Gradient diffusion antibiotic susceptibility testing of potentially probiotic lactobacilli

Area of Science:

• Microbiology
• Probiotics
• Antibiotic Resistance

Background:

• Lactobacillus rhamnosus strains are widely used as probiotics.
• Accurate antibiotic susceptibility testing is crucial for probiotic safety and efficacy.
• Standardized methods are needed for evaluating the antibiotic profiles of Lactobacillus species.

Purpose of the Study:

• To determine the minimum inhibitory concentrations (MICs) of various antibiotics against Lactobacillus rhamnosus strains.
• To compare different inoculum application methods for gradient diffusion antibiotic susceptibility testing.
• To establish reliable methods for antibiotic susceptibility testing of potentially probiotic lactobacilli.

Main Methods:

• Gradient diffusion (E test) on deMan, Rogosa, Sharpe (MRS) agar.
• Testing of Lactobacillus strain GG and 11 Lactobacillus rhamnosus strains against cell wall, protein, and nucleic acid synthesis inhibitors.
• Evaluation of swab and agar overlay inoculum application methods, with and without antibiotic prediffusion.

Main Results:

• All Lactobacillus rhamnosus strains were resistant to vancomycin, co-trimoxazole, metronidazole, gentamicin, and streptomycin.
• Strains were sensitive to penicillin G, ampicillin, rifampicin, tetracycline, chloramphenicol, and erythromycin.
• Antibiotic prediffusion and agar overlay methods influenced MIC values but did not alter susceptibility categories; method agreement varied by strain.

Conclusions:

• The swab and agar overlay gradient diffusion methods are reliable for antibiotic susceptibility testing of rapidly growing, facultative anaerobic lactobacilli on MRS agar.
• These methods are applicable for routine testing of individual Lactobacillus isolates.
• Understanding the antibiotic susceptibility profiles of probiotic lactobacilli is essential for their safe application.