MEDICAL MICROBIOLOGY

MEDICAL MICROBIOLOGY
Case 1
Case 1 is about two brothers in the 1950s, aged 8 and 9 years old, who were suffering from symptoms of severe diarrhoea, abdominal pain and fever. They were treated by antibiotics which got rid of the symptoms, but did not eliminate the infectious bacterium. The brothers were infected by a Gram-negative, rod shaped bacterium, shown in stool samples. The diagnosis for this case would be gastrointestinal infection caused by Salmonella spp, this is a Gram-negative, and rod shaped bacteria that cause food poisoning more commonly in children and elderly. When infected by Salmonella spp is referred to as Salmonellosis, this involves symptoms such as diarrhoea, fever, dehydration and abdominal cramps, these symptoms are similar to the ones the brothers had therefore supporting the diagnosis. The infection is transmitted by the faecal-oral route by ingesting contaminated food such as undercooked poultry, and it occurs worldwide. (Sánchez-Vargas et al., 2011) In the UK in the 1950s, there was an outbreak of Salmonella spp as new foods were introduced, hence supporting the diagnosis of brother being infected by Salmonella spp. (McCoy, 1975)
The bacterium was not eliminated from the faeces of the children even after the antibiotic supplements as antibiotic gets rid of both the beneficial normal flora of the gut (microflora) and the harmful bacteria. Therefore the normal flora could not compete with the harmful Salmonella bacteria; this is why the bacteria were still present in the faeces. Also diarrhoea in the children was caused due to the reduction of microflora, hence why raw yogurt was given to the brothers as it is useful in treating diarrhoea by restoring the microflora. Furthermore microorganisms in the yoghurt called probiotics inhibit the Salmonella bacterium from growing and adhering to intestinal cells and stop their metabolic activity. After the children took a course of raw yoghurt there was no Salmonella bacteria found in the faeces as the restored microflora competed and destroyed the harmful bacteria. (de Vrese and Marteau, 2007)

A variety of laboratory tests are available to diagnose Salmonella spp infections. A common specific method is isolating the organism from the faeces or blood sample and culturing it on an agar plate such as MacConkey agar. The MacConkey agar is useful as it inhibits the growth of Gram-positive bacteria and allows the growth of Gram-negative bacteria so therefore is selective and differentiates between the two types. A Gram-negative bacterium does not ferment lactose therefore changing the colour of the agar to yellow, and a Gram-positive bacterium ferments lactose therefore changing the agar colour to pink. Therefore culturing Salmonella on MacConkey agar will give a yellow colour as it is Gram-negative. (Giannella, 1996)

A further laboratory technique that can be used to confirm the diagnosis of Salmonella is serotyping which identifies the Salmonella serotype an isolated microorganism belongs to; this is carried out by the reference laboratory. The serotyping method tests for the antigens present on the cell wall of microorganisms with different antibodies (antiserum) and detecting antibody-antigen interaction, if antigen is present. Salmonella serotypes have specific (O) antigenic structures on the cell wall and (H) antigenic structures on the flagellar; these are detected by slide agglutination. Agglutination is when an antibody matches with the antigen forming clumps (agglutinate). This results in a specific agglutination pattern that could be used to identify the Salmonella serotype using the Kauffmann-White classification scheme. (McQuiston et al., 2011)
In most cases treatment is not required as symptoms usually disappear between 5-7 days. However intravenous fluids for rehydration may be needed in those who have severe diarrhoea. Also antibiotic may be offered to those with serious infection and those who are at greater risk such as elderly and children. Antibiotics can get rid of the symptoms but the Salmonella bacterium would still show in stool samples for about 2 week after the symptoms disappear, as antibiotics prolong the presence of the organism in the faeces; this was the case for the two brothers. Prevention of Salmonella infection is done by having good hygiene such as washing hands after contact with animals; also ensuing food is refrigerated and cooked thoroughly. (Giannella, 1996)

Case 6

 

