Manual and automated bacterial streak plating using permanent magnet-actuated metal balls

The identification of microorganisms and antimicrobial susceptibility testing usually begins with culture of isolated colonies of the organism by streaking the sample onto petri dishes with appropriate nutrient agar using metal loops. An alternative method of using metal balls actuated by a permanent magnet to successfully perform the streak plate operation was described and demonstrated here. With metal balls of 2.5 mm diameter as carrier of the inoculum, a travel speed of 3 mm/s was found to be optimal for maintaining constant contact with the agar surface and to obtain isolated colonies. In applying this method using both the manual and automated approaches, five repetitions with Escherichia coli and yoghurt samples (likely dominated by Lactobacillus spp) yielded consistent streaking patterns and single colony isolation following incubation. Effective thermal sterilization (180 °C for 3 h) of the used metal balls and restoration of its magnetic properties upon cooling was demonstrated in subsequent streak plating experiments without inoculum.