Virulence of Bacillus cereus as natural facultative pathogen of Anopheles subpictus Grassi (Diptera: Culicidae) larvae in submerged rice-fields and shallow ponds

Abstract

Out of 4407 Anopheles subpictus larvae collected from submerged rice-fields and shallow ponds, 1412 were found to be unhealthy and 2.8% of unhealthy larvae were naturally infected by Bacillus cereus. B. cereus formed circular, white and flat colonies. Bacteria were gram positive, ellipsoidal/oval spore forming aerobic rods. Although the isolate was positive for catalase, urease, gelatinase, lipase, nitrate reduction and H₂S production, it was negative for indole production, Vogues-Proskauer test, oxidase test and acid/gas production from carbon sources. Through biochemical characterization and fatty acid methyl ester (FAME) analysis, the bacterial isolate was identified as Bacillus cereus. In the laboratory condition, B. cereus suspension resulted in 43.57% and 93.78% death of A. subpictus larvae within 3 and 6 h, respectively. The organisms were sensitive to recommended doses of kanamycin, gatifloxacin, gentamycin, levofloxacin, doxycyclin,  tetracyclin, streptomycin, rifampicin, vancomycin, ciprofloxacin, but found resistant to ampicillin.

Key words: Bacillus cereus, fatty acid methyl ester analysis, scanning electron micrograph, biochemical
characterization, pathogen, Anopheles subpictus larva.