 
Charles Darwin (Figure 1) was a British scientist who introduced the theory of evolution and natural selection. In 1831, Darwin went on a scientific voyage on the ship HMS beagle (Figure 2); he travelled around the world (Figure 3) which lasted 5 years. While travelling on his voyage in 1835 Charles Darwin stated in his biography that he was bitten by a ‘great black bug of the pampas’ this insect could have been the blood-sucking triatomine bug (Figure 4) also known as a ‘kissing bugs’. (Darwin, 2008) This triatomine bugs are more commonly found in Latin and South America where Chagas is endemic. The infected bug acts as a vector for the protozoan parasite Trypanosoma cruzi (T. cruzi) which causes Chagas disease (American trypanosomiasis); the disease was discovered in 1909. (Nunes et al., 2013) The bug that Darwin described, bitten him while he was entering Argentina, which is in south America where Chagas is endemic, and this may support the diagnosis of him being infected by Chagas disease.
T. cruzi parasites are most commonly transmitted by the infected faeces of blood-sucking triatomine bugs. These bugs are mostly active at night; they bite on human blood usually an exposed area (face) and defecate near to the bite. The trypomastigotes parasite enters the body and circulates in blood (Figure 5) after the individual rubs the faeces into the bite, eyes, nose or any open wounds on the skin. Other ways of transmission is through blood transfusion from infected donor, contaminated food and from mother to baby. (Rassi and Marin-Neto, 2010)
Charles Darwin experienced symptoms of fever in 1834 when he was in Chile and became ill for a while, Chagas is also endemic in Chile this may suggest that he acquired the infection while he was there. (Bernstein, 1984) Chagas disease has two phases; the acute phase lasts for about 2 months after acquiring the infection and there are increased numbers of parasites in the circulation. The symptoms of this stage are mild or asymptomatic; they include fever, headache, vomiting, pallor, muscle pain, swelling, difficulty in breathing, and chest or abdominal pain. Those who have been bitten by the bug get swelling and lesion on one eyelid (Romaña’s sign). Therefore it could be suggested that the fever Darwin got during the voyage related to the acute phase of the disease. The next phase is the latent stage where no symptoms are experienced this can last for about 10-30 years. This is followed by a chronic phase which could be asymptomatic; however there is a 30% risk of some individuals developing cardiac, intestinal, oesophagus and nervous system complications. Moreover, cardiac complications lead to heart failures and sudden death. (Prata, 2001) Darwin did not suffer from any symptoms between the years of 1835-1841 (latency period), it was later in the years of 1841-1865 were he had symptoms of palpitations, fatigue, trembling, flatulence and vomiting. He then suffered from several heart attacks four months before he died from heart failure on 1882, these signs and symptoms further suggest that he was infected with Chagas. (Bernstein, 1984)
During the time when Charles Darwin may have been suffering from Chagas disease there was no diagnostic test available to confirm this, and diagnosis was based on signs and symptoms. However more recently diagnosis of Chagas can be done by modern laboratory tests such as parasitological, serological and molecular testing. Blood smears can be used to diagnose the acute phase of the infection when circulating parasites in the blood is present. Thick and thin blood smears are stained so that the parasite is visible when examined under the microscope. (Brasil et al., 2010) Enzyme-linked immunoassay (ELISA) and polymerase chain reaction (PCR) is used in order to diagnose chronic phase T. cruzi. As antibodies against the T. cruzi antigens are presents in infected patients few days after the onset of infection. ELISA can be used to detect these antibodies by coating the EISA plate with recombinant antigens of T. cruzi and if there is an antigen antibody interaction the optical density would give a positive result. A variety of PCR techniques can also be used in order to detect the T. cruzi DNA in patient blood samples. Both methods have high sensitivity and specificity in diagnosis of Chagas disease. (Ramírez et al., 2009)
Chagas can be treated and cured by killing the parasite if drugs are given soon after the onset of infection (acute phase). These drugs include benznidazole and nifurtimox, they become less effective if is given long after the onset of infection. In order to treat the chronic phase more specific treatments are needed to help with cardiac and digestive complications. (Prata, 2001)
Prevention is done by vector controls such as spraying the house with insecticides and using bednets in endemic countries, also having good hygiene and screening blood donors, there is no vaccine against Chagas disease. (Nunes et al., 2013)

Medical Microbiology

Case studies:

coursework must be submitted in black ink using Arial font size 12, be double spaced, single sided, have 1” (2.54 cm) margins, numbered pages and a word count (which excludes the References section but not references cited in the main text).

