Which one of the following antibiotics is most likely to cause pseudomembranous colitis?
Correct Answer A: Cephalosporins, not just clindamycin, are strongly linked to Clostridium difficile.
This is a difficult question as both co-amoxiclav and ciprofloxacin are known to cause Clostridium difficile. Studies looking at the relative risk (RR) of developing Clostridium difficile following antibiotic therapy give the following results:
Cefalexin is a first generation cephalosporin and less associated with Clostridium difficile than newer agents such as ceftriaxone.
Clostridium difficile:
Clostridium difficile is a Gram positive rod often encountered in hospital practice. It produces an exotoxin which causes intestinal damage leading to a syndrome called pseudomembranous colitis. Clostridium difficile develops when the normal gut flora are suppressed by broad-spectrum antibiotics. Clindamycin is historically associated with causing Clostridium difficile but the aetiology has evolved significantly over the past 10 years. Second and third generation cephalosporins are now the leading cause of Clostridium difficile.
Features:
Diagnosis is made by detecting Clostridium difficile toxin (CDT) in the stool.
Management:
A 53-year-old woman is diagnosed with left leg cellulitis. A swab is taken and oral flucloxacillin is started. The following result is obtained:
How should the antibiotic therapy be changed?
Correct Answer D: Penicillin is the antibiotic of choice for group A streptococcal infections. The BNF suggests stopping flucloxacillin if streptococcal infection is confirmed in patients with cellulitis, due to the high sensitivity. This should be balanced however with the variable absorption of phenoxymethylpenicillin.
Streptococci: Streptococci are gram-positive cocci. They may be divided into alpha and beta haemolytic types.
Alpha haemolytic streptococci (partial haemolysis):
The most important alpha haemolytic Streptococcus is Streptococcus pneumoniae(pneumococcus). Pneumococcus is a common cause of pneumonia, meningitis and otitis media. Another clinical example is Streptococcus viridans.
Beta haemolytic streptococci (complete haemolysis): These can be subdivided into groups A-H. Only groups A, B & D are important in humans.
Group A:
Group B:
Group D:
A 55-year-old business man presents with a 15 day history of watery, non-bloody diarrhoea associated with anorexia and abdominal bloating. His symptoms started 4 days after returning from a trip to Pakistan. On examination he is apyrexial with dry mucous membranes but normal skin turgor.
Given the likely organism, what is the most appropriate treatment?
Correct Answer E: Although Escherichia coli is the most common cause of travellers' diarrhoea, in this particular case the length of illness and nature of symptoms (bloating, watery diarrhoea) points to a diagnosis of Giardiasis.
Giardiasis: Giardiasis is caused by the flagellate protozoan Giardia lamblia. It is spread by the faeco-oral route.
A 30-year-old man presents for review two weeks after returning from a camping holiday in the New Forest. For the past few days he has felt general unwell with lethargy and arthralgia. On examination he has a rash consistent with erythema chronicum migrans.
What is the most appropriate test to perform given the likely diagnosis?
Correct Answer A: Serological tests are the most appropriate first line investigation for diagnosing Lyme disease. ELISA tests are preferred to Western blots as they are more sensitive.
Lyme disease:
Lyme disease is caused by the spirochaete Borrelia burgdorferi and is spread by ticks.
Investigation:
What is the mechanism of action of macrolides?
Correct Answer E:
Antibiotics: mechanisms of action:
The lists below summarize the site of action of the commonly used antibiotics.
Inhibit cell wall formation:
Inhibit protein synthesis:
Inhibit DNA synthesis:
Inhibit RNA synthesis:
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