Revision with a Master Quiz of 15 questions based on NEET PG Sample Questions on Bacterial Toxins (Microbiology)from Previous Year NEET PG Online Exams.

1. Which of the following statement is CORRECT regarding exotoxins?
A: Highly antigenic
B: Are very stable and resist most physical and chemical agents
C: Produced abundantly by gram negative bacilli
D: Lipopolysaccharide in nature
Correct Ans:A
Explanation
Exotoxins 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: Influenza
B: Diphtheria
C: Brucellosis
D: Cholera
Correct Ans:B

2. Explanation
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 Unit
Polymerase chain reaction (PCR)–based methods
Enzyme-linked immunosorbent assays
Immunochromographic 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.
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
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:D
Explanation
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

3. 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.
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 34 year old man diagnosed of having H. pylori infection after biopsy. Where
can H. pylori be found in the stomach?
A: Epithelial lining
B:
Mucous
layer
C: Lamina propria
D: Local toxins
Correct Ans:B
Explanation
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.
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
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 exoenzymes

4. B:
Both are Gram-positive
cocci
C: Both cause skin, tissue, and systemic infections
D: 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.
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
All of the following statements about superantigens are CORRECT, EXCEPT:
A: Their pathological effects do not depend on antigen processing
B: They include bacterial toxins, especially staphylococcal enterotoxins.
C: They bind to the variable region of the TCR beta chain
D: Their biological effect requires binding to class I MHC molecules
Correct Ans:D
Explanation
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

5. 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.
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
Which of the following is FALSE regarding mycobacterium?
A: Cell wall has high lipid content
B: Mycolic acids and LAM form waxy coat
C: Lack exotoxins or endotoxins
D: None of the above
Correct Ans:D
Explanation
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.

6. 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 toxin
B: Botulinum toxin
C: Cholera toxin
D: Diphtheria toxin
Correct Ans:D
Explanation
This 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.

7. 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
Which of the following structures is found only in Gram-negative
microorganisms?
A: Cell envelope
B:
Exotoxi
n
C: Peptidoglycan
D: Periplasmic space
Correct Ans:D
Explanation
This 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.

8. Ref: Brooks G.F. (2013). Chapter 2. Cell Structure. In G.F. Brooks (Ed), Jawetz,
Melnick, & Adelberg's Medical Microbiology, 26e.
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
All are true about EHEC, except:
A: Sereny test is positive
B:
Ferments
sorbitol
C: Causes HUS
D: Elaborates shiga like exotoxin
Correct Ans:A
Explanation
Sereny 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
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
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: Enterotoxigenic
B:
Enteroinvasiv
e
C: Enteropathogenic
D: Enterohemorrhagic
Correct Ans:A

9. Explanation
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
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
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
Correct Ans:D
Explanation

10. 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.
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
Which of the following statement regarding gas gangrene is TRUE?
A: It is due to Clostridium Botulinum infection
B: Clostridial species are gram-negative spore forming anerobes
C: The clinical features are due to the release of protein endotoxin
D: Gas is invariably present in the muscle compartments
Correct Ans:D
Explanation
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

11. 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
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
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
Correct Ans:D
Explanation
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.
Sample Previous Year Question Of Microbiology on Bacterial Endotoxins and
Exotoxins based on previous Year Questions of NEET PG,

12. USMLE,PLAB,FMGE (MCI Screening). Please visit www.medicoapps.org for
more such Quizzes
The endotoxin of which of the following gram-negative bacteria DOES NOT play any part in
the pathogenesis of the natural disease?
A: Escherichia coli
B:
Klebsiella
sp.
C: Vibrio cholerae
D: Pseudomonas aeruginosa
Correct Ans:C
Explanation
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.
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