Gram-positive

Gram-positive US Medical PG Question 1: A 51-year-old man comes to the physician because of a 4-day history of fever and cough productive of foul-smelling, dark red, gelatinous sputum. He has smoked 1 pack of cigarettes daily for 30 years and drinks two 12-oz bottles of beer daily. An x-ray of the chest shows a cavity with air-fluid levels in the right lower lobe. Sputum culture grows gram-negative rods. Which of the following virulence factors is most likely involved in the pathogenesis of this patient's condition?

• A. IgA protease
• B. Exotoxin A
• C. Capsular polysaccharide (Correct Answer)
• D. P-fimbriae
• E. Heat-stable toxin

Gram-positive Explanation: ***Capsular polysaccharide*** - The patient's symptoms (fever, foul-smelling sputum, cavitation with air-fluid levels) and risk factors (smoking) suggest a **lung abscess** likely caused by **_Klebsiella pneumoniae_**. - **Capsular polysaccharide** is a major virulence factor for _Klebsiella pneumoniae_, providing resistance to phagocytosis and contributing to its invasive potential. *IgA protease* - **IgA protease** is a virulence factor produced by bacteria such as _Neisseria gonorrhoeae_, _Neisseria meningitidis_, and _Haemophilus influenzae_ to cleave IgA antibodies. - While important for mucosal infections, it is not characteristic of the severe lung pathology described, nor a primary virulence factor for a gram-negative rod causing lung abscesses like _Klebsiella_. *Exotoxin A* - **Exotoxin A** is a potent exotoxin produced by _Pseudomonas aeruginosa_, which inhibits protein synthesis by ADP-ribosylation of elongation factor 2. - While _Pseudomonas_ can cause lung infections in compromised patients, the classic description of dark red, gelatinous sputum and the strong association with gram-negative rods causing lung abscesses points more directly to _Klebsiella_. *P-fimbriae* - **P-fimbriae** (pyelonephritis-associated fimbriae) are adhesion factors found on uropathogenic _E. coli_, enabling them to bind to uroepithelial cells and cause urinary tract infections. - These fimbriae are not relevant to the pathogenesis of a lung abscess caused by gram-negative rods in this clinical context. *Heat-stable toxin* - **Heat-stable toxin** is typically associated with enterotoxigenic _E. coli_ (ETEC), causing watery diarrhea by activating guanylate cyclase. - This toxin is involved in gastrointestinal infections and has no role in the pathogenesis of a lung abscess.

Gram-positive US Medical PG Question 2: A 53-year-old diabetic man with cellulitis of the right lower limb presents to the emergency department because of symptoms of fever and chills. His pulse is 122/min, the blood pressure is 76/50 mm Hg, the respirations are 26/min, and the temperature is 40.0°C (104.0°F). His urine output is < 0.5mL/kg/h. He has warm peripheral extremities. The hemodynamic status of the patient is not improving in spite of the initiation of adequate fluid resuscitation. He is admitted to the hospital. Which of the following is the most likely laboratory profile?

• A. WBC count: 11,670/mm3; low CVP; blood culture: gram-negative bacteremia; blood lactate level: 0.9 mmol/L
• B. WBC count: 1234/mm3; high CVP; blood culture: gram-negative bacteremia; blood lactate level: 1.6 mmol/L
• C. WBC count: 16,670/mm3; low CVP; blood culture: gram-negative bacteremia; blood lactate level: 2.2 mmol/L (Correct Answer)
• D. WBC count: 6670/mm3; low CVP; blood culture: gram-positive bacteremia; blood lactate level: 1.1 mmol/L
• E. WBC count: 8880/mm3; high CVP; blood culture: gram-positive bacteremia; blood lactate level: 2.1 mmol/L

Gram-positive Explanation: ***WBC count: 16,670/mm3; low CVP; blood culture: gram-negative bacteremia; blood lactate level: 2.2 mmol/L*** - This profile aligns with **septic shock** driven by gram-negative bacteria, presenting with **tachycardia**, **hypotension**, **fever**, and **poor urine output** despite fluid resuscitation. - A **high WBC count** (leukocytosis), **low CVP** (due to vasodilation and relative hypovolemia), **gram-negative bacteremia** (common in severe sepsis), and **elevated lactate** (indicating tissue hypoperfusion) are characteristic findings. *WBC count: 11,670/mm3; low CVP; blood culture: gram-negative bacteremia; blood lactate level: 0.9 mmol/L* - While most components suggest sepsis (WBC count and low CVP), a **lactate level of 0.9 mmol/L** is within the normal range, contradicting the severe hypoperfusion and shock observed in the patient. - The patient's presentation with **hypotension** unresponsive to fluids and **oliguria** strongly indicates significant tissue hypoperfusion, which would typically result in a higher lactate. *WBC count: 1234/mm3; high CVP; blood culture: gram-negative bacteremia; blood lactate level: 1.6 mmol/L* - A **WBC count of 1234/mm3 (leukopenia)** is an atypical response for severe infection and sepsis; often, sepsis presents with leukocytosis. - A **high CVP** indicates fluid overload or cardiac dysfunction, which is not consistent with the initial low blood pressure and the need for fluid resuscitation seen in distributive shock. *WBC count: 6670/mm3; low central venous pressure (CVP); blood culture: gram-positive bacteremia; blood lactate level: 1.1 mmol/L* - A **WBC count of 6670/mm3** is within the normal range and does not reflect an adequate inflammatory response to severe infection and shock. - A **normal lactate level (1.1 mmol/L)** does not support the clinical picture of shock and tissue hypoperfusion despite the presence of gram-positive bacteremia. *WBC count: 8880/mm3; high CVP; blood culture: gram-positive bacteremia; blood lactate level: 2.1 mmol/L* - A **WBC count of 8880/mm3** is normal, which is unlikely in a severe sepsis presentation. - A **high CVP** is not consistent with the distributive shock state where there is often relative hypovolemia and vasodilation leading to low CVP.

