DMF MSE 1 2011
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
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.
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
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
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
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.
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
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)
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"
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.
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
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.