Bioingeniøren (Aug 2023)
Agar and phosphate - not a harmenious association
Abstract
Background: Heat sterilization of agar and other gelling agents in the presence of phosphate, results in the production of hydogen peroxide (H2O2) and probably other reactive oxygen species (ROS). These substances strongly limit the growth of environmental bacteria. The purpose of this study was to obtain more information about this phenomenon and investigate whether similar problems occur with commercially available media and more patient relevant samples. Materials and methods: Agar media (self-produced or purchased) were used to analyze the bacterial content of bottled waters and samples more representative for clinical specimens. The agars and experiments were designed particularly to see if autoclaving of agar together with phosphate produced growth inhibiting substances, and if the growth inhibition could be removed with catalase. Results: Heat sterilization of agar/phytagel with phosphate led to a strong reduction in the growth of 'water bacteria'. This limitation could be overcome by pre-treating the agar plates with catalase. When testing bacteria of more clinical significance, these effects were negligible within the limitations of the study. Interpretation: The way we prepare agar media, especially if they contain phosphate, has a great impact on the bacterial count (environmental bacteria). If possible, media components should be autoclaved separately and mixed after heat sterilization. The effects are less pronounced with bacteria that are representative of clinical samples.
