Pathogenic Microbiology Exam II

What are the virulence factors for Staphylococcus aureus?

There are four (4) virulence factors that contribute to the pathogenicity of S. aureus:

1. Evasion via capsule and protein A

2. Invasion via surface-attachment proteins (MSCRAMM) and hyaluronidase

3. Toxin production via cytotoxins, enterotoxins, exfoliative toxins (ETA/ETB), and TSS toxin-1

4. Antibiotic resistance via β-lactamase deactivation and alternative penicillin-binding protein

What is the similarities and differences between generalized and localized Staphylococcal scalded skin syndrome (SSSS)?

Both generalized and localized SSSS can arise from the expression of either exfoliative toxin A or B. Both presentations can resolve on their own or can be eliminated with a course of antibiotics.

Generalized SSSS occurs when the toxin spreads throughout the body and causes generalized blistering with no pus present and do not contain bacteria. Conversely, localized SSSS has pus-filled blisters localized around the infection site and may contain bacteria.

What are the virulence factors for Streptococcus pyogenes?

There are four (4) virulence factors that contribute to the pathogenicity of Streptococcus pyogenes:

1. Attachment via lipoteichoic acid, protein M, and protein F

2. Invasion via hyaluronidase and streptokinase

3. Evasion via capsule, protein M, and C5a peptidase

4. Toxin production via streptoccocal pyrogenic exotoxins and streptolysin S/O

Identify the pathogen most known to cause “Necrotizing Fasciitis” and describe the mechanisms by which this pathogen prevents phagocytosis.

Streptococcus pyogenes is responsible for causing streptococcal gangrene, i.e., necrotizing fasciitis.

Streptococcus pyogenes evades phagocytosis through three (3) components:

1. Capsule layer. This thick, sticky polysaccharide layer interferes with complementation.

2. Protein M. This protein not only has serotype variability, but also has a plasma protein that inhibits opsonization (factor H).

3. C5a peptidase. This enzyme disrupts the inflammatory response of the complement pathway.

What are the virulence factors of Streptococcus pneumoniae?

There are four (4) virulence factors that contribute to the pathogenicity of S. pneumoniae:

1. Attachment via pili and choline-containing teichoic acid

2. Evasion via capsule (ALL virulent strains have this), pneumolysin, and pneumococcal surface protein A (PspA)

3. Invasion via pneumolysin

What are the factors by which Streptococcus pneumoniae evades phagocytosis?

Streptococcus pneumoniae evades phagocytosis by evading immune recognition (capsule), inhibiting opsonization (PspA), or phagocyte lysis (pneumolysin).

What are the virulence factors of Bacillus anthracis?

There are two (2) categories of virulence factors that contribute to the pathogenicity of Bacillus anthracis:

1. Evasion through capsule layer via px-02

2. Toxin production of lethal factor, edema factor, and protective antigen via px-01

What are the roles of plasmid px-01 and plasmid px-02 in the pathogenicity of Bacillus anthracis?

px-01 is the plasmid containing the genes responsible for the production of protective antigens, lethal factors, and edema factors.

• The protective antigen binds to a the host anthrax toxin receptor.

• The protective antigen is cleaved by furin proteases, resulting in a receptor-bound protective antigen fragment that self-associates into a prepore.

• This prepore then binds multiple lethal and/or edema factors.

• This bound complex is then endocytosed into the cell, from which it is converted into lethal toxins and/or edema toxins.

The factors themselves are not individually toxic. They only become toxic once a PA-LF/EF complex is intracellularly converted into their toxic forms

px-02 is the plasmid containing the genes responsible for the capsule layer of B. anthracis. Like most pathogens, the capsule layer is crucial for phagocytic evasion.

Fully virulent B. anthracis requires both plasmids.

What are the virulence factors of Bacillus cereus?

The pathogenicity of B. cereus is attributed to the toxin release of emetic toxins and enterotoxins.

• Emetic toxins are heat-stable, acid-stable, and cause emesis (vomiting)

• Enterotoxins are heat-sensitive, acid-sensitive, and cause diarrheal disease (bad poopoo)

What are the differences between the two (2) clinical diseases Bacillus cereus causes?

Bacillus cereus can cause emetic disease and diarrheal disease:

• Emetic disease of B. cereus has a 6-hour incubation period and causes severe vomiting; typically associated with rice.

• Diarrheal disease of B. cereus has a 12-hour incubation period and causes severe diarrhea; typically associated with meat and vegetables.

