The four main methods of microbial analysis include growth measurement, microbial counting, physiological index and smear microscopy.
Growth measurement
Volume measurement: The growth status of microorganisms is reflected by measuring the amount of hyphae in a certain volume of culture medium. This method is simple and fast, but the error is large.
Dry weight method: The dry weight of microorganisms is determined by centrifugation or filtration. This method is more cumbersome and is usually used when the microbial product is a bacterial cell, such as dry yeast, feed, etc.
Turbidimetry: The absorbance value at a certain wavelength is measured by an ultraviolet spectrophotometer to determine the growth status of microorganisms. This method is mainly used for bacterial growth monitoring in the fermentation industry.
Mycelium length measurement: For filamentous fungi and some actinomycetes, the length of hyphae growth within a certain period of time can be measured on the culture medium.
Microbial counting method
Hemocytometer method: simple, intuitive, and quick, but only suitable for counting spores produced by single-cell microorganisms or filamentous microorganisms, and the results include dead and live bacteria23.
Staining counting method: Use dyes such as neutral red, trypan blue, and methyl blue to distinguish dead cells from live cells.
Liquid dilution method: Perform a ten-fold serial dilution of the unknown bacterial sample, record the number of test tubes with growth at each dilution after culture, and then calculate the live bacterial content based on the dilution multiple of the sample. This method is more cumbersome, but the number of live bacteria can be calculated.
Plate colony counting method: Spread the diluted bacterial solution on a solid culture medium, and calculate the number of bacteria in the original bacterial solution after culture. This method can obtain information on live bacteria, but the operation is cumbersome and requires a certain amount of culture time.
Physiological indicator method
Determination of nitrogen content: The nitrogen content of most bacteria is 12.5% of dry weight. The growth status of microorganisms can be indirectly reflected by measuring the nitrogen content.
Determination of carbon content: Use K2Cr2O7 solution to measure absorbance at a specific wavelength, draw a standard curve to calculate the growth amount.
Reducing sugar determination method: Reflect the growth status of microorganisms by measuring the amount of reducing sugar, which is often used in large-scale industrial fermentation production.
Determination of other physiological substances: Including nucleic acid, ATP and other contents, as well as acid production, gas production, oxygen consumption, heat production and other indicators.
Smear microscopy
Smear microscopy is a method for examining bacteria, cytology, enzyme bacteria, microfilariae and cholesterol crystals. By preparing smears and observing them under a microscope, the morphology and structure of microorganisms can be intuitively understood
