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The bacterium that caused field-grown onion decay was identified by nucleotide sequence analysis of the 16S rRNA genes to be Serratia marcescens, a known phytopathogen. However, this isolate did not respond on specific primers designed for pathogenic strains. Inoculation of onion (Allium cepa L.), Arabidopsis thaliana (L.) Heyhn, and lettuce (Lactuca sativa) seeds resulted in biomass promotion of symptomless plants, however, growth promotion attributes were not detected. Polymerase chain reaction revealed the presence of a class 1 integron in S. marcescens IMBG291 which represented the first isolation of this integron in phytopathogenic Serratia species. The gene cassettes harbored by the integron have been represented with the promoterless genes encoded formiminoglutamate deiminase and ascorbate-specific phosphotransferase system enzyme IIC, and with additional three senseless sequences flanked by a 59-bp element. The conclusion was made that S. marcescens IMBG291 exhibited polybiotrophy, expressing either pathogenicity or plant growth promotion, depending on the environmental situation.