Both clinical diseases are typically self-limiting, and resolve on their own without the need for antibiotics. However, ciprofloxacin is prescribed when necessary.

• Antibiotics are useless against emetic disease

What are the virulence factors of Listeria monocytogenes?

There are three (3) broad categories of virulence factors involved in the pathogenicity of Listeria monocytogenes:

1. Invasion via internalins that mediate endocytosis into host cell.

2. Evasion via listeriolysin O and phospholipase C that allows listeria to be released from the vacuole into the cytosol.

3. Translocative invasion via ActA, which polymerizes the host cell’s actin to move between cells.

What is the special characteristic of Listeria monocytogenes when it comes to growth temperature?

Listeria monocytogenes is psychrotolerant and can happily grow in refrigeration temperatures (4°C).

What is the unique mechanism Listeria monocytogenes uses to move into and between cells?

1. Listeria first binds to a host cell receptor protein.

2. Listeria surface proteins (internalins) promote clathrin-mediated endocytosis into the host cell. The bacteria is now enclosed inside a vacuole.

3. Then, listeriolysin O and phospholipase C help to rupture the vacuole and release the bacterium into the cytosol.

4. Listeria will then proliferate (replicate) inside of the host.

5. Then, ActA on the surface of Listeria will then polymerized the host actin to form an actin tail on the bacterium.

6. This actin tail will propel Listeria to the host cell membrane, forming a protrusion called a filopod.

7. This filopod will then touch a neighboring cell, which will endocytose the bacterium.

8. The cycle then repeats.

What are the virulence factors of Corynbacterium diphtheriae?

The major virulence factor in Corynbacterium diphtheriae is a potent exotoxin, of which its expression (tox gene) is mediated by iron concentrations:

• Low iron concentration = toxin is expressed

• High iron concentration = toxin is not expressed

Not all C. diphtheriae posses the tox gene. Lysogenic C. diphtheriae possess the tox gene.

What disease does Corynbacterium diphtheriae cause, and what is the nature of the vaccine used to prevent this disease?

C. diphtheriae causes respiratory and cutaneous diphtheria, whereby humans are the only known reservoir.

Vaccinations in the form of formalin-treated inactive exotoxin are incredibly effective at disease prevention.

• This inactive exotoxin is inside a combination vaccine called DTP (DTaP and Tdap), which is administered 6 times during adolescence.

What is the structure and function of Corynbacterium diphtheriae’s exotoxin?

C. diphtheriae’s exotoxin is an A-B subunit exotoxin that inhibits protein synthesis:

1. The B subunit is the receptor-binding region of the exotoxin that mediates endocytosis into the host cell.

2. Then, the B subunit acidifies the vesicle environment to induce T-domain insertion and pore formation of the vesicle.

3. The B subunit has now formed a channel in which subunit A can be translocated from the vesicle into the host cell cytosol, where it can then actively inhibit protein synthesis.

What are the virulence factors of Mycobacterium tuberculosis?

Mycobacterium tuberculosis possesses a lipid-rich cell wall containing mycolic acid; confers acid-resistance, alkaline-resistance, antibiotic resistance, and phagocytic evasion via survival and replication within the phagocyte.

In detail, describe the “granuloma” formation and breakage in the development of tuberculosis.

Tuberculosis pathogenesis begins at alveolar macrophages in the lungs.

When a phagocyte initially engulfs M. tuberculosis, lipoglycans (lipomannan and lipoarabinomannan) inhibit phagolysosome fusion by inhibiting phagosome acidification and nitric oxide release, thereby arresting phagosomal maturation.

• This neutral pH enables M. tuberculosis to proliferate inside the immature phagocyte.

Then, mycolic acid on MTB will modify cytokine responses to induce an inflammatory response that recruits more macrophages to the infection site, where MTB will continue the spread of infection.

Eventually, the antigen will be presented to naive CD4⁺ T-cells to initiate a THC-1 adaptive, cell-mediated inflammatory response.

• Active T-cells will surround the infection site and release cytokines that activate macrophages.

• These macrophages undergo successful phagolysosome fusion, inhibiting MTB replication; however, MTB continues to persist in the activated macrophages, resulting in the failure to eliminate the infection.

• The accumulation of macrophages and T-cells will result in the formation of a granuloma with an anaerobic caseous center, resulting in an indefinitely latent infection.

Only 5-10% of the infected population will develop an active disease. Active disease arises when the granuloma breaks down and the caseous center liquefies. The caseous material is necrotic and will form cavities in the lungs, resulting in the spread of active infection.

