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Discover the hidden world of microbiology and unlock the secrets of microscopic organisms that shape our lives in ways we never imagined.

USMLE Guide: Microbiology


Microbiology is the study of microorganisms, including bacteria, viruses, fungi, and parasites. It plays a crucial role in understanding the pathogenesis, diagnosis, and treatment of infectious diseases. This USMLE guide aims to provide essential information on microbiology topics frequently tested in the United States Medical Licensing Examination (USMLE).

Bacterial Infections

Gram-Positive Bacteria

  • Staphylococcus aureus: Commonly causes skin and soft tissue infections, pneumonia, and endocarditis. It is coagulase-positive and produces golden colonies on culture media.
  • Streptococcus pyogenes: Causes streptococcal pharyngitis, cellulitis, and necrotizing fasciitis. It is beta-hemolytic on blood agar and can cause rheumatic fever.
  • Clostridium difficile: Causes antibiotic-associated diarrhea and pseudomembranous colitis. It produces toxins A and B, leading to mucosal damage.

Gram-Negative Bacteria

  • Escherichia coli: Causes urinary tract infections, sepsis, and diarrhea. Enterohemorrhagic strains can cause hemolytic uremic syndrome.
  • Neisseria gonorrhoeae: Responsible for gonorrhea, a sexually transmitted infection. It can cause pelvic inflammatory disease and disseminated gonococcal infection.
  • Pseudomonas aeruginosa: Commonly associated with nosocomial infections, particularly in immunocompromised individuals. It produces a characteristic blue-green pigment.

Viral Infections

DNA Viruses

  • Herpes Simplex Virus (HSV): Causes oral and genital herpes. HSV-1 primarily affects the oral cavity, while HSV-2 primarily affects the genital area.
  • Varicella-Zoster Virus (VZV): Causes chickenpox (varicella) and shingles (herpes zoster). Reactivation of VZV leads to the development of shingles.
  • Human Papillomavirus (HPV): Most common sexually transmitted infection. Certain strains are associated with cervical cancer and genital warts.

RNA Viruses

  • Influenza Virus: Causes seasonal influenza with symptoms of fever, cough, and body aches. Influenza A and B viruses undergo frequent antigenic changes, leading to new strains each year.
  • Hepatitis C Virus (HCV): Chronic infection can lead to liver cirrhosis and hepatocellular carcinoma. It is primarily transmitted through blood transfusions and intravenous drug use.
  • Human Immunodeficiency Virus (HIV): The causative agent of acquired immunodeficiency syndrome (AIDS). HIV attacks CD4+ T cells, leading to a weakened immune system.

Fungal Infections

Superficial Fungal Infections

  • Tinea Corporis: Ringworm infection of the body, characterized by a circular rash with a raised border.
  • Tinea Pedis: Athlete's foot, affecting the feet and causing itching, scaling, and redness.
  • Candidiasis: opportunistic infection caused by Candida species, commonly affecting the oral cavity (thrush) and genital area.

Systemic Fungal Infections

  • Histoplasma capsulatum: Causes histoplasmosis, primarily affecting the lungs. Inhalation of bird or bat droppings can lead to infection.
  • Cryptococcus neoformans: Causes cryptococcosis, often presenting as meningitis in immunocompromised individuals. It is associated with bird droppings.

Parasitic Infections


  • Plasmodium species: Responsible for malaria, transmitted through the bite of infected female Anopheles mosquitoes. Symptoms include fever, chills, and anemia.
  • Entamoeba histolytica: Causes amoebic dysentery, characterized by bloody diarrhea. It can lead to liver abscesses if the parasite spreads.


  • Ascaris lumbricoides: Causes ascariasis, the most common helminthic infection worldwide. It can lead to intestinal obstruction and malnutrition.
  • Taenia solium: Causes cysticercosis, a parasitic infection resulting from ingesting undercooked pork. It can lead to neurocysticercosis, affecting the central nervous system.


Understanding microbiology is vital for medical professionals to diagnose and treat infectious diseases effectively. This USMLE guide provides a concise overview of key microbiology topics, including bacterial infections, viral infections, fungal infections, and parasitic infections. By mastering these concepts, you will be well-prepared for microbiology questions on the USMLE.

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