Phoresis between Serratia marcescens and Steinernema carpocapsae (Rhabditida: Steinernematidae) during Infection of Galleria mellonella (Lepidoptera: Pyralidae) Larvae.
Abstract
Infective juveniles (IJs) of an entomopathogenic nematode strain native to southern Mexico transported an associated bacterium that turned the infected Galleria mellonella (L) larvae reddish. The nematode isolate (LBIN-1) was identified as Steinernema carpocapsae (Filipjev) by ITS sequence, and its associated bacterium (LBSe-17) as Serratia marcescens Bizio by 16S rDNA sequencing. Infectivity of S. marcescens LBSe-17 was confirmed by following Koch’s postulates on G. mellonella larvae. Phoresis of the associated S. marcescens bacterium by the nematode into the G. mellonella larvae was corroborated by exposing G. mellonella larvae to S. marcescens either alone or mixed with the nematode. No larval mortality was observed in the first treatment, while 100% mortality was observed in the second treatment. S. marcescens was superficially carried by the IJs, as confirmed by surface sterilization of IJs, which caused total larval mortality but no growth of S. marcescens. Artificial induction of a similar association was achieved by mixing the S. marcescens strain with another Steinernema sp. strain (LBIN-2), showing total larval mortality of G. mellonella larvae and proliferation of S. marcescens in the cadavers. However, 5 consecutive cycles of larval infections showed that colony forming unit (CFU) counts from the larval cadavers declined sharply in the artificially induced association, while the natural association LBIN-1/ LBSe-17 declined, but then tended to stabilize.
Una cepa de nemátodo entomopatógeno, nativa del sur de México, mostró una bacteria asociada que tornaba rojizas a las larvas de Galleria mellonella (L) que infectaba. El nemátodo aislado (LBIN-1) se identificó como Steinernema carpocapsae (Filipjev) por secuenciación de su ITS y a la bacteria asociada (LBSe-17) como Serratia marcescens Bizio por secuenciación de la subunidad 16S. La infectividad de S. marcescens LBISe-17 fue corroborada al aplicar los postulados de Koch sobre larvas de G. mellonella. La foresis de la bacteria por el nemátodo al interior de la larva fue corroborada al exponer larvas de G. mellonella a la bacteria sola y a una mezcla de la bacteria y el nemátodo. No se observó mortalidad larval en el primer tratamiento mientras que ésta fue del 100%, en el segundo. Se comprobó que la bacteria asociada se transporta en la superficie del nemátodo al infectar larvas de G. mellonella con nemátodos esterilizados superficialmente. Todas las larvas fueron infectadas por el nemátodo, pero ninguna desarrolló la infección causada por la bacteria. Una asociación similar se indujo artificialmente al mezclar otra cepa de Steinernema (LBIN-2) con S. marcescens LBISe-17, provocando el 100% de mortalidad en las larvas infectadas y la proliferación de la bacteria en los cadáveres. Sin embargo, al mantener esta nueva asociación por cinco ciclos consecutivos de infección, los conteos de la bacteria asociada declinaron rápidamente, mientras que éstos, en la asociación natural, tendieron a estabilizarse.
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