A comparison of the isolation rates of Salmonella and thermophilic Campylobacter species after direct inoculation of media with a dilute faecal suspension and undiluted faecal material

Regardless of media used, dilution of faecal samples before direct plating may improve isolation rates and reduce subcultures by freeing organisms fro m the faecal mass and diminishing competing flora. Despite the routine use of dilution in many laboratories, it has never been established properly wh ether direct or dilute inocula should be used in primary plating of faeces, a total of 3764 faecal samples was examined in four laboratories with a st andardised methodology, The isolation rates, competing flora and confirmato ry work performed for Salmonella spp, and Campylobacter spp, from primary p lating media with a dilute faecal inoculum were compared with those after d irect inoculation of faecal material. Inoculum effects on the isolation of Shigella spp. could not be assessed as only one isolate occurred during the study period, The overall isolation rates of both major enteric pathogens were unaffected by the inoculum, However, significantly fewer wasted subcul tures were recorded with a dilute inoculum for Campylobacter spp., and comp eting flora was reduced in all cases without diluting out small numbers of the pathogen.