Microbial Pathogenesis and Host Interaction

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Questions and Answers

What characterizes a mutualistic relationship between a host and microbes?

The microbes live independently from the host.

Both host and microbes benefit from the relationship. (correct)

No benefit or harm occurs to either organism.

Both organisms cause harm to each other.

Which factor is primarily responsible for an organism's ability to establish an infection?

Virulence

Infectivity (correct)

Pathogenicity

Evasion mechanisms

What role do exotoxins play in bacterial infections?

They form a protective capsule around bacteria.

They are toxic proteins released from living bacteria. (correct)

They stimulate phagocytosis.

They block the immune response.

Which component of innate defenses acts as a barrier to entry for bacterial infections?

Intact skin and mucosal surfaces Signup and view all the answers

Which method is NOT a bacterial evasion mechanism against host immune responses?

Stimulating an inflammatory response Signup and view all the answers

What is the primary purpose of the streptozyme test?

To detect antibodies to streptococcal products Signup and view all the answers

Which method is NOT commonly used to diagnose H.pylori infection?

Blood culture Signup and view all the answers

What is a significant characteristic of Mycoplasma pneumoniae?

It lacks a cell wall Signup and view all the answers

Which technique is considered the gold standard for diagnosing RMSF?

Serological testing for antibodies by IFA Signup and view all the answers

What is a common issue with diagnosing mycoplasma pneumoniae infections?

It is difficult to grow in culture Signup and view all the answers

What is a major disadvantage of using serology tests for bacterial infections?

They may show delayed antibody production. Signup and view all the answers

How does molecular detection of bacterial DNA or RNA primarily benefit the diagnosis process?

It is fast, typically yielding results in a few hours. Signup and view all the answers

Which clinical manifestation is specifically associated with Group A Streptococcal infections?

Scarlet fever Signup and view all the answers

Which of the following is a common site of infection for Group A Streptococcus?

Skin and upper respiratory tract Signup and view all the answers

What type of bacterial infection is characterized by beta hemolysis and caused by Streptococcus pyogenes?

Gram-positive cocci Signup and view all the answers

What symptom is commonly associated with acute rheumatic fever following a Group A Streptococcal infection?

Joint pain Signup and view all the answers

What occurs as a result of immune responses to streptococcal antigens in acute rheumatic fever?

Inflammation of heart tissue Signup and view all the answers

Which method is used to identify gram-negative rods like E. coli?

Special stains or Gram stain Signup and view all the answers

What is a common laboratory method to detect group A streptococcal infections?

Culture on sheep blood agar Signup and view all the answers

Which serological test is particularly elevated in patients with acute rheumatic fever?

Antistreptolysin O (ASO) Signup and view all the answers

What is the primary cause of transmission for Helicobacter pylori?

Fecal-oral route Signup and view all the answers

What do IgM antibodies indicate in the context of M.pneumoniae infection?

Recent infection Signup and view all the answers

Which of the following is a characteristic of Mycoplasma pneumoniae?

It is the leading cause of respiratory infections Signup and view all the answers

What is the method of choice for detecting active Helicobacter pylori infection?

Urea breath test Signup and view all the answers

What is the typical mechanism by which cold agglutinins affect red blood cells?

Causing agglutination at low temperatures Signup and view all the answers

Which bacterium is transmitted by arthropods and is the causative agent of Rocky Mountain spotted fever?

R.rickettsii Signup and view all the answers

What type of colonies does Mycoplasma pneumoniae produce on specialized media?

Fried egg appearance Signup and view all the answers

Which of the following outcomes is a potential consequence of untreated Helicobacter pylori infection?

Gastric carcinoma Signup and view all the answers

Which diagnostic assay is most useful for detecting R.rickettsii infections?

Serology by Indirect immunofluorescence Assay Signup and view all the answers

What type of relationship can exist between host and microbe?

Symbiotic, commensalistic, mutualistic, or parasitic Signup and view all the answers

How are the antibodies to Mycoplasma pneumoniae typically detected?

Antibody detection assays Signup and view all the answers

Which bacterial component is associated with causing massive cytokine production?

Endotoxin Signup and view all the answers

What laboratory method is primarily used to diagnose acute streptococcal infections?

Culture on sheep blood agar Signup and view all the answers

What do ASO and anti-DNase B assays help diagnose?

Acute rheumatic fever or glomerulonephritis Signup and view all the answers

Study Notes

Host-Microbe Relationships

Symbiotic: Host and microbes live together long term. This includes the indigenous microbiota.
Commensalistic: No benefit or harm to either organism.
Mutualistic: Both host and microbes benefit.
Parasitic: Microbes cause harm to the host.

Infectivity, Pathogenicity, and Virulence

Infectivity: The organism's ability to establish an infection.
Pathogenicity: The ability of an organism to cause disease. This may be increased by virulence factors.
Virulence: The extent of pathology caused by an organism when it infects a host.

Bacterial Virulence Factors and Pathogenicity

Endotoxin: Found in the lipid A portion of lipopolysaccharide (LPS) in gram-negative cell walls. It's a powerful stimulator of cytokine release.
Pili: Adherence to host cells; resistance to phagocytosis.
Flagella Adherence to host cells; motility.
Capsule: Blocks phagocytosis, antibody attachment, and complement.
Exotoxins: Potent toxic proteins released from living bacteria. They include neurotoxins, cytotoxins, and enterotoxins.

Immune Defenses Against Bacteria

Innate Defenses: Intact skin and mucosal surfaces (barriers to entry), antimicrobial defense peptides (e.g., lysozyme, defensins, ribonucleases), complement proteins, cytokines, acute-phase reactants.
Adaptive Defenses: Antibody production (binding of complement, opsonization, neutralization of bacterial toxins), cell-mediated immunity (CD4 T cells produce cytokines that induce inflammation. Cytotoxic T lymphocytes attack host cells that contain intracellular bacteria).

