Effect of cryo preservation on Mycobacterium leprae growth in the footpads of non-immunosuppressed mice.

Objective: To investigate using the mouse footpad system, whether the use of cryopreservants help in retaining the viability of Mycobacterium leprae samples stored at three different temperatures of 4 degrees, -20 degrees and -70 degrees C for 30 days.

Design: Biopsies from eight untreated lepromatous leprosy cases were homogenised and inoculated into footpads of normal Swiss White mice within 24 hours (control) and remaining homogenates in each case was divided and stored at 4 degrees C, -20 degrees C and -70 degrees C respectively for 1 month, using either 10% skimmed milk (SM) or Roswell Park Memorial Institute media + 10% glycerol (RPMI) (test). Homogenates adjusted to contain 1 x 10(4) M. leprae/footpad was inoculated into 10 mice per set. Harvestings were done at 6th, 7th, 8th and 12th months. Footpad counts showing > 1 x 10(5) M. leprae at 6th month or later were considered as positive yield.

Results: Control All the cases showed > 100 fold growth and 100% take. Viability at 4 degrees C: Only one case (SM) showed a 100 fold increase and 23% take. Viability at -20 degrees C: Two cases showed fold growth that was 40-60 fold less with takes of 63% (SM) and 71% (RPMI) respectively. Viability at -70 degrees C: Positivity was 45% but the fold increase was less as compared to control and takes were between 80-20%, except one RPMI where take was 100%.

Conclusion: The viability assessed using the mouse footpad was best and consistent in the inoculas that were injected within 24 hours of harvest from the host tissue (control group). None of the storage temperatures used matched with the controls with respect to bacterial yield or % takes. Among the three storage temperatures, -70 degrees C appeared to be better with 45% of the samples showing growth. There was no significant difference noted between the two preservatives used.