:Culture Media; D000362:Agar