Gram-positive US Medical PG Question 3: A 33-year-old man presents to the emergency department complaining of weakness and fatigue. He states that his symptoms have worsened over the past day. He has a past medical history of IV drug abuse and alcoholism and he currently smells of alcohol. His temperature is 102°F (38.9°C), blood pressure is 111/68 mmHg, pulse is 110/min, respirations are 17/min, and oxygen saturation is 98% on room air. Physical exam is notable for focal tenderness over the lumbar spine. Initial lab values and blood cultures are drawn and are notable for leukocytosis and an elevated C-reactive protein (CRP). Which of the following is the best treatment for this patient?

• A. Nafcillin
• B. Ceftriaxone
• C. Piperacillin-tazobactam
• D. Vancomycin (Correct Answer)
• E. Ibuprofen and warm compresses

Gram-positive Explanation: ***Vancomycin*** - The patient's history of **IV drug abuse**, fever, leukocytosis, elevated CRP, and focal lumbar tenderness is highly suggestive of **vertebral osteomyelitis** or **discitis**, often caused by methicillin-resistant *Staphylococcus aureus* (MRSA). - **Vancomycin** is the appropriate empiric treatment for suspected severe *S. aureus* infections in patients with risk factors for MRSA until culture and sensitivity results are available. *Nafcillin* - **Nafcillin** is effective against **methicillin-sensitive *Staphylococcus aureus* (MSSA)**. - Given the patient's history of IV drug abuse, there's a high likelihood of MRSA, making nafcillin an inadequate empiric choice. *Ceftriaxone* - **Ceftriaxone** is a broad-spectrum cephalosporin effective against many gram-negative and some gram-positive bacteria, but it has **poor coverage against *Staphylococcus aureus***, particularly MRSA. - It would be ineffective as a monotherapy for the suspected staphylococcal infection. *Piperacillin-tazobactam* - This combination provides broad-spectrum coverage, including **Pseudomonas** and many gram-negative and anaerobic bacteria, but its coverage for **MRSA is limited**. - It would not be the first-line empiric choice for a suspected MRSA infection in this setting. *Ibuprofen and warm compresses* - This treatment addresses pain and inflammation but does not treat the underlying **infectious process**. - Overlooking the infection would lead to significant morbidity and potential mortality, making this an inappropriate primary treatment.

Gram-positive US Medical PG Question 4: A 30-year-old man is brought to the emergency department with complaints of fevers to 39.0℃ (102.2℉) and diarrhea for the past 12 hours. There is no history of headaches, vomiting, or loss of consciousness. The past medical history is unobtainable because the patient recently immigrated from abroad and has a language barrier, but his wife says that her husband had a motor vehicle accident when he was a teenager that required emergent surgery. He is transferred to the ICU after a few hours in the ED due to dyspnea, cyanosis, and hemodynamic collapse. There are no signs of a meningeal infection. The blood pressure is 70/30 mm Hg at the time of transfer. A chest X-ray at the time of admission shows interstitial infiltrates without homogeneous opacities. The initial laboratory results reveal metabolic acidosis, leukopenia with a count of 2000/mm3, thrombocytopenia (15,000/mm3), and a coagulation profile suggesting disseminated intravascular coagulation. A peripheral smear is performed as shown in the accompanying image. Despite ventilatory support, administration of intravenous fluids, antibiotics, and vasopressor agents, the patient dies the next day. The gram stain from the autopsy specimen of his lungs reveals gram-positive, lancet-shaped diplococci occurring singly and in chains. Which of the following organisms is the most likely cause for the patient’s condition?

