A 10-year-old girl is brought to the emergency department because of a 2-day history of bloody diarrhea and abdominal pain. Four days ago, she visited a petting zoo with her family. Her temperature is 39.4°C (102.9°F). Abdominal examination shows tenderness to palpation of the right lower quadrant. Stool cultures at 42°C grow colonies that turn black after adding phenylenediamine. Which of the following best describes the most likely causal organism?

Gram-negative, flagellated bacteria that do not ferment lactose

Gram-positive, anaerobic, rod-shaped bacteria that form spores

Gram-positive, aerobic, rod-shaped bacteria that produce catalase

Gram-negative, non-flagellated bacteria that do not ferment lactose

Gram-negative, non-flagellated bacteria that ferment lactose

A 62-year-old man is brought to the emergency department with fatigue, dry cough, and shortness of breath for 3 days. He reports a slight fever and has also had 3 episodes of watery diarrhea earlier that morning. Last week, he attended a business meeting at a hotel and notes some of his coworkers have also become sick. He has a history of hypertension and hyperlipidemia. He takes atorvastatin, hydrochlorothiazide, and lisinopril. He appears in mild distress. His temperature is 102.1°F (38.9°C), pulse is 56/min, respirations are 16/min, and blood pressure is 150/85 mm Hg. Diffuse crackles are heard in the thorax. Examination shows a soft and nontender abdomen. Laboratory studies show: Hemoglobin 13.5 g/dL Leukocyte count 15,000/mm3 Platelet count 130,000/mm3 Serum Na+ 129 mEq/L Cl- 100 mEq/L K+ 4.6 mEq/L HCO3- 22 mEq/L Urea nitrogen 14 mg/dL Creatinine 1.3 mg/dL An x-ray of the chest shows infiltrates in both lungs. Which of the following is the most appropriate next step in diagnosis?

Direct immunofluorescent antibody test

A 21-year-old woman presents with the complaints of nausea, vomiting, and diarrhea for 5 days. She adds that she has fever and abdominal cramping as well. She had recently attended a large family picnic and describes eating many varieties of cold noodle salads. Her past medical history is insignificant. Her temperature is 37.5°C (99.6°F), respiratory rate is 15/min, pulse is 67/min, and blood pressure is 92/68 mm Hg. Physical examination is non-contributory. Given the clinical information provided and most likely diagnosis, which of the following would be the next best step in the management of this patient?

Replacement of fluids and electrolytes

IV antibiotic therapy to prevent disseminated disease

Empiric therapy assuming multi-drug resistance

Short course of oral antibiotics to prevent asymptomatic carrier state

A 38-year-old man complains of a persistent high fever with chills, malaise, and diffuse abdominal pain for over a week. He recently returned from a trip to India. The fever began slowly and climbed its way up to 40.0°C (104.0°F) over the last 4 days. A physical exam reveals a white-coated tongue, enlarged spleen, and rose spots on the abdomen. A bone marrow aspirate was sent for culture which revealed motile gram-negative rods. Which of the following is true about the organism and the pathophysiology of this condition?

It survives intracellularly within phagocytes of Peyer's patches.

Incidence increases after cholecystectomy.

Splenectomy may be necessary for carriers.

It forms blue-green colonies with fruity odor.

It releases a toxin which inactivates 60S ribosomes.

A group of scientists studied the effects of cytokines on effector cells, including leukocytes. They observed that interleukin-12 (IL-12) is secreted by antigen-presenting cells (APCs) in response to bacterial lipopolysaccharide. When a CD4+ T cell is exposed to this interleukin, which of the following responses will it have?

Responds to extracellular pathogens

Waterborne bacterial pathogens for USMLE Step 1: High-Yield Microbiology Lessons

Overview - The Unseen Splash

Primarily transmitted via the fecal-oral route through contaminated water.
Manifests as a spectrum of diarrheal diseases (gastroenteritis).
Key pathogens: Vibrio cholerae, Campylobacter jejuni, Salmonella Typhi, Shigella spp., and Enterotoxigenic E. coli (ETEC).
Clinical range: from asymptomatic carriage to severe, life-threatening dehydration or systemic illness like typhoid fever.

