ABSTRACT
The measurement of physiological components or metabolites may be used to rapidly estimate microbial numbers before bacterial colonies are visible. Traditionally, the microbial quality of food and raw materials has been monitored by standard plating methods that require 1-2 days for bacterial cell counts and 4-5 days for yeast and mold cell counts. Commercially available prepared media and automated counting methods have increased productivity but have not offered substantial time savings because an incu bation period is still required for colonies to become visible. Impedance microbiology utilizes metal electrodes to measure compositional changes occurring within the growth medium long before colonies are able to reach a visible biomass on standard plating media. As bacteria grow, they metabo lize large, weakly charged molecules (polysaccharides, fats, proteins) and produce small, highly charged molecules (organic acids, fatty acids, amino acids) as metabolic by-products. As metabolites are produced by bacteria, the ability of the medium to impede the flow of electricity changes. This change in impedance can be detected by the instrument. The impedance detection time is inversely proportional to the number of microorganisms
present at initial inoculation and can be standardized to a bacterial plate count. Using this method, a single coliform cell can be detected in 9 hours; a single yeast cell in 19 hours. Media components and temperature can be used to select for the growth of specific microorganisms (i.e., coliforms, yeasts and molds, lactic acid bacteria, etc.). Low temperatures can be used to monitor the growth of psychrotrophs, the predominant spoilage bacteria of dairy products and raw meats. Reducing agents may also be incorporated in the growth medium to monitor bacterial growth under reduced oxygen conditions. Additionally, impedance instruments can be helpful in evaluat ing the growth of bacteria in the presence of antibacterial agents such as biocides used in the sanitation of production lines or preservatives used in cosmetic production. In a unique application, the capacitance portion of the impedance signal is used to identify chemical variations in the product relating to product freshness. Recent literature supports the increased interest in impedance microbiology and the potential for multiple uses beyond the enumeration of bacteria. Impedance detection offers a sensi tive, rapid, and automated method for enumeration of bacteria, yeasts, and molds.
