A swab for culture is received from a doctor\'s office. The staff forgot t0 ment
ID: 84424 • Letter: A
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A swab for culture is received from a doctor's office. The staff forgot t0 mention the source from where the swab was taken and the clinic was closed for next few days. What different selective/differential media may be helpful to you to identify the organism. Explain very briefly your rationale for using the particular culture media.? Agar and gelatin are used as a solidifying agent for bacteriological media. Why is gelatin less suitable than the agar? A catalase test is performed on an isolated colony on a blood agar plate and bubbles appear. The test is repeated on another isolated colony form the same plate. This time there are no bubbles. How can you explain the difference in the results? You are given a tube of contaminated water. The number of bacteria in the tube is 100 many to count. You prepare a tenfold dilutions in 5 lubes, so that tube #1 has 1:10 dilution, tube #2 has 1:100 dilution, tube #3 has 1:1000 dilution, tube #4 has 1:10,000 dilution and tube #5 has 1:100,000 dilution. The second lube has 210 cells and the third tube has 26 colonies. What is the number of bacteria per m1 in the original sample?Explanation / Answer
2. In the microbiology laboratory many different types of media are used and divided into several types including nutritive, differential, and selective.
Selective medium are used for growth of certain types of organism while inhibit the growth of other types of organism. EMB agar, Mannitol Salt agar, MacConkey agar, and Phenylethyl Alcohol (PEA) agar are used as a selective media.
Differential medium are used to distinguish microorganisms from one another based on their growth characteristics evident on specific medium types. Examples of differential medium include blood agar, Eosin Methylene Blue (EMB) agar, Mannitol Salt agar, and MacConkey agar.
Eosin Methylene Blue agar (EMB agar) is a type of both selective and differential medium and used for identification and isolation of intestinal gram negative bacteria and inhibit the growth of gram positive bacertia. .It contains the dyes eosin and methylene blue. Lactose-fermenting organisms such like E. coli produce a black colony or dark transparent centers on EMB and non-lactose fermenting organism such as Proteus sp., Salmonella sp., or Shigella sp. produce pink or uncoloured colony . Therefore, the selected medium is dstinguised with respect to lactose fermentation.
MacConkey agar and EMB agar are similar and used for Gram-negative species. MacConkey agar is used for the identification of coliforms and enteric pathogens which is based on their ability to generate lactose. Lactose-fermenting bacteria develop red to pink colonies while non-lactose fermenting bacteria form colorless or transparent colonies. It is generally used for differentiating strains of Salmonella typhosa. the medium supports the growth of all Salmonella and Shigella strains and gives good differentiation between these enteric pathogens and the coliform group.
Mannitol salt agar (7.5% NaCl) is a medium selective which is used for staphylococci and distinguish with respect to produce mannitol. Fermentation of mannitol is noticed in the pathogenic Staphylococcos species and produces acidic end products which change the media from neutral red-orange to bright yellow. Yellow colour indicates mannitol positive .Non-pathogenic staphylococci form red or pink- purple zones small colonies
Phenylethyl Alcohol (PEA) agar is a selective medium that inhibits the growth of Gram negative organisms. Phenylethyl alcohol disrupts lipid structure of Gram negative bacteria and inhibits the protein synthesis which leads to poor growth of the gram positive on the medium.
Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli ,Enterobacter aerogenes Salmonella enteritidis are grow in Mannitol Salt agar, EMB agar, MacConkey's agar and nutrient agar according to their growth medium. MSA and MacConkey's agar plate is divided into two sections and EMB agar plate divide into three sections. The MSA plate Inoculate with S. aureus and S. Epidermidis and MacConkey's agar inoculate with E. coli and Salmonella enteritidis. EMB agar plate inoculates with E. coli, Enterobacter aerogenes, and Salmonella enteritidis and incubates all plates at 37°C for 48 hours.
After incubation observes the growth and appearance of colonies on all plates, we can find out the organism.
3. The basic reason for geletin is less suitable than agar is that both of these medium are solid at room temperature but melting pont is different. Gelatin melts at a low temperature-less than 35 celsius while agar melts at 85 celsius. Another reason is that agar can be easily cut down and used in fewer amounts for solidifying. However, bacteria can digest gelatin more easily than agar.
4. Catalase is an enzyme and used to test the presence of catalase which catalyses the release of oxygen from hydrogen peroxide (H2O2). It is used to distinguish catalase producing bacteria such as staphylococci, from non-catalase producing bacteria such as streptococci. Enterococcus or Streptococcus (catalase negative) and Staphylococcus (catalase positive) are differentiated using the catalase test. Catalase positive result produce bubble while negative produces no bubble.
When a catalase test is performed on a isolated colony on a blood agar plate and give positive result with bubble formation since some blood agar as blood cells are left in that colony which gives false positive result. Because some bacteria have enzymes other than catalase that can degrade hydrogen peroxide and forming a few tiny bubbles after 20 to 30 seconds.
The test is repeated on other isolated colony from the same plate and no bubble is formed because no blood cell is present.
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