Microbiology Study Guide: Pathogenic Microorganisms and Viruses

Chapter 20 – Gram Negative Cocci & Bacilli

Overview of Gram Negative Bacteria

Gram negative cocci and bacilli are a diverse group of bacteria responsible for many significant human diseases. Their cell wall structure, characterized by a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS), contributes to their pathogenicity and resistance to certain antibiotics.

• Gram Negative Cocci: Notable genera include Neisseria (e.g., N. meningitidis, N. gonorrhoeae).

• Gram Negative Bacilli: Includes Escherichia, Salmonella, Shigella, Pseudomonas, Haemophilus, and others.

Key Diseases and Pathogens

• Neisseria gonorrhoeae: Causes gonorrhea, a sexually transmitted infection. Symptoms include urethritis, cervicitis, and pelvic inflammatory disease.

• Neisseria meningitidis: Responsible for meningococcal meningitis and septicemia. Transmission is via respiratory droplets; outbreaks often occur in close-contact settings.

• Enterobacteriaceae Family: Includes E. coli, Salmonella, Shigella, Klebsiella, and Proteus. These bacteria are common causes of urinary tract infections, gastroenteritis, and hospital-acquired infections.

• Pseudomonas aeruginosa: Noted for its resistance to antibiotics and association with hospital-acquired infections, especially in immunocompromised patients.

Pathogenic Mechanisms

• Endotoxin (LPS): Present in the outer membrane, can trigger severe immune responses, including septic shock.

• Capsules: Many gram negative bacteria possess capsules that inhibit phagocytosis.

• Pili and Fimbriae: Facilitate attachment to host tissues.

Clinical Features and Epidemiology

• Common Infections: Urinary tract infections, pneumonia, meningitis, gastroenteritis.

• Transmission: Varies by species; can include direct contact, ingestion of contaminated food/water, or respiratory droplets.

• Antibiotic Resistance: Increasingly common, especially among hospital-acquired strains.

Table: Major Gram Negative Pathogens and Associated Diseases

Pathogen	Major Disease(s)	Transmission
Neisseria gonorrhoeae	Gonorrhea	Sexual contact
Neisseria meningitidis	Meningitis, septicemia	Respiratory droplets
Escherichia coli	UTI, gastroenteritis	Fecal-oral
Pseudomonas aeruginosa	Pneumonia, wound infections	Environmental, hospital
Salmonella	Typhoid fever, gastroenteritis	Contaminated food/water

Additional info: The study guide emphasizes matching organisms to diseases, understanding major effects, routes of infection, and disease vectors.

Chapter 21 – Intracellular Bacteria

Characteristics of Intracellular Bacteria

Intracellular bacteria are pathogens that survive and replicate within host cells, evading many immune defenses. Notable examples include Rickettsia, Chlamydia, and Bartonella.

• Obligate Intracellular Pathogens: Require living host cells for replication (e.g., Rickettsia, Chlamydia).

• Facultative Intracellular Pathogens: Can survive both inside and outside host cells (e.g., Mycobacterium tuberculosis).

Major Diseases

• Rickettsial Diseases: Include Rocky Mountain spotted fever, typhus. Transmitted by arthropod vectors (ticks, lice).

• Chlamydia trachomatis: Causes chlamydia, the most common bacterial STI, and trachoma (eye infection).

• Bartonella henselae: Cat scratch disease; transmitted by cats.

Pathogenesis and Diagnosis

• Cell Tropism: Many intracellular bacteria target specific cell types (e.g., endothelial cells for Rickettsia).

• Diagnosis: Often requires serology or PCR due to difficulty in culturing these organisms.

Chapter 22 – Fungal Pathogens

Overview of Fungal Pathogens

Fungi are eukaryotic organisms that can cause a range of diseases, especially in immunocompromised individuals. Major pathogenic fungi include dermatophytes, Candida, Aspergillus, and Cryptococcus.

• Dermatophytes: Cause superficial infections of skin, hair, and nails (e.g., ringworm, athlete's foot).

• Candida albicans: Responsible for candidiasis, including oral thrush and vaginal yeast infections.

• Aspergillus spp.: Can cause allergic reactions and invasive aspergillosis, especially in immunocompromised hosts.

• Cryptococcus neoformans: Causes cryptococcal meningitis, primarily in HIV/AIDS patients.

Pathogenic Mechanisms

• Dimorphism: Some fungi can switch between yeast and mold forms depending on environmental conditions.

