previous year question on bacterial toxins from microbiology based on neet pg, usmle, plab and fmge...

Which of the following statement is CORRECT regarding exotoxins?

A: Highly antigenicB: Are very stable and resist most physical and chemical agentsC: Produced abundantly by gram negative bacilliD: Lipopolysaccharide in nature

Correct Ans:AExplanationExotoxins are produced by several gram-positive and gram-negative bacteria. The essential characteristic of exotoxins is that they are secreted by the bacteria. Exotoxins are polypeptides whose genes are frequently located on plasmids or lysogenic bacterial viruses (bacteriophages). Some important exotoxins encoded by bacteriophage DNA are diphtheria toxin, cholera toxin, and botulinum toxin.

Exotoxin polypeptides are good antigens and induce the synthesis of protective antibodies called antitoxins, some of which are useful in the prevention or treatment of diseases such as botulism and tetanus. When treated with formaldehyde (or acid or heat), the exotoxin polypeptides are converted into toxoids.

Ref: Levinson W. (2012). Chapter 7. Pathogenesis. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.

Sample Previous Year Question Of Microbiology on Bacterial Endotoxins and Exotoxins based on previous Year Questions of NEET PG, USMLE,PLAB,FMGE (MCI Screening). Please visit www.medicoapps.org for more such Quizzes

Elek test is used for detecting toxins of the following infection:

A: InfluenzaB: DiphtheriaC: BrucellosisD: Cholera

Correct Ans:BExplanation

Elek gel precipitation test is used for detection of Diphtheria toxin. A presumptive C diphtheriae isolate should be subjected to testing for toxigenicity. Such tests are performed only in reference public health laboratories. There are several methods, as follows: The modified Elek method described by the World Health Organization Diphtheria Reference UnitPolymerase chain reaction (PCR)–based methodsEnzyme-linked immunosorbent assaysImmunochromographic strip assay Ref: Brooks G.F. (2013). Chapter 12. Aerobic Non–Spore-Forming Gram-Positive Bacilli: Corynebacterium, Listeria, Erysipelothrix, Actinomycetes, and Related Pathogens. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.


With reference to bacteriodes fragilis, the following statements are true, EXCEPT:

A: Bacteriodes fragilis is a frequent anaerobe isolated from clinical samples

B:Bacteriodes fragilis is not uniformly sensitive to metronidazole

C:The LPS formed by Bacteriodes fragilis is structurally and functionally different from the conventional endotoxins

D: Shock and DIC are common in bacteriodes bacterimia

Correct Ans:DExplanation

The incidence of septic shock and DIC are unusual in patients with bacteremia caused by anaerobic bacteria such as bacteroides. Anaerobic gram negative bacteria such as bacteroides possess lipopolysaccharide (LPS) which is nearly 1000 times less potent compared to endotoxins associated with aerobic gram negative bacteria, accounting for the reason behind its low association with shock and DIC.

Ref: Textbook of Microbiology, Ananthanarayan and Paniker, 7th Edition, Chapter 29, Page 268; Harrisons 16th Edition, Page 941 944.


A 34 year old man diagnosed of having H. pylori infection after biopsy. Where can H. pylori be found in the stomach?

A: Epithelial liningB: Mucous layerC: Lamina propriaD: Local toxins

Correct Ans:BExplanation

H. pylori is a noninvasive organism that colonizes the mucous layeroverlying gastric epithelium. Factors important in the organism’s ability to colonize the stomach include its flagellae, which facilitate its locomotion; its ability to adhere to the mucous layer; and its production of urease.


Staphylococcus aureus and Streptococcus pyogenes are SIMILAR in all of the following features EXCEPT one. Pick the INCORRECT statement:

A: Both produce numerous exotoxins and exoenzymesB: Both are Gram-positive cocciC: Both cause skin, tissue, and systemic infectionsD: Both are catalase positive

Correct Ans:D

Explanation

Both are catalase positive. This is the incorrect pair: S. aureus is catalase-positive, but S. pyogenes (like all streps) is catalase-negative. This is an important point in diagnosis in the lab.

Also Know:The group B streptococcus (S agalactiae) is the most common cause of sepsis in newborns and the pneumococcus (S pneumoniae) a leading cause of both pneumonia and meningitis in persons of all ages. Ref: Ray C.G., Ryan K.J. (2010). Chapter 25. Streptococci and Enterococci. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.


All of the following statements about superantigens are CORRECT, EXCEPT:

A: Their pathological effects do not depend on antigen processingB: They include bacterial toxins, especially staphylococcal enterotoxins.C: They bind to the variable region of the TCR beta chainD: Their biological effect requires binding to class I MHC moleculesCorrect Ans:DExplanation

Their biological effect requires binding to class I MHC molecules. The answer is false, so it is the correct response. Superantigens require binding to MHC Class II (not Class I) and TCR.

A group of antigens have been termed superantigens because they stimulate a much larger number of T cells than would be predicted based on the specificity of combining site diversity. The action of superantigens is based on their ability to bind directly to MHC proteins and to particular V regions of the TCR without involving the antigen combining site.