At what stage of tuberculosis is infectious, whereby the microbe can be transmitted to other hosts?

Tuberculosis is only infectious during granuloma breakdown, whereby the liquified caseous material is necrotic and forms cavities in the lungs.

What are the virulence factors of Neisseria gonorrhoeae?

Lipooligosaccharides, pili, porin proteins, and opa proteins are crucial virulence factors responsible for the pathogenesis of Neisseria gonorrhoeae.

How does Neisseria gonorrhoeae attach, invade, and evade?

Neisseria gonorrhoeae attaches to non-ciliated epithelial cells via pili and opa proteins.

• Pili will anchor the bacterium onto the epithelium. Then, the opa protein will synergistically mediate tight surface attachment to the host to promote endocytosis.

From there, porin proteins will mediate invasion and proliferate inside the cell, and cross the epithelium into subepithelial tissue via transcytosis.

The lipooligosaccharide (LOS) layer of N. gonorrhoeae will then activate toll-like receptor 4 to initiate an inflammatory response that attracts neutrophils to the infection site.

• The neutrophils will then engulf the bacteria; however, N. gonorrhoeae will persist due to por proteins inhibiting phagolysosome fusion, which arrests phagosome maturation.

What are the virulence factors of Enterobacteriaceae?

Endotoxins, capsules, siderophores, type III secretion system (TTSS), and antimicrobial resistance.

What are the various types of pathogenic E. coli, both intestinal and extra-intenstinal?

Intestinal E. coli

• Enteropathogenic E. coli

• Enterohemorrhagic E. coli

• Enterotoxigenic E. coli

• Enteroinvasive E. coli

• Enteroaggregative E. coli

Extra-intestinal E. coli

• Neonatal meningitis

• Urinary Tract Infections

What are the detailed steps by which enteropathogenic E. coli forms attaching and effacing lesions in the host?

1. Enteropathogenic E. coli will facilitate initial contact and adherence via bundle-forming pili.

2. Once the attachment is made, EPEC will cause rapid effacement (destruction) of intestinal microvilli at the attachment site.

3. Then, EPEC’s type III secretion system will inject a translocated intimin receptor (Tir) into the host cell membrane at the locus of microvilli effacement.

4. Tir is tyrosine-phosphorylated in the membrane and becomes the receptor for intimin. The tir-intimin complex mediates the tight adherence of the bacterium to the host cell membrane.

5. Actin polymerizes locally underneath the attachment site to form an actin-rich pedestal.

What are the virulence factors of Yersinia pestis?

Type III secretion system and antiphagocytic protein capsule

What are the virulence factors of Shigella?

Type III secretion system (TTSS), Shiga toxin

What are the differences between typhoidal and non-typhoidal Salmonella?

Typhoidal Salmonella is a bacterium that causes typhoid and paratyphoid. Humans are the only transmission reservoir of this variant and only requires a low infectious dose for infection.

Conversely, non-typhoidal Salmonella is a bacterium that causes Salmonellosis. This is a zoonotic pathogen with diverse transmission reservoirs. Unlike typhoidal Salmonella, non-typhoidal Salmonella requires a high infectious dose for infection.

Both Salmonella variants are primarily foodborne and can be shed for a long period of time after symptoms resolve.

What are the virulence factors of Vibrio cholerae?

Toxin-coregulated pilus and cholera toxin

Describe the epidemiology of cholera.

Cholera is a water-borne pathogen that commonly colonizes marine organisms, i.e., algae and plankton.

Cholera transmission is associated with contaminated food and water supplies, and is a pervasive issue in countries with poor sanitation.

John Snow mapped out the source of London’s cholera outbreak by identifying clusters of infection cases proximal to a community water pump. Upon removing the water pump, cholera cases rapidly declined.

What is the disease caused by Vibrio parahaemolyticus, and how does one acquire the infection?

Gastroenteritis; commonly associated with seafood consumption or contaminated seawater

Define “capnophilic” and “microaerophilic”.

Capnophiles are microorganisms that thrive in the presence of high CO₂ concentrations well above atmospheric levels.

Microaerophiles are microorganisms that thrive in the presence of reduced oxygen concentrations well below atmospheric levels.

Name two bacterial pathogens that are both capnophilic and microaerophilic. Which one of the two pathogens can survive well in the stomach, and why?

1. Campylobacter and helicobacter

2. Helicobacter. This is because H. pylori produces urease, which neutralizes stomach acid.