3.08 Bacillus Species

Microbiology review for the USMLE Step 1 exam

• Bacillus species are aerobic, sporulating, gram + rod-shaped bacteria found in nature such as soil, water, and air
• Two main species to discuss: Bacillus cereus and Bacillus anthracis
• Bacillus cereus is associated with foodborne infections and specifically with reheated rice, as spores can survive cooking and continue to grow in refrigerated rice, leading to enteritis symptoms
• Bacillus anthracis causes anthrax, most commonly contracted through contact with infected animals, particularly grazing animals such as sheep, goats, and cattle.
• There are three forms of anthrax: cutaneous anthrax, inhalation anthrax, and gastrointestinal anthrax
• Cutaneous anthrax is the most common form and results from inoculation of spores through a cut or opening in the skin
• Inhalation anthrax occurs when spores are inhaled and leads to systemic illness, mediastinal widening, septic shock, and death
• Gastrointestinal anthrax results from ingesting spores in contaminated food and leads to severe abdominal symptoms.

3.07 Listeria Monocytogenes

Microbiology review for the USMLE Step 1 exam

• Listeria monocytogenes is an intracellular, aerobic, gram-positive rod that can cause foodborne illness
• It can live inside various types of cells, including epithelial cells, macrophages, and monocytes, and spreads from cell to cell through actin polymerization
• It is most commonly found in refrigerated deli meats and unpasteurized dairy products, and can cause gastroenteritis, meningitis, and premature labor/spontaneous abortion in immunocompromised individuals, pregnant women, and older or young individuals
• It is usually treated with ampicillin, but bactrim (trimethoprim and sulfamethoxazole) can be used in cases of ampicillin allergy

3.06 Staphylococcus Species

Microbiology review for the USMLE Step 1 Exam

• Staphylococcus bacteria are gram-positive cocci that form clusters in the shape of grapes on a vine
• Three pathogenic species of staph: staph aureus, staph epidermidis, and staph saprophyticus
• Differentiation between species can be done using coagulase and novobiocin sensitivity tests
• Staph aureus is a major pathogen that causes many severe infections including toxic shock syndrome, septic arthritis, acute endocarditis, gastroenteritis, pneumonia, and staphylococcal scalded skin syndrome
• Toxic shock syndrome can be caused by tampons, soft tissue infections, post-surgical infections, and retained foreign bodies
• Staphylococcal scalded skin syndrome, or Ritter disease, is characterized by exfoliative damage to the skin and mainly affects newborns within 48 hours of birth
• Staph aureus can spread hematogenously and affect many parts of the body, particularly the heart and joints.
• Gastroenteritis caused by staph aureus is caused by consumption of exotoxins produced by the bacteria.

3.05 Streptococcus Species

Microbiology review for the USMLE Step 1 Exam.

• Streptococcus bacteria are gram positive cocci that form linear chains
• Hemolysis refers to the ability of bacteria to break down red blood cells and is classified as alpha (partial), beta (complete), or gamma (none)
• Alpha hemolytic streptococci include viridans streptococci and Streptococcus pneumoniae
• Viridans streptococci, particularly Streptococcus mutans, are common causes of dental cavities and endocarditis, while Streptococcus pneumoniae is a common cause of pneumonia
• Beta hemolytic streptococci include Group A streptococci (Streptococcus pyogenes) and Group B streptococci (Streptococcus agalactiae)
• Group A streptococci can cause strep throat, impetigo, and cellulitis, as well as more serious infections like scarlet fever and rheumatic fever
• Group B streptococci are commonly found in the vagina and rectum, and can cause infections in newborns and pregnant women, as well as urinary tract infections and pneumonia
• Gamma hemolytic streptococci include the Enterococcus species, which can cause urinary tract infections, endocarditis, and other infections, and can be resistant to some antibiotics
• Lancefield grouping refers to the presence of different types of carbohydrate antigens on the surface of streptococcus bacteria and is used in classification and identification of streptococcus species

3.04 Gram Negative Bacteria

Microbiology review for the USMLE Step 1 Exam.

