Acridine orange as an alternative to optical density to study growth kinetics of Lactobacillus bulgaricus ATCC 7517.

In this study we assessed the use of acridine orange as an alternative to optical density to quantify the growth of Lactobacillus bulgaricus ATCC 7517. The growth of bacteria in Lactobacillus de Man Rogosa Sharpe (MRS) medium was measured by both acridine orange (AO) and optical density (OD) measurements for 24 h. The relationship between both methods was compared via correlation analysis. The doubling time of bacteria based on the values of OD600 and AO obtained during 24 h growth were also calculated. The result shows strong correlation of cell growth between OD600 and AO during the first 10 hours of growth, but the correlation was less strong when analyzing the data from 0 to 24 hours. Growth rates, generation time and lag time were also similar. This study indicates that AO could be used in place of OD to prepare growth curves of Lactobacillus bulgaricus during the exponential phase of growth, and to compare growth rates, generation times or lag times.

Antimicrobial properties of three lactic acid bacterial cultures and their cell free supernatants against Listeria monocytogenes.

Control of Listeria monocytogenes in ready-to-eat (RTE) food products is a significant challenge and improved means for control are needed. In this study, the anti-listerial effects of three lactic acid bacteria (LAB) were investigated. Spot-on-lawn assays demonstrated the largest zones of inhibition against L. monocytogenes were produced by the Pediococcus acidilactici strain, with zone diameters ranging from 13 to 18 mm. Minimum inhibitory concentration (MIC) experiments using cell free supernatant (CFS) from the LAB revealed that while two of the strains were effective at inhibiting L. monocytogenes growth only up to a 1:4 dilution, P. acidilactici was able to inhibit growth up to a 1:256 dilution. Survival assays performed at 7°C determined that the P. acidilactici strain was capable of producing a 4.5 log reduction in L. monocytogenes counts and maintaining the reduction for 21 days. The effectiveness of P. acidilactici was reduced under log phase growth, autoclaving for longer than 15 min (121°C and 15 psi), and treatment with proteinase K (25 mg/mL).