Ref: Ray C.G., Ryan K.J. (2010). Chapter 2. Immune Response to Infection. In

C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e;Immunology, Infection, and Immunity By Gerald Bryan Pier, Jeffrey B. Lyczak.


Which of the following is FALSE regarding mycobacterium?

A: Cell wall has high lipid contentB: Mycolic acids and LAM form waxy coatC: Lack exotoxins or endotoxinsD: None of the above


Of particular importance is the presence of long-chain fatty acids called mycolic acids (for which the mycobacteria are named) and lipoarabinomannan (LAM), a lipid polysaccharide complex extending from the plasma membrane to the surface.

These elements give the mycobacteria a cell wall with unusually high lipid content (more than 60% of the total cell wall mass), which accounts for many of their biologic characteristics. It can be thought of as a waxy coat that makes them hardy, impenetrable, and hydrophobic. Mycobacteria do not produce classic exotoxins or endotoxins. Disease processes are thought to be the result of two related host responses:

Delayed-type hypersensitivity (DTH) Cell-mediated immunity (CMI)

Ref: Ray C.G., Ryan K.J. (2010). Chapter 27. Mycobacteria. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.

Sample Previous Year Question Of Microbiology on Bacterial Endotoxins and Exotoxins based on previous Year Questions of NEET PG,

USMLE,PLAB,FMGE (MCI Screening). Please visit www.medicoapps.org for more such Quizzes

A 54-year-old diabetic patient reports to his physician's office complaining of an unresolved skin lesion on his foot. The lesion began several weeks ago as a blister and has since become a painful, erosive, expanding sore. On examination, the affected site is now 5 cm in diameter, with a black necrotic center and raised red edges. Which of the following toxins has a mechanism of action most similar to the toxin responsible for tissue damage in this patient?

A: Anthrax toxinB: Botulinum toxinC: Cholera toxinD: Diphtheria toxinCorrect Ans:DExplanationThis patient's wound is infected with Pseudomonas aeruginosa, and the characteristic lesion described is called ecthyma gangrenosum. The Pseudomonas alpha toxin, which is responsible for the tissue damage, inhibits protein synthesis by acting on EF-2 with a primary target cell in the liver. The diphtheria toxin has a similar action, although its target cells are heart and nerve. Anthrax toxin is an adenylate cyclase that causes fluid loss from cells. Botulinum toxin is a neurotoxin that decreases acetylcholine synthesis. Cholera toxin acts to increase adenylate cyclase activity by ribosylation of GTP-binding protein. Ref: Brooks G.F. (2013). Chapter 16. Pseudomonads, Acinetobacters, and Uncommon Gram-Negative Bacteria. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.


Which of the following structures is found only in Gram-negative microorganisms?

A: Cell envelopeB: Exotoxin

C: PeptidoglycanD: Periplasmic spaceCorrect Ans:DExplanationThis question requires you to appreciate the key structural difference between gram-positive and gram-negative microorganisms. Most gram-negatives (other than exceptional microorganisms, such as Mycoplasma, that lack a cell wall) have a more complex cell envelope than gram-positive microorganisms. It includes both a cytoplasmic membrane as well as an outer membrane. Between these two membranes is the periplasmic space, which contains enzymes such as phosphatase and penicillinase, binding proteins for the transport of various nutrients, and peptidoglycan, as well as a portion of the lipoprotein that firmly anchors the outer that a baby can form with an acute membrane to the peptidoglycan. Gram-positives do not have outer membranes. The cell envelope is incorrect because both gram-positive and gram-negative microorganisms have this structure, which is defined as all the layers that enclose the cytosol of the bacterium. It is the composition of the envelope that differs between gram-positives and gram-negatives. Exotoxin is not exclusive to gram-negative microorganisms, but is also found in some gram-positives. By contrast, endotoxin (lipopolysaccharide; LPS) is found exclusively in gram- negatives. Peptidoglycan is found in the cell walls of both gram-positive and gram-negative microorganisms. Note that there is a larger amount of peptidoglycan in gram-positive microorganisms. Ref: Brooks G.F. (2013). Chapter 2. Cell Structure. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.


All are true about EHEC, except:

A: Sereny test is positiveB: Ferments sorbitol

C: Causes HUSD: Elaborates shiga like exotoxinCorrect Ans:AExplanationSereny Test is given only by EIEC. Ref: Text Book of Microbilogy By Ananthanarayan, 6th Edition, Page 257 ; Harrison’s Principles of Internal Medicine, 14th Edition, Page 938 and 15th Edition, Page 955


Which of the following strains of Escherichia coli is commonly implicated in traveler’s diarrhea mediated by heat stable enterotoxin and heat labile exotoxin?

A: EnterotoxigenicB: Enteroinvasive

C: EnteropathogenicD: EnterohemorrhagicCorrect Ans:AExplanation

Enterotoxigenic Escherichia coli (ETEC) is common cause of traveler’s diarrhea. It is also associated with diarrhea in infants in developing countries. It causes watery diarrhea, abdominal cramps and low grade fever. The common etiologies of traveler's diarrhea may vary with location and with season. However, on a world-wide basis, the organism most frequently identified in patients with traveler's diarrhea is enterotoxigenic Escherichia coli. This organism accounts for between 30% and 70% of all cases of traveler's diarrhea. Transmission is usually through food or water that is fecally contaminated.

