Assessment of the in vitro antimicrobial activity of Lactobacillus species for identifying new potential antibiotics

• We modified a technique for Lactobacillus antimicrobial activity screening.
• Clear inhibition zones were observed.
• We also developed a method to determine a putative minimum inhibitory concentration (MIC).

The bacteriocin-mediated antimicrobial properties of Lactobacillus spp. have been widely studied, leading to the use of these micro-organisms in the food industry as preservative agents against foodborne pathogens. In an era in which antibiotic resistance is becoming a public health issue, the antimicrobial activity of Lactobacillus spp. could be used for the discovery of new potential antibiotics. Thus, it is essential to have an accurate method of screening the production of antimicrobial agents by prokaryotes. Many in vitro assays have been published to date, largely concerning but not limited to Lactobacillus spp. However, these methods mainly use the spot-on-the-lawn method, which is prone to contamination during the overlay stage, with protocols using methanol vapours or the reverse side agar technique being applied to avoid such contamination. In this study, a method combining the spot-on-the-lawn and well diffusion methods was tested, permitting clear identification of inhibition zones from eight Lactobacillus spp. towards clinical isolates of 12 species (11 bacteria and 1 yeast) commonly found in human pathology. Lactobacillus plantarum CIP 106786 and Lactobacillus rhamnosus CSUR P567 exhibited the widest antimicrobial activity, whereas Lactobacillus acidophilus strain DSM 20079 was relatively inactive. In addition, the putative MIC50 of L. rhamnosus against Proteus mirabilis was estimated at 1.1 × 109 CFU/mL using culture broth dilution. In conclusion, considering the increasing cultivable bacterial human repertoire, these findings open the way of an effective method to screen in vitro for the production of potential antimicrobial compounds.