Bacterial growth defined Since individual cells double in size, then divide into two, the meaningful increase is in the population size. Binary fission: cell divides into two cells. No nucleus, so no mitosis. Cells do not always fully detach; produce pairs, clusters, chains, tetrads, sarcina, etc.
Mathematics of bacterial growth Because bacteria double in number at regular intervals, they grow exponentially: N = N0 x 2n where N is the number of cells after n number of doublings and N0 is the starting number of cells. Thus, a graph of the Log of the number of bacteria vs. time is a straight line.
The Bacterial Growth Curve Bacteria provided with an abundant supply of nutrients will increase in number exponentially, but eventually run out of nutrients or poison themselves with waste products. Lag phase Exponential or Log phase Stationary phase Decline or Death phase. 4 3 2 1
Growth curve (continued) Lag phase: growth lags; cells are acclimating to the medium, creating ribosomes prior to rapid growth. Log phase: cells doubling at regular intervals; linear graph when x-axis is logarithmic. Stationary phase: no net increase in cell numbers, some divide, some die. Cells preparing for survival. Decline phase: highly variable, depends on type of bacteria and conditions. Death may be slow and exponential.
More about Growth The Growth curve is true under ideal conditions; in reality, bacteria are subject to starvation, competition, and rapidly changing conditions. Generation time: the length of time it takes for the population to double. Growth of bacteria is nonsynchronous, not every bacterium is dividing at the same time. Instead of stepwise curve, smooth curve
Measurement of growth Direct methods: cells actually counted. Petroff-Hausser counting chamber (right), 3D grid. Count the cells, multiply by a conversion factor. Dry a drop of cells of known volume, stain, then count. Coulter-counter: single-file cells detected by change in electric current.
Measurement of growth -2 Viable plate count Relies on bacteria being alive, multiplying and forming colonies. Spread plate: sample is spread on surface of agar. Pour plate: sample is mixed with melted agar; colonies form on surface and within agar. biology.clc.uc.edu/.../Meat_Milk/ Pour_Plate.htm
Filtration: Membrane filters are very thin with a defined pore size, e.g. 0.45 µm. Bacteria from a dilute sample are collected on a filter; filter placed on agar plate, colonies counted. http://dl.clackamas.cc.or.us/wqt111/coliform-8.jpg http://www.who.int/docstore/water_sanitation_health/labmanual/p25bs.jpg
Spectrophotometry Bacteria scatter light, making a turbid (cloudy) suspension. Turbidity is usually read on the Absorbance scale Not really absorbance, but Optical Density (OD) More bacteria, greater the turbidity (measured as OD) Based on www.umr.edu/~gbert/ color/spec/Aspec.html
More about Spectrophotometry Does NOT provide an actual number unless a calibration curve (# of bacteria vs. O.D.) is created. Indirect counting method Quick and convenient, shows relative change in the number of bacteria, useful for determining growth (increase in numbers). Does NOT distinguish between live and dead cells. To create a calibration curve, best to plot OD vs. number of cells determined with microscope (not plate count).
Biomass: Measure the total mass of cells or amount of any component such as protein, PS, DNA, KDO. Especially when cells are doubling, the amounts of all the components of a cell are increasing at the same rate, so any could be measured. Not so in stationary phase. Note different ratios of carbohydrate (slime) to protein in these cells.