Plagiarism and deviation from the word limit will, as always, be penalized; the former is an assessment offence and can result in exclusion from the course

for advice on how to avoid plagiarism and cite references. Remember that material from the World Wide Web must also be acknowledged and that the URL alone is insufficient (give URL in full, title of web site, name of institution/organization, and its source/author). Also note that such material is generally much less reliable than that from journals because it is not usually refereed.

Properly constructed and grammatically correct sentences and paragraphs must be used throughout. Avoid “bullet points”, numbered lists, non-scientific language, slang and clichés. Do not use any photocopies of material from reference sources in your essay. Submit your work stapled together in the top left-hand corner and without a folder or binder.

You should address the bullet points as on the web site, viz. evidence for the diagnosis, further tests, aetiology, therapy, symptoms (including ones not already given), prophylaxis, prognosis, epidemiology. You may provide various “facts” but they must mainly address the bullet points as on the web site. The emphasis should be on the laboratory investigation of each case and too much clinical detail and minute description of disease symptoms is only wasting words. Also, the over-use of medical terms (some of which we suspect some students do not know the meaning of!) do not impress us. Treatments, particularly antimicrobial drugs, that you find in the literature could vary considerably but brand names should not be used. If there is more than one possible ID or diagnosis, you should discuss them all and give confirmatory tests. You should use as much of the detail given as possible to arrive at a diagnosis of the disease and an identification of the likely microorganism(s). Your 2 case studies follow.

CASE STUDY A
Subject is a 45-year-old farmer. Eight months ago he had a renal transplant for end-stage renal disease from rapidly progressive glomerulonephritis. He has been on immunosuppressive drugs including prednisone. He presents to the hospital with a three-day history of progressive shortness of breath, productive cough, and fevers. On examination, he is tired, anxious, and in moderate respiratory distress. He can only speak in 2- to 3-word sentences and is using accessory muscles to breathe. Chest X-ray shows bilateral fluffy infiltrates. Bronchoscopy is done and sputum is sent for culture. Several days later, colonies appear on Sabouraud agar but nothing unexpected grows on bacterial growth media.

CASE STUDY B (intentionally this has no text)

 

Criteria for assessment of the Case Study assignment

In this you
have to arrive at a reasoned diagnosis based on the details provided about a patient or
discuss a disease outbreak, perhaps an historical one. The case studies should be
widely researched and relevant to the investigation of infectious disease by the medical
microbiology laboratory. You are expected to find your own references and cite them
fully at the end of the study (remember that journal articles and textbooks are a much
more reliable source than the Web). Detailed description of laboratory methods is NOT
required – only the basic principles need be covered. Similarly, avoid too much clinical
detail and ‘medi-speak’. Concentrate on the investigation of the disease by the modern
medical microbiology laboratory (although with historical case studies you should also
cover the state of knowledge at the time). You should consider covering: evidence for
the diagnosis, further laboratory tests that could be carried out to confirm your
diagnosis, therapy, symptoms, prophylaxis, prognosis and epidemiology. You must
always ‘discuss’ and not just ‘describe’ and give full evidence for your conclusions. Note
that microorganisms are identified whilst infectious diseases are diagnosed.
Case studies word length = 1200 – 1500 words in total for BOTH case studies
(excluding references listed in the References section at the end but including
references cited in the text). You must provide a word count at the end.
Your case studies will be graded into the appropriate class depending on how well you:
• Conform to the word limit and provide an accurate word count.
• Interpret clinical background and laboratory data.
• Arrive at a reasoned diagnosis and suggest suitable treatment, therapy, and
prophylaxis, as appropriate.
• Describe symptoms and discuss prognosis.
• Use logical arguments and justify conclusions.
• Show evidence of background reading with critical understanding and
independence of thought and argument.
• Demonstrate thorough understanding of key concepts and theories and
evidence of wide reading with critical understanding.
• Show independence of thought and argument.
• Illustrate all points by reference to named examples from as wide a range as
possible.
• Evaluate contrasting viewpoints.
• Use clear, concise and correct expression.
• Provide well-structured, flowing argument.
• Demonstrate evidence of appropriate planning.
• Give clear evidence with definite conclusions.
• Show a high standard of presentation, including good use of English (correct
spelling, good grammar, appropriate use of paragraphs) and approved reference
formats both in the text and in the References section.

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