missed questions, review Flashcards
if your E. coli culture looks blue after Gram staining, what did you most likely do wrong?
failure to decolorize with alcohol
(it’s not failure to counter-stain with safranin because the culture would be colorless in that case, assuming you did prior steps correctly)
for each of these technical errors of Gram staining, predict what would happen:
a. failure to use fresh culture
b. failure to heat fix
c. failure to stain with Gentian violet
d. failure to apply iodine
e. failure to decolorize with alcohol
f. failure to counter-stain with safranin
(assume all other steps were done correctly)
a. failure to use fresh culture = dead cells
b. failure to heat fix = no cells
c. failure to stain with Gentian violet = all pink cells
d. failure to apply iodine = all pink cells
e. failure to decolorize with alcohol = all blue cells
f. failure to counter-stain with safranin = colorless Gram(-)
what part of the envelope of Shigella (Gram-) contributes most to its toxicity?
Lipid A endotoxin of outer membrane (contained in LPS, lipopolysaccharide)
*remember Lipid A can cause toxicity whether bacteria is dead or alive
what is the structure of Lipid A, the part of LPS responsible for causing toxicity?
disaccharide substituted with saturated fatty acids
R-factor (resistance genes) are usually transferred via a ____
plasmid, via conjugation
what needs to happen for stable maintenance of a transferred antibiotic resistance gene?
plasma genes need to integrate into a chromosome (DNA which replicates)
requires homologous recombination
which of these processes is NOT involved in moving genes from one cell to another?
a. conjugation
b. homologous recombination
c. transduction
d. transformation
e. movement of a transposon
e. movement of a transposon - moving DNA from one plasmid to another or to the chromosome (within a cell)
which of these processes of genetic transfer in bacteria requires homologous recombination?
a. transformation
b. conjugation
c. transduction
a. transformation (uptake of genetic material from environment)
A culture of Salmonella is found to have a mixture of cells expressing different H (flagellar) antigens. What genetic process accounts for this?
inversion of DNA segment which contains the promoter for the flagellin gene
where in the body does Neisseria meningitidis establish asymptotic carrier state?
nasopharynx
which of the following is essential for the generation of antibodies against bacterial surface proteins?
a. B lymphocytes are stimulated in a “T-independent” fashion
b. CD8+ lymphocytes proliferate
c. DC cells phagocytose and digest bacterial proteins
d. peptides from bacterial proteins are presented by Class I MHC
c. DC cells phagocytose and digest bacterial proteins
on what kind of plate and what conditions can Neisseria meningitidis be cultured?
Thayer-Martin agar and elevated CO2: chocolate agar containing antibiotics to inhibit normal flora
what component of Neisseria meningitidis causes septic shock?
LOS (lipo-oligosaccharide) - contains Lipid A endotoxin
[remember that septic shock is due to endotoxins, while toxic shock is due to exotoxin superantigens]
what does it mean when a bacteria is catalase positive? (what does this enzyme do?)
catalase = hydrogen peroxidase
catalyzes H2O2 —> H2O + O2
protective against ROS
contrast the causes of septic shock and toxic shock syndrome
septic shock: due to endotoxins that cause inflammatory response
toxic shock syndrome: due to superantigen exotoxins that cause polyclonal T cell activation
match the pathogen with the more likely mode of infection:
staphylococcus or streptococcus
a. contamination of a needle or injection site with oral flora
b. entry of skin flora through a needle-stick site
staph.: skin flora
strep.: oral flora (alpha-hemolytic)
which Gram+ coccus is uniformly sensitive to penicillin G?
a. staph. aureus
b. staph. epidermidis
c. strep. pneumoniae
d. strep. pyogenes
d. strep. pyogenes
You are culturing a bacteria which you suspect to be Strep. pyogenes, Group A streptococcus that is beta-hemolytic. To confirm your suspicion, you do a bacitracin test. What do you expect the result to be?
