Rational evaluation of antimicrobial properties of lactobacilli isolates against some pathogenic microorganisms: a new method comparing the susceptibility of indicator microorganisms

Lactobacilli are known as a valuable source of antimicrobial compounds and have a high potential of use in food biopreservation against food related microorganisms.

Antimicrobial potency of 63 dairy lactobacilli isolates against four highly important food-related microorganisms were evaluated. In addition، a new way in data organization was introduced، which led to a more informative and rational comparison of indicator microorganisms'' susceptibilities to a set of compounds. Correlation of pH and antimicrobial properties was investigated.

Microbroth dilution assay was used to evaluate indicator microorganisms'' susceptibility to lactobacilli CFCS (cell free culture supernatant). Results were organized by both the conventional way - demonstrating the minimum inhibitory and lethal concentrations of CFCS - and a new suggested method، representing comparative effectiveness of each CFCS specimen against indicator microorganisms of comparison interest.

Susceptibilities of tested strains were in the following order: Escherichia coliO157: H7 > Listeria monocytogenes > Aspergillus parasiticus> Candida parapsilosis. Despite the high susceptibility of L. monocytogenes، it showed the highest resistance to death among the tested microorganisms. Eefficiency of Lactobacilli CFCS in killing the tested strains showed the following susceptibility order: E. coli O157: H7 > A. parasiticus> C. parapsilosis> L. monocytogenes. Antimicrobial property was in correlation with the pH value of CFCS. PH had a pronounced impact on susceptibilities of C. parapsilosisand E. coli in pH values of concentrated CFCS lower than 4 and 4. 5، respectively.

Potency of lactobacilli isolates in growth inhibition of the indicator microorganisms was found promising، and the suggested data organization method provided additional information، leading to more precise comparison of indicator microorganisms.