⭐ The presence of fecal leukocytes is a key diagnostic clue to differentiate inflammatory (e.g., Shigella, Campylobacter) from non-inflammatory (Vibrio, ETEC) diarrhea.

Fecal-oral transmission routes and prevention barriers

Gram-Negative Rods (Curved) - Comma-Shaped Killers

Vibrio cholerae
- Oxidase ⊕, grows in alkaline media.
- Source: Contaminated water/seafood.
- Pathogenesis: Cholera toxin activates Gs, ↑cAMP → profuse, secretory "rice-water" diarrhea.
- Culture: Grows on Thiosulfate-Citrate-Bile Salts-Sucrose (TCBS) agar.
Campylobacter jejuni
- Oxidase ⊕, thermophilic (grows at 42°C).
- Source: Undercooked poultry, unpasteurized milk.
- Causes inflammatory bloody diarrhea.
- Major antecedent to Guillain-Barré syndrome (GBS).
Helicobacter pylori
- Oxidase ⊕, strongly Urease ⊕ (neutralizes stomach acid).
- Causes gastritis, peptic ulcers, and is a risk factor for MALT lymphoma & gastric adenocarcinoma.

⭐ The urease produced by H. pylori is a key virulence factor, creating an alkaline microenvironment that allows it to survive in the acidic stomach.

Gram stain of comma-shaped Gram-negative bacteria

Gram-Negative Rods (Straight) - Enteric Invaders

General: All are oxidase-negative, non-lactose fermenters that invade intestinal epithelium.
Salmonella
- Motile, H₂S positive, acid-labile (high inoculum needed).
- S. enteritidis: Self-limiting gastroenteritis from poultry/eggs.
- S. typhi: Typhoid fever (rose spots, fever, headache). Associated with osteomyelitis in sickle cell disease.
Shigella
- Non-motile, H₂S negative, acid-stable (low inoculum).
- Produces Shiga toxin → inhibits 60S ribosome → bloody dysentery (bacillary dysentery).
Yersinia enterocolitica
- Causes mesenteric adenitis (pseudoappendicitis), often from pork or pet feces.
- Grows in cold temperatures ("psychrophile").

⭐ High-Yield: Salmonella typhi can establish a chronic carrier state by colonizing the gallbladder, leading to persistent shedding and transmission.

Atypical Bacteria - The Air-Con Menace

Legionella pneumophila: Poorly staining Gram-negative rod.
- Transmission: Inhaled aerosols from contaminated water (AC units, hot tubs). No person-to-person spread.
- Lab Dx: Urine antigen test; culture on buffered charcoal yeast extract (BCYE) agar (requires L-cysteine, iron).
- 📌 Mnemonic: A Legionnaire with a silver helmet, an iron dagger, and a call of “Cys-teine!”
Clinical Syndromes:
- Legionnaires' Disease: Atypical pneumonia + diarrhea + confusion.
- Pontiac Fever: Self-limited, flu-like illness.

⭐ High-Yield: Suspect Legionella in a patient with pneumonia, hyponatremia (low sodium), and prominent GI symptoms (diarrhea).

Legionnaires' Disease: Transmission, Symptoms, and Risks

Diagnosis & Management - Identify & Annihilate

Primary goal: Aggressive oral/IV rehydration to correct electrolyte imbalance.
Antibiotics (if severe/high-risk): Guided by sensitivity.
- Vibrio cholerae: Doxycycline
- Campylobacter: Azithromycin
- Salmonella Typhi: Ceftriaxone

⭐ Campylobacter jejuni infection is a major antecedent to Guillain-Barré syndrome (GBS) due to molecular mimicry.

Vibrio cholerae causes profuse rice-water stool; its toxin permanently activates Gs, leading to a dramatic ↑ in cAMP.

Campylobacter jejuni is a leading cause of bloody diarrhea and the most common bacterial antecedent to Guillain-Barré syndrome.

Salmonella typhi classically presents with "rose spots," constipation followed by diarrhea, and can establish a carrier state in the gallbladder.

Shigella requires a very low infectious dose to cause inflammatory bloody dysentery.

ETEC is the primary cause of Traveler's Diarrhea, mediated by its heat-labile (LT) and heat-stable (ST) toxins.

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