Bacitracin sensitive
antibodies to which antigen of Streptococcus pneumoniae are most important for effective immunity?
a. teichoic acid
b. peptidoglycan
c. C-carbohydrate
d. Type III system
e. polysaccharide capsule
e. polysaccharide capsule - most important virulence factor of Strep. pneumoniae (no capsule = non-infectious)
which is more likely to be highly antibiotic resistant, Enterococcus or Group D Streptococcus?
Enterococcus
Which test result would best distinguish Strep. pneumoniae from other alpha-hemolytic Streptococci?
a. sensitivity to bacitracin
b. sensitivity to optochin
c. sensitivity to penicillin G
d. sensitivity to methicillin
e. sensitivity to rifampin
b. sensitivity to optochin
recall that other alpha-hemolytic Strep, Viridans group, is optochin resistant
What bacterial component defines the Lancefield Groups?
cell-associated or C-carbohydrate
what does it mean when colonies on MacConkey agar plates turn dark pink?
strain ferments lactose, which causes pH to drop and pH indicator dye turns pink
what does it means when a TSI (Triple Sugar Iron) agar slant turns yellow?
TSI slants contain pH indicator that turns yellow if pH drops because of fermentation of either/both lactose or sucrose
what is the clinical outcome of the heat stable toxin of E. coli stimulating guanyl cyclase?
stabile toxin: structural analogue of gut peptide hormone which stimulates guanyl cyclase coupled receptor
*increase in either cAMP (labile toxin, resembles cholera toxin) or cGMP causes salt/osmotic outflow into gut lumen and watery diarrhea
Verotoxin =
Shiga-like toxin
where would a young child most likely contract Shigella?
a. reptile such as snake or iguana
b. dog or cat
c. cow, horse, sheep
d. parakeet or other bird
e. another child
f. spores present in soil
e. another child - Shigella found only in humans, transmitted via fecal oral route
*note that Shigella is resistant to stomach acid and has a low infectious dose
Yersinia enterocolitica and Yersinia pseudotuberculosis both use a Type III secretion system to prevent:
a. phagocytosis by neutrophils
b. fusion of phagosomes with lysosomes
c. processing and presentation of antigens
a. phagocytosis by neutrophils
[remember that Yersinia enterocolitica and Yersinia pseudotuberculosis are acquired from animals or ingestion of contaminated meat or dairy]
what is the antigenic component of Haemophilus influenzae vaccine?
Hib conjugate vaccine: purified capsular polysaccharide covalently bound to diphtheria toxoid
severe projectile vomiting a few hours after eating contaminated food is a hallmark of food poisoning caused by
Staphylococcus aureus
primary mechanism for Enterococcal resistance to vancomycin is via expression of
VanHAX operon (D-Ala-D-Ala —> D-Ala-D-Lac)
when a person living in a closed community (military, college) presents with severe headache + neck stiffness + fever, your first thought should be
Neisseria meningitidis
increased production of endogenous metabolite PABA (p-aminobenzoic acid) causes resistance to
sulfamethoxazole - sulfonamides are anti-metabolite antibiotics that block the first enzyme in folate synthesis, which uses PABA as a reactant
therefore, resistance works by overwhelming drug with increased PABA
the most important virulence factor for the development of UTIs
pili (adhesins): necessary for adhesion to urethra
name the toxin which:
a. induces adenylate cyclase
b. inhibits protein synthesis on 60s ribosomal subunit
c. ribosylates EF-2 (elongation factor 2)
a. induces adenylate cyclase —> Cholera toxin and labile toxin of E. coli
b. inhibits protein synthesis on 60s ribosomal subunit —> Shiga toxin (and Shiga-like toxin)
c. ribosylates EF-2 (elongation factor 2) —> pseudomonas exotoxin A and diphtheria toxin
what bacterial infection will cause copious amounts of watery diarrhea but no fever
Vibrio cholerae
endotoxins are only found in Gram(+/-)
endotoxins only found in Gram (-), in LPS
exotoxins found in both Gram+/-
how do beta-lactamases inactivate penicillin?