• Spore Formation: Fungi reproduce via spores, which can be inhaled and cause infection.

Table: Major Fungal Pathogens and Associated Diseases

Fungus	Disease	Transmission
Candida albicans	Candidiasis	Endogenous, contact
Aspergillus fumigatus	Aspergillosis	Inhalation of spores
Cryptococcus neoformans	Meningitis	Inhalation of spores
Dermatophytes	Ringworm, athlete's foot	Contact, fomites

Chapter 23 – Protozoans

Overview of Protozoan Pathogens

Protozoa are unicellular eukaryotes responsible for several major human diseases, often transmitted by vectors or contaminated water.

• Major Protozoan Diseases:

  • Malaria: Caused by Plasmodium species, transmitted by Anopheles mosquitoes.

  • Amebiasis: Caused by Entamoeba histolytica, transmitted via contaminated water.

  • Giardiasis: Caused by Giardia lamblia, waterborne.

  • Trypanosomiasis: Includes African sleeping sickness (Trypanosoma brucei) and Chagas disease (Trypanosoma cruzi).

• Transmission: Often via vectors (mosquitoes, tsetse flies) or contaminated water/food.

• Life Cycles: Many protozoa have complex life cycles involving multiple hosts.

Chapter 24 – DNA Viruses

Overview of DNA Viruses

DNA viruses are a group of viruses that use DNA as their genetic material. They are responsible for a variety of human diseases, ranging from mild to severe.

• Herpesviridae: Includes Herpes simplex virus (HSV), Varicella-zoster virus (VZV), and Cytomegalovirus (CMV).

• Poxviridae: Includes Variola virus (smallpox), Molluscum contagiosum.

• Hepadnaviridae: Includes Hepatitis B virus (HBV).

• Papillomaviridae: Includes Human papillomavirus (HPV), associated with warts and cervical cancer.

Key Features

• Replication: DNA viruses typically replicate in the host cell nucleus.

• Latency: Some DNA viruses (e.g., HSV, VZV) can establish latent infections and reactivate later.

Table: Major DNA Viruses and Associated Diseases

Virus	Disease	Transmission
Herpes simplex virus	Oral/genital herpes	Contact
Varicella-zoster virus	Chickenpox, shingles	Respiratory droplets, contact
Hepatitis B virus	Hepatitis B	Blood, sexual contact
Human papillomavirus	Warts, cervical cancer	Contact, sexual

Chapter 25 – RNA Viruses

Overview of RNA Viruses

RNA viruses use RNA as their genetic material and are responsible for many acute and chronic diseases. They often mutate rapidly, leading to challenges in vaccine development and treatment.

• Picornaviridae: Includes Poliovirus, Rhinovirus (common cold).

• Orthomyxoviridae: Includes Influenza virus.

• Flaviviridae: Includes Dengue virus, Yellow fever virus.

• Paramyxoviridae: Includes Measles, Mumps, Rubella.

• Filoviridae: Includes Ebola, Marburg viruses.

Key Diseases and Features

• Poliovirus: Causes poliomyelitis; transmitted via fecal-oral route. Vaccines include inactivated (IPV) and oral (OPV) forms.

• Influenza: Causes seasonal flu; characterized by antigenic drift and shift, leading to epidemics and pandemics.

• Dengue Fever: Transmitted by Aedes mosquitoes; symptoms include high fever, rash, and hemorrhagic manifestations.

• Measles, Mumps, Rubella: Vaccine-preventable diseases; transmitted via respiratory droplets.

• Ebola, Marburg: Cause severe hemorrhagic fevers; outbreaks often occur in Africa.

Table: Major RNA Viruses and Associated Diseases

Virus	Disease	Transmission
Poliovirus	Poliomyelitis	Fecal-oral
Influenza virus	Influenza (flu)	Respiratory droplets
Dengue virus	Dengue fever	Mosquito vector
Measles virus	Measles	Respiratory droplets
Ebola virus	Ebola hemorrhagic fever	Contact with bodily fluids

Summary

• Be able to match organisms to diseases and understand their major effects, routes of infection, and vectors.

• Recognize the importance of antibiotic resistance, especially among gram negative bacteria.

• Understand the unique features of intracellular pathogens, fungi, protozoa, and viruses.

• Familiarize yourself with major vaccine-preventable diseases and differences between vaccine types.