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Molecular Microbiology - Case Study Example

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Case 1: 1. A 67 year old leukaemic patient developed bacteriaemia in hospital. It was suspected that his plastic central line catheter was colonized as indicated by the swelling at the point of entry in his skin. Blood culture showed Gram-positive cocci in clusters.
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Molecular Microbiology

Download file to see previous pages... The commoner intravenous catheter-related infections are exit-site infections, as in this case, often with erythema around the area where the line penetrates the skin.
Bacterial blood stream infections are common in this given scenario, and Staphylococci are the second most prevalent bacteria. However, a smear suggesting the staphylococci or Gram-positive cocci in clusters in blood culture as in here, is not sufficient for the diagnosis of true bacterial blood stream infection before the species is identifiable, since the most frequent of this species, Coagulase-negative staphylococci or CoNS usually habituate in the skin, and there is always a chance of contamination of the culture bottles during the venipuncture. In contrast, such an infection due to Staphylococcus aureus is virulent by its intrinsic nature, and isolation in one blood culture bottle is clearly diagnostic and is an indication of initiation of antibiotic therapy. Thus the therapeutic decision making is based on identification of the organism (Beekmann, S. E., Diekema, D. J. and Doern, D. J., 2005).
The first test obviously would be to do a light microscopic examination. Direct microscopic examination may provide a rapid, presumptive report of Gram-positive cocci resembling staphylococci. Isolation of S. aureus should be performed using 5% blood agar following an incubation period of 18-24 h in air at 35-37 C. Staphylococcus aureus ferments mannitol, resulting in a change in the colour of the medium from pink to yellow. Colony morphology may be used by the experienced observer to define presumptive staphylococci. A Gram stain appearance of cocci in clusters and a positive catalase test provide rapid indicators of staphylococci. However, in order to be able to distinguish between Staphylococcus aureus and the remaining members of the staphylococcus species, other tests are necessary. For clinical microbiological purposes, two or three simple tests suffice. The coagulase test detects the production of coagulase by S. aureus. In this test, one colony is mixed with plasma, incubated at 37 C for 4 h and observed for clot formation. Samples that are negative at 4 h are incubated and observed again for clotting at 24 h. The slide agglutination test detects clumping factor (ClfA). This is performed by making a heavy homogenous suspension of cells in distilled water on a glass slide to which a drop of plasma is added. Within 10 s, the mixture is examined for presence of clumping (Chapin, K., and M. Musgnug, 2003).

How would you differentiate the cocci in clusters from those in chain

Cocci in clusters are named as staphylococci. All staphylococci have the ability to convert hydrogen peroxide into nontoxic H2O and O2. Both coagulase positive and negative staphylococci produce catalase. This test differentiates them from cocci in chain or streptococci, which cannot produce catalase and hence are catalase negative (Chapin, K., and M. Musgnug, 2003).

What is the principle of DNase test and what is the identity of this organism and why Support your answer with microbiological diagnostic facts.

DNase or deoxyribonuclease is an extracellular enzyme that can hydrolyze deoxyribonucleic acid to oligonucleotides. Several varieties of deoxyribonucleases are distinguished on the basis of antigenic properties, response to inhibitory substances, hydrolytic end products, and ...Download file to see next pagesRead More
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