transient covalent linkage
Hemolytic Uremic Syndrome (HUS) is typically caused by ____ toxin, produced by ______ bacteria
HUS: caused by Shiga toxin produced by Shigella or STEC E. coli
what would you expect to see after an acid-fast stain of tuberculosis sputum?
red-staining bacilli
what is the purpose of culturing Mycobacterium tuberculosis on Lowenstein-Jensen agar and in the BACTEC system?
culture allows testing for antibiotic sensitivity/resistance
what characteristic of M. tuberculosis is most essential for its transmission?
impermeable lipid-rich envelope
A first-year resident tests tuberculin-positive and requires prophylactic treatment - which drug should be given?
a. streptomycin (aminoglycoside)
b. ciprofloxacin (fluoroquinolone)
c. isoniazid
d. rifampin
e. ethambutol
c. isoniazid
what is the most important type of immune defense against M. tuberculosis?
killing by macrophages activated by TH1 cells via gamma interferon
in what kind of immunodeficient patient might a PPD test produce a false-negative?
patients with impaired cell-mediated immunity may produce false-negative PPD because immune response to tuberculosis is TH1 mediated (activate macrophages via IFNy)
if cells infected with Chlamydia trachomatis were examined by staining and light microscopy, what would be observed?
membrane-bounded inclusion body within the cytoplasm
what property of Rickettsia (causing RMSF) prevents its recovery by culture?
it can only be grown in living mammalian cells
if cells infected with Rickettsia were examined by electron microscopy, what would be observed?
small rod-shaped bacilli free in the cytosol (intracellular)
what structural feature is unique to bacteria such as Borrelia burgdorferi and Treponema pallidum?
a. outer membrane with porins
b. pili for tight adherence
c. flagella within the outer membrane
d. high-affinity iron-transport systems in the cell envelope
c. flagella within the outer membrane - endoflagella
A 25yo pregnant woman develops chorioamnionitis from Bacteroides fragilis - how did the organism reach the fetal membranes?
Bacteroids fragilis is normal intestinal flora - spread from endogenous source to her genitourinary tract
which bacteria is most likely to be acquired from home-canned foods?
Clostridium botulinum - produces spores not easily killed by boiling
Your patient presents with a lesion they said began as a small red swelling which enlarged and then ulcerated. A swab of the ulcer contains large Gram+ rods and aerobic culture on sheep blood agar produces non-hemolytic colonies. When the ulcer heals, it produces a thick black scab.
What is most likely going on?
Bacillus anthracis cutaneous infection
Bacteroides fragilis is isolated from your patient’s abscess - what condition most likely facilitated the infection?
a. treatment with sulfonamide
b. tissue ischemia
c. antibiotic treatment for another bacterial infection
d. colonization of the large intestine with spores of this organism
tissue ischemia - Bacteroides fragilis is anaerobic
to what do neutralizing antibodies for Polio virus bind?
a. matrix protein
b. nucleocapsid protein
c. membrane glycoproteins
d. non-structural proteins
nucleocapsid protein
which of these steps only occurs in “early” phase of viral replication?
a. production of virion structural proteins
b. synthesis of viral-encoded enzymes
c. transcription of viral genes
d. production of new viral genome
b. synthesis of virus-encoded enzymes
which of the following will detect only infectious virions?
a. ELISA
b. PCR
c. hemagglutination
d. plaque assay
e. western blot
plaque assay - non-infectious viruses will not be able to lyse cells
where does variation that differentiates serotypes occur?
a. structural variation of proteins at viral surface
b. variation in symmetry of nucleocapsid
c. presence or absence of particular proteins in virions
d. structural variations of viral internal proteins
structural variation of proteins at virus surface
some viruses spread through inducing fusion of uninfected cells with infected cells - what mode of immune defense does this escape?
neutralization of virions by antibody
