3.28 Klebsiella Pneumoniae 

Microbiology review for the USMLE Step 1 Exam

• Klebsiella pneumoniae is a lactose-fermenting, gram-negative bacilli that causes pneumonia and is part of the normal GI flora in humans.
• It causes infection when introduced to the respiratory tract and is commonly found in hospitals, making it a common cause of nosocomial pneumonia.
• Risk factors for klebsiella pneumonia include alcoholism and diabetes.
• Symptoms of klebsiella pneumonia include cough, fever, pleuritic chest pain, and shortness of breath, as well as rust-colored "currant jelly" sputum and cavitary lesions in the lungs.
• Klebsiella can also cause urinary tract infections, especially in hospital settings.
• Treatment typically involves antibiotics, but resistance testing may be necessary due to the bug's high antibiotic resistance

3.27 Chlamydia Species

Microbiology review for the USMLE Step 1 exam

• Chlamydia is a gram-negative bacteria that is an obligate intracellular organism.
• It has a two-part life cycle involving the reticular body (active form) and elementary body (inactive form).
• There are three main species of chlamydia that cause human infections: chlamydia trachomatis, chlamydia pneumoniae, and chlamydia psittaci.
• Chlamydia trachomatis is the most common sexually transmitted infection in the US and causes genital tract infections, lymphogranuloma venereum, pelvic inflammatory disease, and reactive arthritis.
• Treatment for chlamydia trachomatis includes azithromycin or doxycycline, and ceftriaxone is often added to cover Neisseria gonorrhoeae.
• Chlamydia trachomatis can also infect neonates during delivery, causing conjunctivitis and pneumonia.
• Chlamydia pneumoniae causes community-acquired pneumonia spread through respiratory droplets.
• Chlamydia psittaci is associated with pet birds and also causes pneumonia.

3.26 Pseudomonas Aeruginosa

Microbiology review for the USMLE Step 1 Exam

• Pseudomonas aeruginosa is an encapsulated gram-negative oxidase positive rod that causes a variety of diseases.
• It can be found in soil, hospitals, and water, and it particularly likes water.
• P. aeruginosa can enter the body through contact with water, inhalation of respiratory droplets, or by medical equipment.
• The symptoms of infections caused by P. aeruginosa are caused by the toxins it secretes, including phospholipase C, exotoxin A, and endotoxin.
• P. aeruginosa causes community-acquired infections such as hot tub folliculitis, otitis externa, lung infections in cystic fibrosis patients, osteomyelitis, and skin infections.
• It causes nosocomial infections such as ventilator-associated pneumonia and catheter-associated urinary tract infections.
• P. aeruginosa is associated with skin infections in burn victims

 3.25 Proteus Species

Microbiology review for the USMLE Step 1 exam

• Proteus species are gram-negative rods that cause hospital acquired urinary tract infections
• They have swarming motility and can move quickly in a coordinated fashion across surfaces due to their numerous flagella
• They can self-elongate and secrete a sticky polysaccharide when in contact with solid surfaces, allowing for attachment and easy motility
• Proteus mirabilis and Proteus vulgaris are the two main species that cause infections in humans
• Proteus mirabilis tends to cause hospital acquired UTIs and can cling to medical devices such as catheters, where it can infect the epithelium of the urinary tract and ascend up to the kidneys
• Proteus mirabilis expresses urease, converting urea to ammonia, which can cause the formation of struvite kidney stones
• Proteus vulgaris is less common than Proteus mirabilis and tends to be more difficult to treat due to drug-resistance plasmids
• Treatment for Proteus mirabilis infection depends on the type of infection, and for uncomplicated UTIs, patients can be given bactrim, while fluoroquinolones are given for pyelonephritis.

3.24 Salmonella Species

Microbiology review for the USMLE Step 1 Exam

• Salmonella are gram-negative bacteria that cause gastrointestinal disease.
• They have flagella that enable them to move around and infect cells, and can live inside macrophages.
• There are two categories of salmonella species: typhoidal and non-typhoidal.
• Typhoidal salmonella, such as salmonella typhi, cause typhoid fever and are usually acquired through contaminated water.
• Non-typhoidal salmonella, such as salmonella enteritidis, are usually acquired through contaminated poultry or reptiles.
• Typhoidal salmonella cause high fever and a pink spotted rash, while non-typhoidal salmonella cause diarrhea, nausea, and vomiting.
• Both types of salmonella require large quantities of bacteria to make you sick, and supportive care is usually all that is required.
• Fluoroquinolones like ciprofloxacin are the go-to antibiotics for typhoidal salmonella.

3.23 Yersinia Species

Microbiology review for the USMLE Step 1 Exam

• Yersinia are gram-negative, non-motile bacteria that cause zoonotic diseases.
• Yersinia produce urease enzyme to survive in the stomach and are facultative intracellular bugs.
• Three species of yersinia causing disease in humans are Yersinia pestis, Yersinia enterocolitica, and Yersinia pseudotuberculosis.
• The Yersinia species inhibit phagocytosis, the capsule contains F1 and V proteins and the injectosome injects Yersinia outer proteins.
• Yersinia pestis spreads through fleas, causing bubonic, pneumonic, or septicemic plague with symptoms like fever, chills, and swollen lymph nodes.
• Yersinia enterocolitica is spread through contaminated food or dog feces, causing enteric infection, and symptoms like diarrhea, abdominal pain, and vomiting.
• Yersinia pseudotuberculosis causes enteric disease, similar to Y. enterocolitica, and granulomatous lesions that look like tuberculosis.
• Yersinia pestis is rare with 500 cases reported since 1970, compared to Y. enterocolitica, which causes 100,000 cases each year

3.22 Escherichia Coli (E. Coli)

Microbiology review for the USMLE Step 1 Exam

• Esherichia coli (E. coli) is a lactose-fermenting, gram-negative rod causing various diseases, ranging from mild gastroenteritis to septic shock.

