Transcript

INSTRUCTOR

When a bacterial culture grows to cover the entire surface of a plate, it is called a lawn. To grow good lawn, we can use a swab. We immerse a pre-sterilised swab in a broth culture and drain the excess liquid against the side. We make a streak down the centre of the plate and then rub the swab across the surface of the plate. First in one direction and then in another to ensure maximum coverage. After overnight incubation, the lawn is visible. Lawns can be used to test the sensitivity of the bacteria to different substances, such as antibiotics. Here, specially prepared paper discs impregnated with a known quantity of antibiotic are used. The discs are placed on the surface of the plate after inoculation. After overnight incubation, clear zones-- indicating inhibition of bacterial growth-- can be seen around some or all of the discs. In this example, we can see that the culture contains at least two populations of bacteria. One that forms small, yellowish colonies and is sensitive to the antibiotic. And one that forms larger white colonies and is not sensitive to the antibiotic. This technique is very useful for testing antibiotic sensitivity and resistance patterns of pathogens.