DMF Week 3 SAQ


Select the CORRECT statement regarding the characteristics of Pseudomonas aeruginosa:

  It is a highly motile gram negative coccus.

  It produces two water soluble pigments, pyocyanin and fluorescein.

  It is an opportunistic pathogen which is commonly found as a component of the normal flora.

  It is a common cause of urinary tract infections.

  It is susceptible to most antibiotics and disinfectants.

A is incorrect: The organism is a highly motile Gram negative bacillus (rod). B is correct: These pigments provide the characteristic colour associated with its colonial morphology in vitro and inflammatory exudates due to this organism in vivo. C is incorrect: This organism can transiently colonise people, but is not a major component of the normal microbiota. D is incorrect: It is a rare cause of community acquired UTI. It more commonly causes nosocomial UTI, probably facilitated by its ability to produce biofilms on catheter surfaces. E is incorrect: Ps. aeruginosa is a highly resistant organism.


Which one of the following statements is CORRECT regarding bacteria which cause opportunistic infections?

  Bacteria which cause opportunistic infections are not encountered outside hospitals.

  Bacteria which cause opportunistic infections are predominantly highly virulent organisms which cause more severe disease in compromised patients.

  Bacteria which cause opportunistic infections do so by exploiting defects in one or more host defences.

  Bacteria which cause opportunistic infections are typically carried as aerosols, thus providing efficient transmission.

  Bacteria which cause opportunistic infections only colonise compromised patients.

A is incorrect: Bacteria which cause opportunistic infections are usually ubiquitous organisms. B is incorrect: Whilst it is true that virulent bacteria cause infections in compromised patients as they do in patients with intact immune systems, opportunistic pathogens are by definition, avirulent in patient’s with intact immune systems. C is correct. D is incorrect: Opportunistic pathogens are most commonly spread from the hands of health care workers. E is incorrect: These bacteria are commonly found colonising the skin and mucous membranes of normal, healthy people.


Following a hip replacement, the bone and surrounding area became infected, with ensuing tissue necrosis. Gram positive rods were detected in the foul-smelling discharge which was associated with the surgical site. The laboratory cultured organisms which grew on media incubated in the absence of air, but not in air. Which one of the following statements best reflects the most likely cause of the problem?

  It is the most common component of the gut flora.

  It is a common component of the oral flora.

  It is a sporing organism.

  It is a facultative anaerobe.

  It contains a powerful endotoxin.

A is incorrect: Whilst this organism may be found in the gut, the most common member of the gut flora are members of the Bacteroides genus. B is incorrect. C is correct: The most likely cause of the patient's condition is Clostridium perfringens. It is a sporing Gram positive rod. It is most likely that spores or vegetative organisms from the gut or environment have contaminated the wound or surgical instruments. D is incorrect: The culture characteristics described above, together with the finding of foul-smelling pus, suggest that the organism is a strict anaerobe. E is incorrect: Whilst C. perfringens produces a wide range of exo-toxins, as a Gram positive rod it does not posses lipopolysaccharide.


Which one of the following statements regarding innate and adaptive immunity is INCORRECT?

  The cells of the innate and adaptive immune system are activated by binding molecular patterns which are present on microbes, but absent on host cells.

  The engagement of Toll Like Receptors (TLR) on macrophages results in their activation.

  Toll Like Receptors undergo gene rearrangement to allow discrimination between self and foreign antigen.

  The adaptive immune response improves in magnitude after a repeated exposures to an infectious agent.

  The cells of the adaptive and innate immune system are activated by infectious agents at different sites.

A is correct: The innate immune system has evolved to recognise molecular patterns unique to microbes. These include molecules such a mannose and similar sugars, peptidoglycan and lipopolysaccharide, nucleic acid motifs such as CpG. B is correct: Macrophages and other cells of the immune system express members of the Toll Like Receptor family of receptors. Signalling through these receptors following interaction with pathogen associated molecular patterns (PAMPs), such as peptidoglycan, lipopolysaccharide and others results in activation of the cells. C is incorrect. D is correct: The magnitude of the adaptive response increases when it is exposed to the same organism. This phenomenon is termed "memory". E is correct: The cells of the adaptive immune system are activated by infectious agents in the secondary lymphoid tissue, whereas the cell of the innate immune system are activated at the site of infection.


Mr. W.B. has presented to his local GP with persistent fever, malaise, and anorexia. He has tachycardia, and a temperature of 38.3 degrees Celsius. His blood pressure and respiratory rate are normal. He has no lymphadenopathy, and examination of abdomen, lungs and rectum are normal. However, a cardiac murmur is heard over the pericardium. He has had a history of poor dental hygiene and gum problems. A diagnosis of infective endocarditis is considered by his GP. Which one of the following options is the least appropriate for the management of his current problem?

  Collect three sets of blood cultures for microbiological investigation.

  Order an echocardiogram.

  Collect blood for a full blood examination.

  Collect urine for urinalysis.

  Treat him with empirical antimicrobial therapy immediately.

A is incorrect: This is the appropriate investigation to isolate the causative organism. B is incorrect: This is an appropriate investigation to visualise any vegetations. C is incorrect: This is an appropriate investigation to detect any leucocytosis. D is incorrect: This is an appropriate investigation to detect any nephritis. E is correct: Antimicrobials should be withheld until blood cultures have been collected.


(continued) An organism is cultured from all three sets of the blood cultures. It is found to be a catalase negative Gram positive coccus, which grows on HBA as a small, greyish colony with greening (alpha) haemolysis. From the data provided above, and your knowledge of organisms implicated in the aetiology of endocarditis, select the likely organism which has caused Mr. W.B's disease?

  Staphylococcus aureus

  Staphylococcus epidermidis

  Streptococcus pyogenes

  “Viridans” streptococci

  Streptococcus pneumoniae

A is incorrect: Staph. aureus is a catalase positive Gram positive coccus, which is pigmented (white, cream or gold) and is non-haemolytic. B is incorrect: Staph. epidermidis is a catalase positive Gram positive coccus. C is incorrect: Strep. Pyogenes is catalase negative Gram positive coccus, which produces complete (beta-haemolysis) on HBA. D is correct: “Viridans” streptococci are Gram positive cocci which cause greening haemolysis on HBA. They are commonly associated with endocarditis. The history provided in Question 5 suggests a portal of entry for flora from the oral cavity. E is incorrect: Although Streptococcus pneumoniae is a Gram positive coccus which causes greening haemolysis on HBA and which may colonise the pharynx , it is not associated with endocarditis.


Regarding sterilisation processes, which one of the following statements is INCORRECT?

  Sterilisation processes are affected by the nature of the infectious agents which might be present on an article to be sterilised.

  Sterilisation processes are affected by the initial number of infectious agents present on an article to be sterilised.

  The nature of the article to be sterilized determines which process will be chosen.

  Sterilisation processes are affected by the duration of exposure of an article to be sterilised to the sterilizing process chosen.

  The level of sterility assurance specified for any article is independent of its intended use.

A is correct: The possible presence of spores or prions will affect the efficiency of a sterilization process. B is correct: An increase in the number of organisms contaminating an article will result in an increase in the sterilization time. C is correct: Some sterilization processes may utilize conditions which may compronise the integrity of the article to be sterilized. D is correct: The number of microbes surviving a sterilization process decreases with time, and the probability that none will survive (the sterility assurance) increases with time. E is correct.