• Gram negative bacteria are characterized by a thin cell wall surrounded by a lipid outer membrane
• They do not retain the violet gram stain and appear pink after staining
• Gram negative bacteria can be classified by shape: cocci, coccobacilli, bacilli, and spirals
• Gram negative cocci include Neisseria species, which can be further categorized as maltose fermenters or non maltose fermenters
• Gram negative coccobacilli include: Bordetella pertussis, Haemophilus influenzae, Pasteurella, Francisella tularensis, Brucella, and Acinetobacter
• Gram negative bacilli can be divided into lactose fermenting and non lactose fermenting categories
• Lactose fermenting gram negative bacilli include: Escherichia coli, Klebsiella, Enterobacter, Citrobacter, and Serratia
• Non lactose fermenting gram negative bacilli can be further divided into oxidase positive and oxidase negative
• Oxidase positive non lactose fermenting gram negative bacilli include Pseudomonas
• Oxidase negative non lactose fermenting gram negative bacilli include Shigella, Yersinia, and Salmonella
• Gram negative spirals can be divided into oxidase positive and oxidase negative categories
• Oxidase positive gram negative spirals include Vibrio species, Helicobacter pylori, and Campylobacter jejuni
• Oxidase negative gram negative spirals include Borrelia burgdorferi (cause of lyme disease) and Treponema pallidum (cause of syphilis)

3.03 Gram Positive Bacteria

Microbiology review for the USMLE Step 1 Exam.

• Gram positive bacteria are characterized by their ability to retain the violet stain during the gram stain procedure, due to their thick peptidoglycan cell walls
• Gram positive bacteria can be cocci (spherical) or bacilli (rod-shaped)
• Cocci can be grouped in clusters, pairs, or chains
• Gram positive cocci in pairs or chains include streptococcus (pairs or chains), enterococcus (pairs), and staphylococcus (clusters)
• Hemolysis describes the ability of a bacteria species to break down red blood cells and is categorized as alpha (partial), beta (complete), or gamma (none)
• Coagulase activity refers to the presence of an enzyme that causes blood to coagulate and is present in some staphylococcus species
• Gram positive bacilli can be aerobic (listeria, bacillus, corynebacterium) or anaerobic (clostridium)

3.02 Bacterial Toxins

Microbiology review for the USMLE Step 1 Exam.

• Bacterial toxins are harmful compounds produced by bacteria that cause damage to the host
• Exotoxins are toxins that are actively secreted by some species of gram positive and gram negative bacteria
• Examples of exotoxins include botulinum toxin, Corynebacterium diphtheriae toxin, and cholera toxin
• Endotoxins are toxins that are contained within the cell wall of gram negative bacteria and are released when the bacteria are lysed or fragmented
• Structurally, endotoxins consist of the O antigen, the core oligosaccharide, and Lipid A, with the Lipid A component being responsible for most of the toxicity
• Endotoxins are less virulent and more heat stable than exotoxins, and are usually contained in the bacteria's own genetic material rather than on a plasmid

3.01 Bacteria Basics

Microbio review for the USMLE Step 1 Exam:

• Bacteria are single-celled organisms that can cause infection
• Structural components of bacteria include: capsule, cell wall, flagella, fimbriae, plasma membrane, ribosomes, endospores, and DNA
• Capsules help bacteria evade the immune system by inhibiting phagocytes
• Flagella are tail-like appendages that allow bacteria to move, fimbriae help bacteria adhere to surfaces
• Most bacteria have a cell wall, which can be either gram positive or gram negative
• Plasma membrane is composed of proteins and phospholipids and performs various functions
• DNA is the genetic material, ribosomes are used to build proteins, some bacteria have endospores which are heat-resistant resting cells
• Bacteria can be classified by cell wall composition, morphology, metabolism, and based on their ability to grow at different temperatures and pH levels
• Gram staining is a laboratory technique used to differentiate between gram positive and gram negative bacteria
• Morphology of bacteria includes cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped)
• Metabolism of bacteria can be aerobic (uses oxygen) or anaerobic (doesn't use oxygen)
• Temperature and pH ranges at which bacteria can grow vary widely, some can grow at extreme conditions
• Pathogenic bacteria cause disease, while non-pathogenic bacteria do not
• Bacteria can be treated with antibiotics, but overuse can lead to antibiotic resistance

2.12 Idiopathic Pulmonary Fibrosis

Pulmonary system review for the USMLE Step 1 exam.

2.11 Bronchiectasis

Pulmonary system review for the USMLE Step 1 exam.