Determination of available breaking stress of agar and gellan gum plate culture methods and the duration of bacterial culture under strong acidic conditions

Abstract

Aims: Several acidophilic bacteria have not been cultured, primarily owing to the lack of suitable culture methods under strong acidic conditions. This study aimed to quantitatively evaluate the strengths of the agar plates (AP) and gellan gum plates (GP), and optimal culture periods under strong acidic conditions. Methods and Results: To define the lower limit of plate strength for bacterial isolation culture, the diameter of Escherichia coli K12 colonies and the breaking stress of plates at different concentrations of gelling agents, medium composition and pH conditions were determined. The lower limit of available strength of AP and GP was 19Á6 and 14Á8 kPa, respectively. Medium composition slightly affected AP breaking stress, although GP with a high cationic concentration medium could not be prepared. Conclusions: Assessment of the strength limits of AP and GP revealed that AP is not suitable for prolonged bacterial culture (≥72 h). Furthermore, GP was completely ineffective for bacterial culture under highly acidic conditions (≤pH 1Á0). Significance and Impact of the Study: Our quantitative evaluation method based on breaking stress is a potentially valuable tool to understand the state and the suitable limit of plate culture methods in more detail under various conditions.

Improvement of the Pour Plate Method by Separate Sterilization of Agar and Other Medium Components and Reduction of the Agar Concentration

“…Using a durometer, the strength of the culture medium was assessed after it was cooled and poured onto a sterile petri dish without a sample. The limit for colony isolation without swarming of 1.0% (wt/vol) agar has been established by a previous study ( 40 ), and the better growth of microorganisms despite the significant lower strength values found only in TSA culture medium has been demonstrated. In TSA, the lower strength is correlated with a lower agar concentration.…”