Listeria detection in microscale solid state inoculation with minimal selective agents

The food industry needs a simple, reliable, and cost-effective primary screening protocol for routine inspection of bacterial contamination. Microscale inoculation technique is proposed as an alternative method for Listeria detection. This proposed novel technique shows a good correlation with standard spread plate technique for the enumeration of pure Listeria cultures (R2 = 0.96, P < 0.0001). The commonly used selective agents in Listeria media (PALCAM, MOX, and OCLA) were assessed for their inhibitory effect on Listeria monocytogenes, Listeria innocua, and Listeria ivanovii using the microscale inoculation technique. The concentration of selective agents was lowered to inhibit competitive bacteria with minimum impact on the growth of Listeria. At the standard concentration, all three media showed less inhibition on L. monocytogenes and L. innocua than on L. ivanovii. OCLA had less inhibitory effect on the three species than did MOX and PALCAM. The comparison between ISO 11290-1 and microscale with 25% of selective agent concentration on detection of Listeria in 36 naturally contaminated food samples revealed that the microscale technique agreed well with the ISO method (Cohen KAPPA = 0.83). Reduction of selective agent concentration to 25% of the conventional formulas resulted in substantial improvement in detectability of Listeria spp. without significantly reducing specificity. The detection sensitivity of the Listeria colonies in food samples was significantly improved by microscale inoculation with 25% of the inhibitors on the three selective media at 24 and 48 h incubation (ANOVA, P = 0.039). At 48 h incubation the improvement was from 95%, 90%, and 85% (ISO method with regular strength inhibitors) to 100%, 100%, and 90% (microscale) on OCLA, MOX, and PALCAM respectively. There was no discrepancy between the microscale and the ISO method in detection of L. monocytogenes in the food samples. The optimization of Listeria detection improves sensitivity of detection as well as reducing media volume which may reduce costs and waste generated.