Abstract
Proteus mirabilis was the predominant cause of acute diabetes-associated pyelonephritis occurring spontaneously in male MM mice until they were segregated in a new environment. Thereafter Pasteurella pneumotropica and Streptococcus faecalis emerged collectively as the most common causal organisms, the pyelonephritis became more chronic and Proteus mirabilis isolates from faeces and urine produced atypical non-swarming colonies on blood agar plates. This did not account for the reduced pathogenicity of Proteus mirabilis; when MM males were returned to the original environment the pyelonephritis again became acute but was associated with the atypical type of Proteus mirabilis although the normal type was abundant in the environment. The MM mice were Caesarian-derived and cross-fostered shortly before their transfer to the second environment, which probably accounts for their changed microbial status, but the reason for the emergence of the atypical type of Proteus mirabilis is not understood. The acute nature of the male MM pyelonephritis when caused by Proteus mirabilis parallels the situation described in other animals and humans.