Microbiology

Microbiology Study Guide

rapidly. This phase is represented by a straight line and the population is at its metabolic peak. Research experiments are often performed at this time. During the next phase, termed as stationary phase, the reproduction of bacterial cells is offset by their death and the population reaches a plateau. The reasons for bacterial death include the accumulation of waste, the lack of nutrients, and the unfavorable environmental conditions that may have developed. If the conditions are not altered, the population will enter its decline , or death phase. The bacteria die off rapidly, the curve turns downward, and the last cell in the population soon dies. Microbial Measurements In order to measure the number of bacteria in a population, various methods are available. In one method, known as the plate count method, a sample of bacteria is diluted in saline solution, distilled water, or other holding fluid. Samples of the dilutions are then placed in Petri dishes with a growth medium and set aside to incubate. Following incubation, the count of colonies is taken and multiplied by the dilution factor represented by that plate. Generally, plates with 30-300 colonies are selected for determining the final count, which is expressed as the number of bacteria per original ml of sample. Another measuring method is to determine the most probable number . This technique is often used to determine the number of bacteria in a sample of contaminated water. Water samples are added to numerous tubes of single-strength and double-strength lactose broth. If coliform bacteria (such as E. c li ) are present, they will ferment the lactose and produce gas. Judging by the number of tubes that contain gas at the end of the test, one may approximate the original number of bacteria in the water sample. Another evaluative method is by a direct microscopic count. A specially designed counting chamber called a Petroff-Hausser counter is used. A measured sample of the bacterial suspension is placed on the counter, and the actual number of organisms is counted in one section of the chamber. Multiplying by an established reference figure gives a number of bacteria in the entire chamber and in the sample counted. The disadvantage of this method is that both live and dead bacteria are counted. Turbidity methods can also be used to assess bacterial growth. As bacteria multiply in liquid media, they make the media cloudy. Placing the culture tube in a beam of light and noting the amount of light transmitted gives an idea of the turbidity of the culture and the relative number of bacteria it contains. The dry weight of a culture can also be used to determine microbial numbers. The liquid culture is dried out, and the amount of microbial mass is weighed on a scale. It is also possible to measure the oxygen uptake of a culture of bacteria. If more oxygen is used by culture A than by culture B and all other things are equal, then it may be deduced that more microorganisms are present in culture A. A variation of this method, called the biochemical oxygen demand (BOD), is used to measure the extent of contamination in a water sample

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