DMF Week 1 SAQ


Which one of the following statements regarding prokaryotic cells is incorrect?

  Prokaryotic cells do not possess a cytoskeleton

  Prokaryotic cells possess genetic material in the form of circular DNA.

  Prokaryotic cells have no nucleus

  Prokaryotic cells possess the same ribosomes as those in eukaryotic cells.

  Prokaryotic cells do not contain mitochondria.

A is correct: Lack of cytoskeleton means that bacteria must divide by binary fission. B is correct. C is correct: The genetic material of prokaryotes is found free in the cytoplasm. D is incorrect: Bacterial ribosomes are smaller than host cell ribosomes, and although the process of protein synthesis in bacteria follows essentially the same pathway as in host cells, differences exist which can be targeted by some classes of antimicrobial agents.


Regarding the structure of Gram-negative cell walls, which one of the following is not found in the wall of Gram-negative bacteria?

  Lipopolysaccharide or lipooligosaccharides

  A thick layer of peptidoglycan

  Porins which act to transport nutrients

  An inner and outer lipid membrane

  A periplasmic space

A is incorrect: This feature is found in the wall of Gram-negative bacteria. The Gram negative outer membrane do possess lipopolysaccharides (LPS), or, in some bacteria, lipo-oligosaccharides (LOS). These act as endotoxins.

B is correct: This feature is NOT found in the wall of Gram-negative bacteria. The peptidoglycan in Gram negative cell walls is thin, (5 – 10nm) and is located inside an outer membrane. In contrast Gram positive cell walls are characterised by a thick outer layer of peptidoglycan, around 20 – 80nm thick.

C is incorrect: The outer membrane of Gram negative bacteria contains channels called porins. These channels allow passive diffusion of various small hydrophilic compounds, including sugars, amino acids and some ions. Porins are an essential component of Gram negative bacteria because the high lipid content of the outer membrane would otherwise restrict the entry of hydrophilic compounds, and make entry of many nutrients difficult.

D is incorrect: This feature is found in the wall of Gram-negative bacteria. These structures are characteristic of Gram negative bacteria.

E is incorrect: This feature is found in the wall of Gram-negative bacteria. The periplasmic space of Gram negative bacteria is a site for the accumulation of many degradative enzymes and binding proteins which facilitate microbial nutrition. It also contains enzymes which inactivate antibiotics.


Regarding the roles of bacterial pili (fimbriae), which one of the following is correct?

  Bacterial pili increase bacterial surface area.

  Bacterial pili provide a means of movement.

  Bacterial pili contain endotoxin.

  Bacterial pili transport nutrients.

  Bacterial pili provide a means of attachment to surfaces.

B is incorrect: Although, some pili or fimbriae do contribute to a very limited movement. When certain bacteria grow in a biofilm (you'll hear about biofilms in Week 3), the pili "twitch" so that the bacteria are orientated in an appropriate position to interact with others in the biofilm. However, pili do not allow bacteria to move distances in a solution in the same way that flagella do.

E is correct: Bacterial pili or fimbriae may have several functions. One most important function is allow bacteria to adhere to the surface of host cells. Also, some bacteria produce 'sex pill' which link recipient and donor bacterial cells to enable the transfer of DNA by conjugation. In some cases pili may aid bacteria in the evasion of the immune system, by being anti-phagocytic, or by changing their surface molecules to provide antigenic variation.


Which one of the following assays has the least value in aiding the diagnosis of a current infection caused by a particular micro-organism?

  The detection of antigens from the micro-organism in an appropriate specimen

  The detection of nucleic acid from the micro-organism in an appropriate specimen

  Culture of the micro-organism from an appropriate specimen

  The detection of specific antibodies in a serum sample collected from the patient

  An appropriate history

A is incorrect: The detection of antigens which are specific for a micro-organism implies the presence of the micro-organism. This detection will be significant assuming microbe is known to be associated with the patient's symptoms, and that it does not cause chronic or persistent infections.

B is incorrect: The detection of nucleic acid which is specific for a micro-organism implies the presence of the micro-organism. This detection will be significant assuming the microbe is known to be associated with the patient's symptoms, and that it does not cause chronic or persistent infections.

C is incorrect: However, there are some qualifications. Culture of the micro-organism will be significant assuming the specimen is appropriate, the micro-organism is known to be associated with the patient's symptoms, it is present in significant numbers if the specimen also contains normal flora, and that it does not cause chronic or persistent infections.

D is correct: The detection of specific antibodies in a single sample of serum is of least diagnostic value of the options provided because the detection of antibodies are specific for the micro-organism which stimulated their production, their presence may imply either historic or current infection. In most circumstances, diagnosis of current infection using antibody detection requires the detection of specific antibodies of the IgM isotype or the detection of a rising antibody titre in two appropriately collected serum samples.

E is incorrect: A good history is one of the most important pieces of information available in the diagnosis of an infection.


The presence of a particular microorganism in a patient with appropriate symptoms may sometimes be inferred by the use of an appropriate stained specimen combined with microscopy. Which one of the following combinations would not help in the specific diagnosis of an infection caused by that microbe?

  Gram stain of Neisseria meningitidis in CSF

  Gram stain of Streptococcus pyogenes in a throat swab

  Gram stain of Streptococcus pyogenes in discharge from cellulitis.

  Ziehl-Neelsen stain of Mycobacterium tuberculosis in sputum

  Direct immunofluorescence of Treponema pallidum in a lesion.

