Drop-plate versus spread-plate enumeration for assessing antimicrobial effects of a portable point-to-ring non-thermal plasma.

Non-thermal plasma (NTP) is a promising antimicrobial approach with potential applications in microbial decontamination and infection control. This study evaluated the antimicrobial effects of NTP against bacteria and yeast, comparing two microbial enumeration methods used after plasma exposure. A portable point-to-ring bipolar corona discharge source (7 kV, 150 μA, 3.3 mm gap) was employed to compare drop plate (DP) and spread plate (SP) methods following a maximum exposure time of 600 s across six microorganisms. Subsequently, the time-dependent antimicrobial effects of NTP were assessed at exposure times of 150, 300 and 600 s using 12 microbial strains, including Gram-negative and Gram-positive bacteria, as well as Candida yeast species. No significant differences were observed in colony-forming unit (CFU; log scale) counts between the DP and SP methods (p > 0.05). Bland-Altman analysis demonstrated an acceptable agreement between the two methods, indicating that they can be considered comparable within the context of the present study. However, under the experimental conditions evaluated, the DP method was faster and more cost-effective. Following 150 s of NTP treatment, all tested strains exhibited at least a 99% reduction in viability. The most susceptible strains were Listeria monocytogenes 5576, Staphylococcus epidermidis 4418 and Candida albicans 8261. These findings demonstrate that the DP method can be considered a reliable and economical alternative to the SP method for enumeration following plasma treatment.