<P> The sample is spread across one quadrant of a petri dish containing a growth medium . Bacteria need different nutrients to grow . This includes water, a source of energy, sources of carbon, sulfur, nitrogen, phosphorus, certain minerals, and other vitamins and growth factors . A very common type of media used in microbiology labs is known as agar, a gelatinous substance derived from seaweed . The nutrient agar has a lot of ingredients with unknown amounts of nutrients in them . On one hand, this can be a very selective media to use because as mentioned bacteria are particular . If there is a certain nutrient in the media the bacteria could most certainly not grow and could die . On the other hand, this media is very complex . Complex media is important because it allows for a wide range of microbial growth . The bacteria growth can be supported by this media greatly due in part to the high amounts of nutrients . Choice of which growth medium is used depends on which microorganism is being cultured, or selected for . </P> <P> Dependent on the strain, the plate may then be incubated, usually for 24 to 36 hours, to allow the bacteria to reproduce . At the end of incubation there should be enough bacteria to form visible colonies in the areas touched by the inoculation loop . From these mixed colonies, single bacterial or fungal species can be identified based on their morphological (size / shape / colour) differences, and then sub-cultured to a new media plate to yield a pure culture for further analysis . </P> <P> Automated equipment is used at industrial level for streak plating the solid media in order to achieve better sterilization and consistency of streaking and for reliably faster work . While streaking manually it is important to avoid scratching the solid medium as subsequent streak lines will be damaged and non-uniform deposition of inoculum at damaged sites on the medium yield clustered growth of microbes which may extend into nearby streak lines . </P> <P> Bacteria exist in water, soil and food, on skin, and intestinal tract normal flora . The assortment of microbes that exist in the environment and on human bodies is enormous . The human body has billions of bacteria which creates the normal flora fighting against the invading pathogens . Bacteria frequently occur in mixed populations . It is very rare to find a single occurring species of bacteria . To be able to study the cultural, morphological, and physiological characteristics of an individual species, it is vital that the bacteria be divided from the other species that generally originate in the environment . This is important in determining a bacterium in a clinical sample . When the bacteria is streaked and isolated, the causative agent of a bacterial disease can be identified . </P>

When does the sterile swab use for isolation of microorganisms