Proficiency test specimens for water bacteriology

A procedure for the preparation and distribution of simulated water specimens for coliform density testing is described. Lyophilization of Escherichia coli in a cooked meat-glucose supporting substrate provides stable samples which may be distributed to participants in a proficiency testing program. Logarithmic conversion of the data allows statistical evaluation of the results for inter- and intralaboratory variation. Comparisons between the most probable number and membrane filter techniques indicate no significant difference in the accuracy of these techniques, although the membrane filter technique is more precise.