Bacterial Evasion Mechanisms

Inhibit chemotaxis.
Block adherence of phagocytes.
Resist digestion by phagocytic cells.
Block the action of complement.
Degrade IgA molecules.

Laboratory Detection of Bacterial Infections

Culture of the causative agent: Grow on broth or solid media. This is the major means of diagnosis, but it may take time or not be possible.
Microscopic examination: Gram stain or special stains.
Detection of bacterial antigens: Rapid testing with techniques like ELISA, LFA, or LA.
Molecular detection of bacterial DNA or RNA: Results can be obtained within hours using PCR.
Proteomics: Analysis of proteins produced by specific bacteria.

Microscopic Examination of Bacteria

Gram stain: Used to identify gram-positive cocci (e.g., Staphylococcus aureus) and gram-negative rods (e.g., E. coli).
Special stains like acid-fast stain are used for Mycobacterium tuberculosis.

Serology Tests for Bacterial Infections

Detect antibodies to bacterial antigens.
Used to:
- Detect and confirm infections when other laboratory methods are unavailable.
- Diagnose infections when clinical symptoms are nonspecific.
- Diagnose current infection with findings of IgM, a high IgG titer, or a fourfold rise in antibody titer between acute and convalescent samples.
- Determine past exposure to an organism (IgM-, IgG+).
- Assess reactivation or reexposure.
Disadvantages:
- A delay between the onset of infection and antibody production.
- Low antibody production by immunosuppressed patients.

Group A Streptococci (GAS)

Streptococcus pyogenes: Gram-positive cocci arranged in pairs or chains.
Transmission: Person-to-person.
Major sites of infection: Upper respiratory tract and skin.
Beta hemolytic: Produces streptolysin, an exotoxin.

Clinical Manifestations of Acute GAS Infection

Pharyngitis (“strep throat”).
Pyoderma (impetigo).
Scarlet fever.
Toxic shock syndrome.
Necrotizing fasciitis.
Treatment: antibiotics.

Group A Streptococcal Sequelae

Acute Rheumatic Fever: Develops 1 to 3 weeks after pharyngitis or tonsillitis. Features: fever, joint pain, inflammation of the heart.
Poststreptococcal glomerulonephritis: May follow GAS infection of the skin or pharynx. Damages glomeruli, producing hematuria, proteinuria, edema, hypertension, malaise, backache, abdominal discomfort, and impairment in renal function.

Laboratory Diagnosis of Acute Group A Streptococcal Infections

Culture on sheep blood agar: Small translucent colonies surrounded by a clear zone of beta hemolysis.
Rapid assays to detect group A streptococcal antigens: LFA (Lateral flow immunochromatographic assay).

Serological Detection of Group A Streptococcal Sequelae

Antistreptolysin O (ASO): Nephelometric methods are currently used to measure light scatter produced by immune complexes containing streptolysin antigen.
Anti-DNase B: Produced by patients with both rheumatic fever and impetigo. Tested by EIA and nephelometric methods.
Streptozyme test: A rapid slide agglutination test that detects antibodies to five streptococcal products: ASO, anti-hyaluronidase (AHase), anti-streptokinase (ASKase), anti-nicotinamide-adenine dinucleotide (anti-NAD), and anti-DNase B.

Helicobacter pylori

Gram-negative microaerophilic spiral bacterium.
Transmission likely by fecal-oral route.
Major cause of gastric and duodenal ulcers.
Can survive in the acid environment of the stomach because it produces urease, which provides a buffering zone around the bacteria.
Treatment: antibiotics and anti-ulcer medications.
If untreated, it can lead to gastric carcinoma or mucosa-associated lymphoid tumors (MALT).

Detection of Helicobacter pylori Infection

Detect urease in stomach biopsy (CLOtest):
Urea breath test.
H. pylori antigens.
H. pylori antibodies: ELISA is the method of choice. IgG in serum indicates an active infection. Titers decrease after successful treatment.

Mycoplasma pneumoniae

Tiny bacteria that lack a cell wall.
Leading cause of respiratory infections (fever, headache, malaise, cough).
Commonly referred to as “atypical” or “walking” pneumonia.
Can cause Raynaud syndrome.
Causes Stevens-Johnson syndrome in a minority of cases.
Spread by respiratory droplets.

Laboratory Diagnosis of M. pneumoniae Infection

Culture: Produces mulberry colonies with a “fried egg” appearance on specialized media. Is the gold standard but rarely performed in clinical laboratories because the organism is difficult to grow.
Antibodies to M. pneumoniae: Most useful diagnostic assay. IgM antibodies indicate a recent infection while IgG antibodies indicate possible reinfection.
Cold agglutinins: Present in about 50% of patients with M. pneumoniae but not specific for the infection.
Molecular methods: Film array respiratory panel.

Rickettsial Infections

Obligate intracellular gram-negative bacteria.
Transmitted by arthropods (ticks, mites, lice, or fleas) through biting after feeding on an infected animal.
Two main groups: Spotted fever group (e.g., Rocky Mountain spotted fever) and Typhus group (e.g., epidemic typhus)

Rocky Mountain Spotted Fever (RMSF)

Caused by R. rickettsii.
Transmitted by three species of ticks.
Symptoms include headache, nausea, vomiting, diarrhea, skin rash; can rapidly progress to death.
Diagnosis: Clinical presentation and serology by Indirect Immunofluorescence Assay.

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Description

Explore the complex relationships between hosts and microbes in this quiz. Discover concepts like symbiosis, infectivity, pathogenicity, and various virulence factors. Test your knowledge on how microbes can benefit or harm their hosts.