• E. coli diseases can be divided into two categories: intestinal infections and extraintestinal infections (outside the GI system).

• E. coli is part of the normal flora in the human GI tract, but some strains are pathogenic.

• Four pathogenic strains of E. coli are Entertoxigenic E. coli (ETEC), Enterohemorrhagic E. coli (EHEC), Enteroinvasive E. coli (EIEC), and Enteropathogenic E. coli (EPEC).

• ETEC causes watery diarrhea and is commonly found in food and water in areas with inadequate sanitation and is the most important cause of travelers' diarrhea.

• EHEC, also called Shiga toxin-producing E. coli (STEC), causes bloody diarrhea and is associated with contaminated food (spinach, sprouts, lettuce, fruit, undercooked beef).

• Shiga toxin produced by EHEC causes cell death in the GI tract and can lead to hemolytic uremic syndrome (HUS) in about 10% of cases.

• EIEC causes dysentery-like symptoms such as abdominal pain, fever, and bloody diarrhea.

• EPEC causes diarrhea, malnutrition, and growth retardation, especially in children.

• Extraintestinal infections include urinary tract infections (UTIs), neonatal sepsis, and pneumonia (incomplete list)

2.22 Tuberculosis

Pulmonary system review for the USMLE Step 1 Exam

• Tuberculosis is caused by Mycobacterium tuberculosis bacteria and mainly affects the lungs. It is more common in developing countries, with India, Indonesia, China, Nigeria, Pakistan, and South Africa having the most TB deaths worldwide.

• TB is spread through inhaling infected aerosolized droplets, making crowded places hotspots for TB spread.

• Primary TB infection occurs when a person inhales infected droplets and the bacteria become dormant in a granuloma. Secondary TB infection occurs when the bacteria disseminate to surrounding tissues, affecting various parts of the body.

• Symptoms of active TB include night sweats, weight loss, fever, and cough (sometimes with blood). TB can present with various symptoms, making it known as the "great imitator."

• Diagnosis methods include chest X-rays, sputum acid-fast staining, and real-time nucleic acid amplification. Chest X-rays showing cavitary lesions and positive acid-fast staining are indicative of TB.

• Treatment involves a 4-drug combination of rifampin, isoniazid, pyrazinamide, and ethambutol (RIPE) for an extended period of time (2-4 months) to get rid of the infection.

2.21 Pneumonia: Lobar, Broncho, and Interstitial

Pulmonary system review for the USMLE Step 1 Exam

• Pneumonia is an infection of the lung parenchyma that deals with gas exchange
• Three classic patterns of pneumonia: lobar, bronchopneumonia, and interstitial pneumonia
• Lobar pneumonia involves a single lobe of the lung and has four stages: congestion, red hepatization, gray hepatization, and resolution
• Most commonly associated with strep pneumoniae, also caused by Legionella and Klebsiella
• Bronchopneumonia is an infection of the bronchi that leads to diffuse infection of adjacent alveoli
• Causative organisms include strep pneumoniae, klebsiella, H influenza, and staph aureus
• Interstitial pneumonia is an ongoing infection in the interstitial areas of alveolar walls
• Causative organisms tend to be different compared to other types of pneumonia and include Mycoplasma pneumoniae, Legionella, chlamydophilia bugs, Coxiella burnetti, and some viruses (influenza and RSV)
• On chest X-ray, lobar pneumonia shows haziness in a single lobe with sharp demarcation, while bronchopneumonia and interstitial pneumonia show more diffuse haziness throughout the lungs
• Interstitial pneumonia tends to be more indolent, and patients may have not so good looking lungs on chest X-ray but are up and walking around (walking pneumonia).

2.20 Pneumothorax

Pulmonary system review for the USMLE Step 1 Exam

• Pneumothorax is a collection of air outside the lung in the pleural cavity, caused by a hole in the pleurae that allows air to infiltrate and disturb normal respiratory function.
• There are three categories of causes: primary spontaneous, secondary spontaneous, and trauma.
• Trauma, such as blunt force or penetrating wounds, is the most common cause, as well as iatrogenic causes like transthoracic needle aspiration and central venous catheterization.
• Spontaneous pneumothorax occurs without any trauma and can be classified as primary (no underlying lung disease) or secondary (with underlying lung disease).
• Smoking greatly increases the risk for spontaneous pneumothorax, especially in tall men with Marfan's syndrome or Ehlers-Danlos syndrome.
• Tension pneumothorax is a more emergent and dangerous type of pneumothorax, where air enters the pleural cavity but cannot exit, causing pressure to build up and compress nearby structures like the lungs, blood vessels, and heart.