Effects of sub-MIC antibiotic concentrations on biofilm production of Salmonella Infantis

In the present study, 13 Salmonella Infantis strains, which have been originated from Turkey, were selected due to their clinical and industrial relevance, sufficient biofilm producing capability and multidrug resistance. Although all tested strains were built up of thin pellicle, optimum pellicle formation has occurred at 28 °C. All S. Infantis biofilms were categorized as ‘bdar’ morphotype following the incubation at both 20 and 28 °C, while they were categorized as ‘saw’ morphotype at 37 °C. Under a certain incubation temperature (28 °C), 84.62% of strains have formed strong biofilm structures. By using the disk diffusion method, high levels of resistance have been observed among tested bacteria against nalidixic acid (100%), spectinomycin (100%), streptomycin (92.3%), tetracycline (92.3%), kanamycin (76.9%) and neomycin (76.9%). Further studies were performed with S. Infantis DMC 12 strain, due to its capability to produce biofilm and multidrug resistance phenotype. Gentamycin (>64 µg/mL, 2 × MIC) and tetracycline (>128 µg/mL, 4 × MIC) were determined as the most effective antibiotics against biofilm formation. The biofilm forms have showed increased antimicrobial resistance when it was compared to the planktonic bacteria. The highest resistance rates of the biofilm bacteria were observed to neomycin (12 × MIC) followed by spectinomycin (10 × MIC) and streptomycin (10 × MIC). Biofilm structure was induced as a result of nalidixic acid, spectinomycin, tetracycline and neomycin treatment at sub-MIC concentrations of tested antibiotics.