Microbial Diseases, Vaccines, and Viral Pathogenesis: Study Guide

Vaccines: Types and Examples

Overview of Vaccine Types

Vaccines are biological preparations that provide acquired immunity to specific infectious diseases. They can be classified based on their composition and method of production.

• Subunit Vaccines: Contain purified components of a pathogen (such as proteins or polysaccharides) rather than the whole organism. Example: Hepatitis B vaccine (contains recombinant surface antigen).

• Toxoid Vaccines: Contain inactivated bacterial toxins (toxoids) that stimulate immunity without causing disease. Example: Tetanus and diphtheria vaccines.

• Inactivated (Killed) Vaccines: Contain pathogens that have been killed by heat or chemicals. Example: Inactivated polio vaccine (IPV).

• Live Attenuated Vaccines: Contain live pathogens that have been weakened so they do not cause disease in healthy individuals. Example: Measles, mumps, and rubella (MMR) vaccine.

• Conjugate Vaccines: Link antigens or toxoids to polysaccharides to enhance immune response. Example: Haemophilus influenzae type b (Hib) vaccine.

• mRNA Vaccines: Contain messenger RNA encoding a pathogen antigen. Example: COVID-19 vaccines (Pfizer-BioNTech, Moderna).

Additional info: Subunit and conjugate vaccines are often used when whole-cell vaccines are too reactogenic or when targeting specific components of a pathogen.

Major Microbial Diseases and Organisms

Overview

This section summarizes key pathogens, their modes of transmission, and associated signs and symptoms. Understanding these features is essential for diagnosis, treatment, and prevention of infectious diseases.

Candida albicans

• Type: Fungal (yeast)

• Transmission: Endogenous (normal flora overgrowth), direct contact

• Signs/Symptoms: Oral thrush, vaginal yeast infections, systemic candidiasis in immunocompromised patients

Vibrio cholerae

• Type: Gram-negative bacterium

• Transmission: Fecal-oral route, contaminated water/food

• Signs/Symptoms: Profuse watery diarrhea ("rice-water stools"), dehydration, electrolyte imbalance

Neisseria gonorrhoeae

• Type: Gram-negative diplococcus

• Transmission: Sexual contact, perinatal transmission

• Signs/Symptoms: Urethritis, cervicitis, pelvic inflammatory disease, neonatal conjunctivitis

Coccidioides immitis

• Type: Dimorphic fungus

• Transmission: Inhalation of arthroconidia from soil (airborne)

• Signs/Symptoms: Valley fever (coccidioidomycosis): fever, cough, chest pain, sometimes disseminated disease

Escherichia coli (E. coli)

• Type: Gram-negative rod

• Transmission: Fecal-oral route, contaminated food/water, person-to-person

• Signs/Symptoms: Diarrhea (watery or bloody), urinary tract infections, neonatal meningitis

Salmonella, Shigella, and Campylobacter

• Type: Gram-negative rods

• Transmission: Fecal-oral route, contaminated food (especially poultry, eggs), water

• Signs/Symptoms: Gastroenteritis: diarrhea, abdominal cramps, fever; Shigella may cause dysentery (bloody diarrhea)

Helicobacter pylori

• Type: Gram-negative spiral bacterium

• Transmission: Oral-oral or fecal-oral route

• Signs/Symptoms: Gastritis, peptic ulcers, increased risk of gastric cancer

Yersinia pestis

• Type: Gram-negative rod

• Transmission: Flea bites (vectorborne), respiratory droplets (pneumonic plague)

• Signs/Symptoms: Bubonic plague (swollen lymph nodes), septicemic and pneumonic forms

Haemophilus (e.g., Haemophilus influenzae)

• Type: Gram-negative coccobacillus

• Transmission: Respiratory droplets

• Signs/Symptoms: Meningitis, epiglottitis, pneumonia, otitis media

Trichomonas vaginalis

• Type: Protozoan parasite

• Transmission: Sexual contact

• Signs/Symptoms: Vaginitis: itching, discharge, irritation

Human Herpesviruses (HHV 1-4)

• HHV-1 (Herpes Simplex Virus 1): Oral herpes (cold sores), transmitted by oral contact

• HHV-2 (Herpes Simplex Virus 2): Genital herpes, transmitted sexually

• HHV-3 (Varicella-Zoster Virus): Chickenpox and shingles, transmitted by respiratory droplets or direct contact

• HHV-4 (Epstein-Barr Virus): Infectious mononucleosis, transmitted by saliva

Coronavirus

• Type: Enveloped RNA virus

• Transmission: Respiratory droplets, close contact

• Signs/Symptoms: Range from mild respiratory illness to severe pneumonia (e.g., COVID-19)

Rabies Virus

• Type: Enveloped, negative-sense RNA virus (Rhabdoviridae)

• Transmission: Bite from infected animals (zoonotic)

• Signs/Symptoms: Neurological symptoms, hydrophobia, fatal if untreated after symptom onset

Poliovirus

• Type: Non-enveloped, positive-sense RNA virus (Picornaviridae)

• Transmission: Fecal-oral route

• Signs/Symptoms: Asymptomatic or mild illness; can cause paralytic poliomyelitis

Influenza A and B

• Type: Enveloped, segmented negative-sense RNA viruses (Orthomyxoviridae)

• Transmission: Respiratory droplets

• Signs/Symptoms: Fever, cough, sore throat, myalgia, can cause severe pneumonia

Antigenic Drift vs. Antigenic Shift (Influenza)

Mechanisms of Influenza Virus Variation

• Antigenic Drift: Gradual accumulation of point mutations in the viral genome, leading to minor changes in surface antigens (hemagglutinin and neuraminidase). Responsible for seasonal influenza epidemics.

• Antigenic Shift: Abrupt, major change in influenza A virus resulting from reassortment of gene segments between different strains. Can lead to pandemics due to the emergence of novel subtypes.

Feature	Antigenic Drift	Antigenic Shift
Mechanism	Point mutations	Genetic reassortment
Frequency	Continuous, gradual	Occasional, sudden
Impact	Seasonal epidemics	Pandemics
Viruses affected	Influenza A and B	Influenza A only

Additional info: Antigenic shift is possible only in influenza A because it infects multiple species, allowing for reassortment between human and animal strains.