The virulence factor implicated are as follows:Heat stable enterotoxin stimulates fluid secretion by activating guanylate cyclase in enteric epithelial cells. Heat labile exotoxin binds to the ganglioside at the brush border of epithelial cells of small intestine and leads to activation of adenylate cyclase. Colonization factors facilitate the attachment of ETEC to epithelial cells of intestine.

Ref: Textbook of Microbiology ANANTHANARAYANAN, 8th Ed, page 274-277


The exotoxin implicated in this patient's disease is unusual in that it is a superantigen. This means which of the following?

A:It can bind to the inside surfaces of the normal peptide grooves of both lgA and the TCR molecule

B:It can bind to the inside surfaces of the normal peptide grooves of both lgE and the TCR molecule

C:It can bind to the inside surfaces of the normal peptide grooves of both the MHC class l molecule and the TCR molecule

D:It can bind to the outside surfaces of the normal peptide grooves of both the MHC class ll molecule and the TCR molecule


Both the staphylococcal toxic shock syndrome toxin-1 (TSST-1) and the streptococcal superantigen (SSA) are unusual in that they tend to trigger a massive release of cytokines in response to relatively small amounts of toxin. The reason is that, unlike normal antigens that bind inside the normal peptide grooves of the MHC class II molecule on the antigen presenting cell and the T cell receptor (TCR) molecule of the T lymphocyte, these antigens recognize sites outside the grooves on these two molecules. The result is that they can, in effect, "turn on" almost every T cell they find, thereby producing the massive release of cytokines.

IgA, IgE, MHC class I molecules, and IgG are not involved in these processes.

Ref: Levinson W. (2012). Chapter 58. Cellular Basis of the Immune Response. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.


Which of the following statement regarding gas gangrene is TRUE?

A: It is due to Clostridium Botulinum infectionB: Clostridial species are gram-negative spore forming anerobesC: The clinical features are due to the release of protein endotoxinD: Gas is invariably present in the muscle compartments


Gas gangrene is a rapidly developing and a spreading infection caused by the toxin producing clostridial species namely, Clostridium perfringens ( formerly known as C. welchii). It is highly fatal if left untreated and accompanied by profound toxicity. If the oxidation–reduction potential in a wound is sufficiently low, C perfringens spores can germinate and then multiply, elaborating alpha-toxin.

The process passes along the muscle bundles, producing rapidly spreading edema and necrosis as well as conditions that are more favorable for growth of the bacteria. As the disease progresses, increased vascular permeability and systemic absorption of the toxin and inflammatory mediators leads to shock.

Ref: Oxford Textbook of Medicine, Vol 1 By D. A. Warell, Timothy M. Cox, Page 553


With reference to Bacteroides fragilis the following statements are true, EXCEPT:

A: B. fragilis is the same frequent anaerobe isolated from clinical samples.B: B. fragilis is not uniformly sensitive to metronidazole

C:The lipopolysaccharide formed by B. fragilis is structurally and functionally different from the conventional endotoxin

D: Shock and disseminated intravascular coagulation are common in Bacteroides bacteremia


Bacteroides species have lipopolysaccharides, but lack the lipopolysaccharide structures with endotoxic activity (including beta-hydroxymyristic acid). The lipopolysaccharides of B fragilis are much less toxic than those of other gram-negative bacteria. Thus, infection caused by Bacteroides does not directly produce the clinical signs of sepsis (eg, fever and shock) so important in infections caused by other gram-negative bacteria. When these clinical signs appear in Bacteroides infection, they are a result of the inflammatory immune response to the infection.

Ref: Brooks G.F. (2013). Chapter 21. Infections Caused by Anaerobic Bacteria. In G.F. Brooks (Ed), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.


The endotoxin of which of the following gram-negative bacteria DOES NOT play any part in the pathogenesis of the natural disease?

A: Escherichia coliB: Klebsiella sp.

C: Vibrio choleraeD: Pseudomonas aeruginosaCorrect Ans:CExplanation

Vibrio cholerae possess the lipopolysaccharide O antigen(LPS, endotoxin). It does not play any role in the pathogenesis of cholera but is responsible for the immunity induced by the killed vaccines.

The watery diarrhea of cholera results from the action of a cholera toxin which is a potent protein enterotoxin produced by the organism in the small intestine. It has 2 subunits A and B, Ganglioside GM1 serves as receptor for subunit B which in turn promotes the entry of subunit A into the cell. Activation of subunit A1 cause increase in level of cAMP and results in prolonged hypersecretion of water and electrolytes.

Ref: Textbook of Microbiology By Ananthanarayan And Paniker page 311. Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e chapter 17.


previous year question on bacterial toxins from microbiology based on neet pg, usmle, plab and fmge...

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