A is incorrect: Pathogenic Neisseria are recognised as Gram negative intracellular diplococci so the presence of such organisms in CSF is highly suggestive of infection with N. meningitidis, a common cause of bacterial meningitis. And, of course, CSF should be sterile, so the microscopic determination of numbers of micro-organisms is suggestive of infection.

B is correct: Whilst Streptococcus pyogenes is the most common bacterial cause of pharyngitis and tonsillitis, its presence in the throat is impossible to diagnose by a Gram stain of a throat swab, because most the normal oral flora are members of the genus Streptococcus, and hence have similar staining characteristics.

C is incorrect: Although streptococci may be found on skin in low numbers, the numbers of Str. pyogenes, recognised as Gram positive cocci in chains found in discharge from a case of cellulitis would be far in excess of those found as components of the normal skin flora.

D is incorrect: Mycobacteria are rarely found as part of the normal respiratory flora, hence the detection of acid-fast bacilli following a Ziehl-Neelsen stain of sputum suggests infection with Mycobacterium tuberculosis.

E is incorrect: Spirochaetes are not components of the normal genital or skin flora, hence their presence in a scraping from a lesion is suggestive of Treponema pallidum.


Regarding pathogenic bacteria, which one of the following statements is incorrect?

  Invasive bacteria which evade phagocytic action are termed pyogenic

  Phagocytes can be poisoned by toxic bacterial metabolites

  Bacteria which are able to resist killing by phagocytes are termed facultative intracellular pathogens.

  Facultative intracellular pathogens are predominantly removed by the action of antibodies and complement.

  Facultative intracellular pathogens include Mycobacterium tuberculosis and Salmonella Typhi.

A is correct: Pyogenic (pus forming) bacteria are able to evade the action of phagocytes, resulting in an accumulation of bacteria and white cells at the site of infection (pus).

B is correct: Many bacteria produce toxins which kill phagocytes, e.g. Staph. aureus produces 6 different leucocidins. Other bacteria produce toxins which inhibit the function of phagocytes e.g. Bacillus anthracis, Bordetella pertussis.

C is correct: These bacteria are able to survive inside phagocytes, and hence may replicate in these cells to avoid components of the immune system.

D is incorrect: Antibodies and complement are important in the removal of extracellular organisms, but facultative intracellular organisms are protected from their action.

E is correct: These organisms possess mechanisms which inhibit phagocyte killing, enabling the long term survival and replication of these organisms inside the macrophages.


Which one of the following microorganisms is not associated with food or water borne gastroenteritis?

  Giardia lamblia.

  Salmonella spp.

  Staphylococcus epidermidis.

  Norovirus (calicivirus, Norwalk virus)

  Campylobacter spp.

A is incorrect: Giardia lamblia colonises the small bowel mucosa, and may produce diarrhoea characterised by slimy, foul smelling stools. It is an anaerobic protozoan parasite, predominantly found in water.

B is incorrect: Salmonella spp. are important causes of food associated gastroenteritis, which, in rare cases may become invasive. Remember that Salm. Typhi and Salm. Paratyphi cause systemic disease (Typhoid fever, Paratyphoid fever), not localised gastroenteritis.

C is correct: Although Staph. epidermidis may be detected in various foods, it is not associated with food or water born gastroenteritis. However, its presence may be suggestive of significant contamination of food due to poor hand hygiene.

D is incorrect: Noroviruses are associated with water and aerosol born gastroenteritis. They induce vomiting and diarrhoea. The viruses are non-enveloped small RNA viruses, which survive well in the environment. Virus particles are thought to be aerosolised during vomiting, and may be inhaled, or deposited on surfaces where they may persist for some time.

E is incorrect: Campylobacters are one of the most frequently dentified cause of food associated gastroenteritis.


Regarding the pathogenesis of bacterial gastroenteritis, which one of the following statements is not correct?

  Vibrio cholerae produces a toxin which acts as an ADP-ribosylating toxin, which activates adenylate cyclase and disrupts normal transport of electrolytes and water.

  Vibrio cholerae adheres to intestinal mucosa, then invades the epithelium to produce disease.

  The toxins of V. cholerae and enterotoxigenic E. coli (ETEC) act by the same mechanism.

  Salmonellae stimulate a strong inflammatory response resulting in prostaglandin release which in turn activates adenylate cyclase.

  Staphylococcus aureus produces an enterotoxin which acts on the CNS.

B is incorrect: Vibrio cholerae is not an invasive bacterium. Cholera toxin is produced after the organism has adhered to the mucosal epithelium and binds to its receptor on host cells (GM1 ganglioside), and is endocytosed.

D is correct: Adenylate cyclase can be activated by both toxin (e.g. Cholera toxin, LT of ETEC) and prostaglandin mediated pathways.

E is correct: Staphylococcal enterotoxin (SE) is produced by bacteria replicating in contaminated food. When ingested, the SE stimulates the vagus nerve endings resulting in vomiting (often projectile) and abdominal pain.


Regarding the genus Salmonella, which one of the following statements is incorrect?

  It is a gram-negative rod (bacillus)

  It is recognized as a lactose non-fermenter on MacConkey agar.

  Members of the genus can be subdivided by using antibodies directed against "O" and "H" antigens.

  Serovars classified by "O" and "H" antigen typing can be further typed by bacteriophage ("phage") typing.

  It is a member of the normal flora of the gastrointestinal tract of humans.

Salmonellae are common normal flora of many animals but not humans, although some people can carry the organisms for some time.