How can streak plate be contaminated

Some individual bacterial cells are separated and well-spaced from each other. As the original sample is diluted by streaking it over successive quadrants, the number of organisms decreases. Usually, by the third or fourth quadrant, only a few organisms are transferred which will give discrete colony forming units CFUs. While streaking in successive areas of the plate, the inoculum is diluted to the point where there is only one bacterial cell deposited every few millimeters on the surface of the agar plate.

When these lone bacterial cells divide and give rise to thousands and thousands of new bacterial cells, an isolated colony is formed. Pure cultures can be obtained by picking well-isolated colonies and re-streaking these on fresh agar plates.

A common assumption is an isolated colony of bacteria is the progeny of a single bacterial cell i. However, this is not necessarily true. With species in which the cells form a characteristic grouping during cell divisions, the colony-forming unit may develop from a group of cells rather than form a single cell. For example, clusters of staphylococci, chains of streptococci, etc. Note: Bi-plate inoculation of samples from the sterile sites is often done in diagnostic laboratories to save handling time and space.

Examine the colonies grown in the plate carefully. As I learned, the streak plate method is a rapid qualitative isolation method. The techniques commonly used for isolation of discrete colonies initially require that the number of organisms in the inoculums be reduced.

It is essentially a dilution technique that involves spreading a loopful of culture over the surface of an agar plate. The resulting diminution of the population size ensures that, following inoculation, individual cells will be sufficiently far apart on the surface of the agar medium to effect a separation of the different species present.

Although many type of procedures are performed, the four ways or quadrant streak is mostly done. Quadrant Streak Plate Method Step 1: Obtain the proper media plate for your bacterial sample, such as an LB agar plate; label the bottom with your name, date and bacterial species, and set up your materials in a sterile environment such as a laminar flow hood or next to a Bunsen burner with a clean bench top.

Diagrams a The diagram on the left shows the four quadrants and their zig-zag streaked patterned mentioned in steps two through four. Note the thickest, densest amount of bacteria in quadrant one no isolated, individual colonies and the correspondingly fewer number of colonies in quadrants two, three and four. With isolated colonies in quadrant four, that would be easy to obtain without touching any other colony using a sterile loop to start an experiment!

Source: Elte. Show More. Related Articles. This means passing rims and lids through the flame produced by a Bunsen burner in order to kill microorganisms coming in contact with those surfaces. Sterile technique, in general, is a learned state-of-being, or mantra, where every utilization of any sterile material comes with the caveat of taking every precaution to ensure it remains as free of contaminants as possible for as long as possible.

A serial dilution is the step-wise dilution of a substance in solution. Usually the dilution factor at each step is constant, resulting in a geometric progression of the concentration in a logarithmic fashion.

A ten-fold serial dilution could be 1 M, 0. A culture of microbes can be diluted in the same fashion. For a ten-fold dilution on a 1 mL scale, vials are filled with microliters of water or media, and microliters of the stock microbial solution are serially transferred, with thorough mixing after every dilution step.

The dilution of microbes is very important to get to microbes diluted enough to count on a spread plate described later. In microbiology, streaking is a technique used to isolate a pure strain from a single species of microorganism, often bacteria. Samples can then be taken from the resulting colonies and a microbiological culture can be grown on a new plate so that the organism can be identified, studied, or tested.

The streaking is done using a sterile tool, such as a cotton swab or commonly an inoculation loop.