Multiplication and persistence of Spiroplasma melliferum strain A56 in experimentally infected suckling mice.

Strain A56 of the bee pathogen Spiroplasma melliferum was isolated from a honeybee (Apis mellifera) during ecological studies on mosquito spiroplasmas in Savoie (France). When inoculated intracerebrally (i.c.) into 48-h old suckling mice, this strain was found to replicate to very high titres and to persist in brain for up to 9 months in one individual. We attempted to increase the "neurotropism" of A56 by sequential i.c. passages. During the first two passages, multiplication of the organisms was observed at very high titres in suckling mouse brain (10(7) to 10(11) CCU/ml) generally without the appearance of antibody, thus mimicking the so-called "immunological tolerance" phenomenon. Intracerebral multiplication of A56 decreased during the third passage (10(2) to 10(4) CCU/ml) and ceased during the fourth passage. Spiroplasma multiplication in brain was apparently well tolerated, since brain lesions were minimal and clinical symptoms were limited to a clear, but only rarely significant, delay in growth curves of inoculated versus non-inoculated mice. Progressive spongiform encephalopathy was never observed. Strain A56 S. melliferum appears as the second spiroplasma, after the tick-derived Spiroplasma mirum capable of multiplying in both invertebrate and vertebrate hosts.