If a bacterial culture is left in the same media for too long, the cells use up the available nutrients, excrete toxic metabolites, and eventually the entire population will die. Thus bacterial cultures must be periodically transferred, or subcultured, to new media to keep the bacterial population growing.

How can a streak plate become contaminated? If the loop is not sterilized. If you drop the plate. If lid isn’t on.

Streak plate technique is used for the isolation into a pure culture of the organisms (mostly bacteria), from a mixed population. The inoculum is streaked over the agar surface in such a way that it “thins out” the bacteria. Some individual bacterial cells are separated and well-spaced from each other.

The pour plate technique can be used to determine the number of microbes/ mL in a specimen. It has the advantage of not requiring previously prepared plates and is often used to assay bacterial contamination of foodstuffs.

The main difference between pour plate and spread plate is that the molten agar is poured on to the inoculum during the preparation of the pour plate whereas inoculum is spread on the surface of the solidified agar during the preparation of the spread plate.

The pour plate method of counting bacteria is more precise than the streak plate method, but, on the average, it will give a lower count as heat sensitive microorganisms may die when they come contact with hot, molten agar medium.

Disadvantages. The streak plate method does not work with high volumes of organisms. It will not enable you to get a concentration count. It requires huge storage space and there is a possibility that your incubator cannot accommodate a large volume of petri plate.

After incubation, bacterial growth is visible as colonies in and on the agar of a pour plate. Can some bacteria grow on the streak plate and not be seen if the pour plate technique is used? Yes, the pour plate is O2 limited. Thus, some bacteria will only grow on the streak plate as it provides ample O2.

If the plate has not been inoculated, the presence of any bacterial colonies indicates contamination. On an inoculated plate, look for colonies that display morphology different than what you would expect from the type of bacteria used to inoculate the plate.

Flaming the loop between streaks ensures that the loop starts clean and that only this small amount of bacteria is used to inoculate the next quadrant.

What must be done between each phase in a streak plate? Examine the plate macroscopically for difference in colony characteristics. The medium clears around the colony from opaque to transparent. Gamma-hemolysis: no hemolysis.

As you might guess, the purpose of streaking for isolation is to produce isolated colonies of an organism on an agar plate. This is useful when you need to separate organisms in a mixed culture or when you need to study the colony morphology of an organism.

Streak plating is a microbiology laboratory method that has two major disadvantages. Firstly, users will not be able to grow obligate anaerobes using this method. Secondly, only organisms that were viable in the original sample are able to be grown.

What is a pure culture? What is a bacterial colony? What would happen if you forgot to sterilize your loop in between each quadrant streak? You would spread a lot of bacteria back into quadrant one and probably not see isolated colonies.

Streak-plate Technique. A sample application for streak plating is shown in Figure 1. This procedure is used for isolating bacterial colonies from mixed cell cultures and is by far one of the most important techniques to master in microbiology and molecular genetics.

Petri dishes need to be incubated upside-down to lessen contamination risks from airborne particles landing on them and to prevent the accumulation of water condensation that could disturb or compromise a culture.

What advantage does the pour-plate method have over the streak plate method? The pour plate method requires less skill, has optimization built in, and will more likely produce the desired result. Why is the loop flamed before it is placed in a culture tube?

What would a streak plate look like if you had not sterilized the loop between streaking into the new areas of the plate? While the number of bacteria colonies seen in each area may decrease, the numbers would not decrease enough for individual colonies to form.

first quadrant

Optimal Growth Conditions Thus, a microbiologist will incubate a particular strain of bacteria at its optimal temperature so that he can study it when it is healthy. Organisms that grow best at human body temperature, which is approximately 37 degrees Celsius (98.6 degrees Fahrenheit), are called mesophiles.

Another commonly practiced technique is to take a jar, something wider than your petri dish and fill it with hot water and once you’ve poured your stack of dishes, put the jar of hot water on the top dish and leave it all to cool.

Move the flame quickly over the surface of the agar or you will melt the petri dish. lids are free of moisture. Store plates upside down in a refrigerator or cold room. If they are stored in a room, check the plates after a few hours for condensation in the lid.

It should go away eventually once the agar completely cools. Condensation does not mean it’s a more humid environment. It only means the inside of the dish is warmer than the outside.

Excess condensation means you poured your agar too hot. When you finish your pours, stack a cup with cool water on top of your last Petri while it cools. After it’s solid, store inverted and don’t stress it.