Microbiology Study Guide: Infectious Diseases by Body System

Skin, Eyes, and Cutaneous Infections

Normal Skin Microbiota

The skin is a dry, salty, and acidic environment where only well-adapted microbes can survive. The normal microbiota of the skin includes:

• Staphylococcus epidermidis: A common, generally harmless skin bacterium.

• Staphylococcus aureus: Can be pathogenic, causing various skin infections.

• Micrococcus: Another genus of skin-dwelling bacteria.

• Corynebacterium: Part of the normal flora, but some species can be opportunistic pathogens.

Major Skin Diseases

• Staphylococcus aureus infections: Can cause impetigo, cellulitis, and scalded skin syndrome (SSSS).

• Impetigo: Superficial skin infection, often in children, causing pustules and yellow crusts.

• Cellulitis: Deeper skin infection, can spread rapidly.

• Scalded Skin Syndrome (SSSS): Exfoliative toxins cause skin peeling, mostly in infants.

Other Cutaneous Pathogens

• Measles: Causes a maculopapular rash.

• Rubella: Also causes a rash, but milder than measles.

• Varicella-Zoster Virus (VZV): Causes chickenpox and shingles.

• Herpes Simplex Virus (HSV): Can cause cold sores and eye infections.

• Ophthalmia neonatorum: Eye infection in newborns, often due to Neisseria gonorrhoeae or Chlamydia trachomatis.

• Trachoma: Chronic eye infection by Chlamydia trachomatis, leading to blindness if untreated.

Nervous System & Meningitis

Key Concepts

• Meningitis: Inflammation of the meninges, the protective membranes covering the brain and spinal cord.

• Encephalitis: Inflammation of the brain itself.

• Common Pathogens:

  • Neisseria meningitidis: Gram-negative diplococcus, causes meningococcal meningitis.

  • Streptococcus pneumoniae: Gram-positive diplococcus, causes pneumococcal meningitis.

  • Haemophilus influenzae: Gram-negative coccobacillus, causes meningitis in children.

  • Clostridium botulinum: Produces botulinum toxin, causes flaccid paralysis.

  • Clostridium tetani: Produces tetanospasmin, causes spastic paralysis.

Transmission and Symptoms

• Spread via respiratory droplets, direct contact, or contaminated food (e.g., Listeria monocytogenes).

• Symptoms: Fever, headache, stiff neck, photophobia, altered mental status.

Cardiovascular & Lymphatic System

Key Terms and Diseases

• Viremia: Presence of viruses in the blood.

• Sepsis: Bacteria growing in the bloodstream, leading to systemic inflammation.

• Acute endocarditis: Rapid infection of heart valves, often by Staphylococcus aureus.

• Subacute endocarditis: Slower infection, often by less virulent bacteria on previously damaged valves.

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

• Lyme Disease: Caused by Borrelia burgdorferi, transmitted by ticks.

• Plague: Caused by Yersinia pestis, transmitted by fleas.

• Tularemia: Caused by Francisella tularensis, transmitted by rabbits and rodents.

Respiratory System

Normal Flora and Defenses

• Includes nasal, oral, and pharyngeal microbiota.

• Alveolar macrophages provide immune defense in the lungs.

Upper vs. Lower Respiratory Tract Infections

• Upper Respiratory Tract: Includes nose, throat, sinuses, and pharynx.

• Lower Respiratory Tract: Includes trachea, bronchi, and lungs.

Major Diseases

• Pharyngitis: Sore throat, often caused by Streptococcus pyogenes (Group A Strep).

• Pneumonia: Infection of the lungs, can be caused by bacteria, viruses, or fungi.

• Influenza: Viral infection, causes fever, cough, and body aches.

• Tuberculosis: Caused by Mycobacterium tuberculosis, chronic lung infection.

Gastrointestinal System

Normal Flora and Defenses

• Includes a diverse microbiota that aids in digestion and protects against pathogens.

• Defenses: Stomach acid, peristalsis, GALT (gut-associated lymphoid tissue), and lysozyme.

Intestinal Diarrheal Diseases

Pathogen	Key Features
Vibrio cholerae	Rice-water stools, severe dehydration
E. coli O157:H7	Shiga-like toxin, no antibiotics recommended
Campylobacter jejuni	Most common U.S. bacterial diarrhea, can cause Guillain-Barré syndrome
Clostridioides difficile	Pseudomembranous colitis after antibiotics

Viral and Parasitic Diarrhea

• Rotavirus: Leading cause of severe diarrhea in children.

• Giardia: Protozoan, causes greasy, foul-smelling stools.

• Cryptosporidium: Protozoan, causes watery diarrhea, especially in immunocompromised patients.

• Entamoeba histolytica: Causes amoebic dysentery and liver abscesses.

• Taenia: Tapeworm, can cause abdominal pain and malnutrition.

Urinary & Reproductive Systems

Normal Urinary Microbiota

• Includes Lactobacillus and Staphylococcus species.

Major Urinary Tract Infections (UTIs)

• Escherichia coli: Most common cause of UTIs.

• Staphylococcus saprophyticus: Common in young women.

• Proteus mirabilis: Can cause complicated UTIs.

Sexually Transmitted Infections (STIs)

• Chlamydia trachomatis: Most common bacterial STI, can cause pelvic inflammatory disease.

• Neisseria gonorrhoeae: Causes gonorrhea, can lead to infertility if untreated.

• Treponema pallidum: Causes syphilis, characterized by distinct stages.

• Herpes Simplex Virus (HSV): Causes genital herpes, can be transmitted even when asymptomatic.

• Human Papillomavirus (HPV): Causes genital warts, some types associated with cervical cancer.

Additional info:

• Some STIs can be vertically transmitted from mother to child during childbirth.

• HPV vaccines are available and recommended for prevention of certain cancer-causing types.