Modified culture protocol for isolation of Mycobacterium avium subsp. paratuberculosis from raw milk

A modified culture method using C₁₈-carboxypropylbetaine (CB-18) and microscopic screening was evaluated for time to and limit of detection of Mycobacterium avium subsp. paratuberculosis (MAP) in raw milk. Bulk-tank milk samples weie spiked with six different concentrations (10¹ to 10 ⁶ CFU/mL) of MAP. Samples were processed using two different protocols. The first protocol involved specimen processing with the zwitterionic detergent C₁₈-carboxypropylbetaine (CB-18) and lytic enzymes followed by culture on modified Middlebrook 7H10 agar plates with microscopic screening. The second protocol used 0.75% hexadecylpyridinium chloride (HPC) for specimen processing, followed by culture on Herrold's egg yolk medium (HEYM). Both protocols were repeated eight independent times, and the detection limit and time to detection were compared. The presence of MAP in spiked milk samples was detected between 14 and 45 days (N [number of samples], 46; mean, 22.7; median, 19.5) using the CB-18 and microscopic screening method, and between 21 and 63 days (N, 47; mean, 31; median, 28) using HEYM (P < 0.001). Time to detection also varied with MAP concentration (P < 0.001). Higher concentrations of MAP were detected earlier than lower concentrations and this finding was independent of the method used (P = 0.479). The two methods had similar detection limits but the modified culture method reduced the time to detect MAP in raw milk for the majority of concentrations.