QVprep Microbiology Lessons

Bacterial Growth



Relationship of the bacterial concentration to optical density: The relationship of the bacterial concentration to optical density is they are inversely related; as one increases, the other decreases. As bacteria grow in liquid medium, the turbidity increases. Turbid solutions have a high absorbance (optical density) and therefore a low % transmittance of light, which can be correlated to growth or bacterial density.

Sterile Medium: When a medium or item has been rendered “sterile”, this means that the item is free of living microorganisms. Sterile means that all living organisms have been removed or killed.

Statistically accurate, viable (quantitative) plate count: The range of colony forming units (cfu) that must be counted in order to get a statistically accurate, viable (quantitative) plate count is 30-300 cfu; in some industries, such as the food industry, a standard of 25-250 cfu is used. The purpose of a plate count is to correlate the final number of cells/mL to the original sample.

Tonicity: High solute concentrations, such as those present with an excess of glucose, cause an increase in osmotic pressure because of tonicity (effects of hyper- or hypotonic environments).