DMF MSE 1 2011

1

Which one of the following statements about bacterial capsules is correct?


  Capsules are an important virulence factor of facultative intracellular bacteria because they protect them from intracellular killing by macrophages and polymorphonuclear leukocytes

  Polysaccharaide capsules of bacteria are unsuitable for use as vaccines, because most of them are extremely weak antigens

  Exceptions to the rule in the previous statement are capsules of group A streptococci and group B meningococci, which can be purified to make high effective vaccines

  Capsules of Gram negative bacteria may mask pathogen-associated molecular patterns (PAMPs) on the bacterial surface, thus concealing the bacteria from the innate immune system

  When injected into animals, the capsules of Gram negative bacteria such as E. coli, typically produce fever and shock

2

Which one of the following statements about exotoxins is correct?


  Shiga toxin binds to its target cells, is endocytosed and, following release of the A-subunit, activates adenylate cyclase

  Tetanus and diphtheria toxins are used as vaccines because they can be safely injected into humans and induce the production of antibodies that neutralised the corresponding toxin

  A-B exotoxins must bind to a receptor on the surface of the host cell before being able to induce damage

  Although they have different mechanisms of action, all bacterial exotoxins eventually kill host cells

  Exotoxins are proteins, and are harmless when ingested, because they are destroyed by stomach acid and proteolytic enzymes


Table shown in review lecture - should know this.

3

Choose the correct statement regarding urinary tract infection:


  To diagnose a urinary tract infection in a child less than one year of age, it is advisable to obtain a sample of mid-stream urine

  It is important to obtain a specific aetiological diagnosis of urinary tract infection before commencing treatment with antibiotics

  The finding of Gram negative bacilli in urine passed per urethra indicates a high bacterial count, and it is a sufficient indication to start treatment with antibiotics

  Urine collected by puncture of the bladder through the lower abdominal wall should normally be sterile

  The isolation of E. coli from urine passed per urethra is a sufficient indication to start treatment with antibiotics

4

Which of the following statements about bacterial virulence determinants is not correct?


  You can obtain evidence that something e.g. Factor Y, is a virulence factor of a bacterium by inactivating the gene for the Factor Y and showing that the resultant mutant is less virulent in a suitable animal model

  In the above experiment, you could strengthen the evidence by by re-activating the gene for Factor Y and showing that virulence is restored

  If Factor Y is a surface molecule, you could make a vaccine from it, and show that immunisation with it protects animals from colonisation with bacteria that require Factor Y for adhesion

  If Factor Y is a toxin, you could inject antibodies to it into an experimental animal and show that the animal is protected against the effects of the toxin

  If Factor Y is a lipid toxin you could convert it to a toxoid, conjugate it to a polysaccharide and make an effective vaccine from it

5

Miss LC, a 24 yr old student, presented to her GP with a 2 day history urinary frequency and dysuria. She had been previously well, having just returned from an interstate visit with her boyfriend. A mid-stream specimen of urine was collected and sent promptly to the laboratory for microscopy and culture. Which one of the following sets of results from laboratory testing of the urine sample would confirm the diagnosis of acute cystitis?


  Microscopy: 5x106 leukocytes/mL, moderate number of epithelial cells. Culture: <104 Staphlococcus saprophyticus/mL

  Microscopy: <104 leukocytes/mL, no epithelial cells seen. Culture: 103-104 Escherichia coli/mL

  Microscopy: <104 leukocytes/mL, 2x104 erythrocytes/mL, few epithelial cells. Culture: 104-105 Escherichia coli and Staphylococcus epidermidis/mL

  Microscopy: 2x105 leukocytes/mL, no epithelial cells seen. Culture: >105 Staphylococcus saprophyticus/mL

  Microscopy: 2x105 leukocytes/mL, moderate number of epithelial cells. Culture: >105 Escherichia coli and Staphylococcus epidermidis/mL

6

Which one of the following statements regarding Clostridium botulinim and/or botulism is incorrect?


  The term "C. botulinum" refers to heterogenous group of bacteria that produce a similar toxin

  Botulism may occur by ingesting preformed toxin in vacuum-packed fish products, because these packages contain low concentrations of oxygen and also because the intenstinal tract of flush is a natural habitat of some varieties of C. botulinum

  Botulinum toxin acts by disinhibiting neurones that modulate excitatory impulses, causing anterior horn cells and autonomic neurones to become overactive

  If spores of C. botulinum are ingested by an infant they can germinate in the gut and cause botulism

  If spores of C. botulinum infect a wound with necrotic tissue, they can germinate in tissues, secrete toxin and cause botulism


Option 3 describes the mechanism of tetanus toxin.