• A. Neisseria meningitidis
• B. Non-typeable H. influenzae
• C. Streptococcus pneumoniae (Correct Answer)
• D. Staphylococcus aureus
• E. Streptococcus pyogenes

Gram-positive Explanation: ***Streptococcus pneumoniae*** - The patient's history of a prior **motor vehicle accident (MVA) with emergent surgery** as a teenager suggests a possible **splenectomy**, making him susceptible to infections by **encapsulated organisms**. - The presentation with **sepsis**, profound **leukopenia** and **thrombocytopenia**, **DIC**, **interstitial infiltrates** on CXR, and **gram-positive, lancet-shaped diplococci** in lung tissue is classic for severe **pneumococcal sepsis** in an asplenic individual. *Streptococcus pyogenes* - While *S. pyogenes* can cause severe infections, it typically presents with conditions like **necrotizing fasciitis** or **streptococcal toxic shock syndrome**, which would involve different clinical features. - It is a **coccus** that grows in **chains**, but the characteristic **lancet-shape** and **diplococci** are not typical for *S. pyogenes*. *Neisseria meningitidis* - Although an encapsulated organism that can cause severe sepsis in asplenic patients, it is typically a **gram-negative diplococcus**. - Symptoms often include **meningitis** (though not always present) and a **petechial rash**, neither of which are described here. *Non-typeable H. influenzae* - This is a **gram-negative coccobacillus** and would not present as gram-positive, lancet-shaped diplococci. - While it can cause pneumonia, it is less commonly associated with the fulminant sepsis and DIC seen here, especially in an asplenic patient. *Staphylococcus aureus* - *S. aureus* is a **gram-positive coccus** that typically clusters, not as lancet-shaped diplococci or chains. - While it can cause severe sepsis and DIC, the morphology described in the Gram stain is inconsistent with *S. aureus*.

Gram-positive US Medical PG Question 5: A 57-year-old HIV-positive male with a history of intravenous drug abuse presents to the emergency room complaining of arm swelling. He reports that he developed progressively worsening swelling and tenderness over the right antecubital fossa three days prior. He recently returned from a trip to Nicaragua. His past medical history is notable for an anaphylactoid reaction to vancomycin. His temperature is 101.4°F (38.6°C), blood pressure is 140/70 mmHg, pulse is 110/min, and respirations are 20/min. Physical examination reveals an erythematous, fluctuant, and tender mass overlying the right antecubital fossa. Multiple injection marks are noted across both upper extremities. He undergoes incision and drainage and is started on an antibiotic that targets the 50S ribosome. He is discharged with plans to follow up in one week. However, five days later he presents to the same emergency room complaining of abdominal cramps and watery diarrhea. Which of the following classes of pathogens is most likely responsible for this patient’s current symptoms?

• A. Gram-negative curved bacillus
• B. Gram-negative bacillus
• C. Anaerobic flagellated protozoan
• D. Gram-positive bacillus (Correct Answer)
• E. Gram-positive coccus

Gram-positive Explanation: ***Gram-positive bacillus*** - The patient was administered an antibiotic targeting the **50S ribosomal subunit** following incision and drainage for a suspected skin infection (likely **MRSA** given IV drug abuse). This strongly suggests **clindamycin** was used. - **Clindamycin** is a known risk factor for developing **Clostridioides (formerly Clostridium) difficile infection (CDI)**, which is caused by a **Gram-positive, spore-forming bacillus** and manifests with **abdominal cramps and watery diarrhea**. *Gram-negative curved bacillus* - This class of pathogens includes organisms like **Vibrio cholerae** or **Campylobacter jejuni**, which can cause diarrhea. - However, the patient's presentation with **colitis** after antibiotic use is more consistent with **Clostridioides difficile**, not typically a curved Gram-negative bacillus. *Gram-negative bacillus* - While some Gram-negative bacilli (e.g., E. coli, Salmonella) can cause diarrhea, their association with **antibiotic-induced colitis** following treatment for a skin abscess is less direct than that of *Clostridioides difficile*. - The initial skin infection in IV drug users is most commonly staphylococcal (Gram-positive coccus), for which a 50S targeting antibiotic would be prescribed. *Anaerobic flagellated protozoan* - This description often refers to pathogens like **Giardia lamblia** or **Trichomonas vaginalis**, which are not bacteria. - While *Giardia* can cause diarrhea, it typically causes **malabsorption** and **greasy stools**, and wouldn't be triggered by recent antibiotic use for a skin infection. *Gram-positive coccus* - **Gram-positive cocci** (e.g., Staphylococcus aureus) are the likely cause of the initial skin infection/abscess. - However, they do not typically cause **antibiotic-associated colitis** with watery diarrhea; rather, the *antibiotic treatment itself* for these organisms can predispose to *Clostridioides difficile*.

Gram-positive Flashcard 1 of 5

Question: Gram _____ bacteria have a much thicker cell wall

Answer: positive

Gram-positive Flashcard 2 of 5

Question: _____ is a filamentous-branching gram-positive organism that can cause brain abscess.

Answer: Nocardia

Gram-positive Flashcard 3 of 5

Question: Staphylococcus aureus is gram _____

Answer: positive

Gram-positive Flashcard 4 of 5

Question: Strep pneumonia contains _____, allowing bacteria to adhere and colonize the mucous membranes.

Answer: IgA protease

Gram-positive Flashcard 5 of 5

Question: Enterococcus spp. can become resistant to ampicillin due to _____ production

Answer: beta-lactamase