Measuring Microbial Growth Lab 6

Measuring Cell Growth Number of Cells per milliliter of liquid –cells/ml Bacterial cultures contain millions of cells Direct Vs. indirect counts –Direct Counts mean actually counting cells More accurate, but takes a lot longer –Indirect Counts mean estimating the number of cells based on turbidity in liquid or dry weight. Less accurate, but quick Total Count vs. Viable count –Total count  all cells in a culture –Viable count  only live cells

Turbidity Method Spectrophotometer Measure % transmittance –Amount of light going through culture –Transmittance is inversely proportional to turbidity (cell concentration) As turbidity increases cell number increases and transmittance decreases Indirect and Total Count

Turbidity and Transmittance

Plate Counts Direct Viable counts The idea is to evenly spread the culture across a plate and count the colonies Each colony represents a cell in the original culture. Problem: If we have a culture that has 1x10 6 cells/ml and we put 1 ml on the plate will we have isolated colonies?

Plate Count We must dilute first If we put 1ml of culture into 9 ml of sterile media we have performed a 1/10 serial dilution (SD) Formula: SD=AT/TV –AT=amount of culture transferred –TV=Total volume after transfer (1ml + 9 ml) What if we did 5 serial dilutions on that culture that had 1 x 10 6 cells/ml