Concomitant infections of Anopheles stephensi with Plasmodium berghei and Serratia marcescens: additive detrimental effects.

The mortality rate of Anopheles stephensi increased after infection with Plasmodium berghei and correlated negatively with temperature. Development of oocysts is inhibited at temperatures above 21 degrees C. We tested the hypothesis that microorganisms were involved in killing the mosquitoes. In fact we were able to demonstrate that in our A. stephensi colony great numbers of Serratia marcescens could be found in the midgut of the insects. The highest value was 2.3 x 10(7) cfu/ml. Other bacteria were rarely seen (1 out of 30 females had flavobacteria). Serratia was neither found in larvae and pupae nor in the water of the breeding dishes. Moderate numbers were detectable in glucose solutions (for feeding of adult mosquitoes) as well as in jars where pupae emerged. Isolated Serratia strains grew faster at 25 degrees C than at 21 degrees C. In glucose solutions alone growth rates were low but they rose rapidly after the addition of blood. -In experimental infections of A. stephensi with S. marcescens (1 x 10(7) bacteria/ml glucose solution) the mortality increased at 25 degrees C. At 21 degrees C the effect of Serratia was insignificant whereas in P. berghei-infected A. stephensi the damaging effects of migrating ookinetes were obvious. Additive detrimental effects were observed at 25 degrees C in mosquitoes infected with P. berghei and Serratia concomitantly.