7

Which one of the following is not known to be a resistance strategy of Gram negative rods to aminoglycoside antibiotics, such as streptomycin?


  Failure to activate an inactive form of streptomycin

  Acetylation of streptomycin mediated by a bacterial enzyme

  Activation of an efflux pump which transports streptomycin out of the bacterial cell

  Modification of the bacterial outer membrane in a way that reduces its permeability to streptomycin

  Mutation of the binding site for stretomycin on the 30S subunit of the bacterial ribosome

8

The general antimicrobial action of metronidazole is best described as:


  Acting on micro-organisms with soft or absent cell walls

  Acting on obligately anaerobic micro-organisms

  Active on the outer membrane of Gram negative micro-organisms

  Altering the activity of 70S ribosomes

  Acting on the plasma membrane of protozoa


Must be activated by nitro-reductases, which only occur in anaerobic organisms (occur in both bacteria AND parasites)

9

Which one of the following is not a mechanism of intrinsic resistance to antimicrobials?


  Enterococcus faecalis to sulphonamides because it is unable to synthesis folic acid

  Pseudomonas aeruginosa to ampicillin because it produces chromosomally encoded beta-lactamase

  Gram negative bacteria to vancomycin because their outer membrane is impermeable to glycopeptides

  Enterococcus foecium to vancomycin because it produces a cell wall precursor comprising D-ala-D-lac instead of D-ala-D-ala

  Mycoplasma species to penicillins because they lack cell walls containing peptidoglycan


Intrinsic: "Something the bug is born with" Acquired: "Gene that has been picked up or mutated in a subpopulation of the organism"

10

Which one of the following strategies cannot be used to identify a specific antigen?


  Smearing a clinical sample on a slide, then staining it with a specific fourescently labelled antibody preparation, and examining it under a UV microscope

  Incubating a clinical sample with latex beads coated with specific antibody to the antigen of interest, and looking for clumping (agglutination) of the latex beads

  Incubating a clinical sample with a specific capture antibody, e.g. bound to a plastic wall; then, after washing, adding a second, labelled antibody and detecting the presence of the second antibody

  If the antigen is present on whole bacteria, adding a specific antibody to a bacterial suspension and looking for agglutination of the bacteria

  If the amino acid sequence of the antigen is known, detecting it by using reverse transcriptase PCR


Ought to know what the tests you are requesting are actually detecting. Option 5 is incorrect because you would be detecting nucleic acid in this case, not antigens.

11

Which one of the following is not characteristic of Pseudomonas aeruginosa growing in a biofilm?


  More capsular material is produced leading to a mucoid phenotype

  Genes regulated by quorum sensing are activated

  The bacteria are more adherent, but less invasive

  Bacteria synthesis shorter LPS molecules with no O-antigen

  The bacteria grow more rapidly than normal and show increased susceptibility to antibiotics

12

The medical officer in a regional town was notified of a cluster of cases of gastroenteritis (with symptoms of profuse vomiting and some diarrhoea) among children attending a local pre-shcool. Laboratory investigations indicated that Staphylococcus aureus was the cause. Which one of the following activities undertaken by all the affected children would best explain how they may have been infected?

Notes on questions 13-15:
These were not included in the recording, but here are some useful points:

  • Q13 covered bacterial toxins. A main learning point was that LT has the same mechanism as cholera toxin.
  • Q14 covered a case with a heart murmur/endocarditis. Main learning points were that it was important to get culture results before AB therapy, high and prolonged doses are required, and that intermittent shedding of organisms from the heart valve puts the patient at risk of abscesses in distant organs.
  • Q15 covered sterilisation procedures. Main learning points included the information required to determine sterilisation time (amount of pre-existing organism etc), the D value and fact that even steam sterilisation cannot and will not kill everything.


  Consuming slices of cream-filled birthday cake at morning tea, 4 hours before the onset of symptoms

  Drinking cordial (made up with rain water fromt he pre-school's water tank) at morning tea, 4 hours before the onset of symptoms

  Handling baby goats and ducklings at a petting zoo visit, on the day before the onset symptoms

  Eating chicken sausages at a BBQ lunch following the petting zoo visit, on the day before the onset of symptoms

  Consuming egg sandwiches that had been prepared by a parent who was recovering from a gastro-intestinal illness, at afternoon tea on the day before the onset of symptoms


Staph. aureus onset is usually short (within hours) and it likes to live in lipid-rich environments.