Review Question - QID 217275

This patient with cystic fibrosis presents with an acute cough productive of green sputum, fever, tachycardia, tachypnea, and focally decreased breath sounds and dullness to percussion, which are indicative of pneumonia. The organism that is most strongly associated with pulmonary infections in patients with cystic fibrosis is Pseudomonas aeruginosa, which is a gram-negative, oxidase-positive, non-lactose fermenting rod.

Pseudomonas aeruginosa is a gram-negative rod that produces several virulence factors that are responsible for its infectivity, including phospholipase C, endotoxin, and exotoxin A. It also produces pyoverdine and pyocyanin pigments, which result in a greenish hue of infected fluids (e.g., sputum). In the general population, it commonly causes catheter- and ventilator-associated infections, osteomyelitis, hot tub folliculitis, and ecthyma gangrenosum. In patients with cystic fibrosis, it is the most frequently isolated microbe and a common cause of pneumonia. Treatment of Pseudomonas infections includes source control (e.g., removal of infected catheters) and antibiotic therapy. Anti-pseudomonal antibiotics include the carbapenems, aminoglycosides, monobactams, polymyxins, fluoroquinolones, ceftazidime, cefepime, and extended spectrum penicillins.

Smith et al. studied the genetic adaptations that allow Pseudomonas aeruginosa to colonize the airways of patients with cystic fibrosis. They separate its virulence factors into those that permit acute infections and those that facilitate chronic infections, such as those seen in cystic fibrosis.

Incorrect Answers:
Answer 1: Gram-negative, oxidase-negative, lactose fermenting bacteria refer to Klebsiella species. Klebsiella pneumonia is productive of red, “currant jelly” sputum and is commonly seen in patients with alcoholism. It is also a common cause of community-acquired pneumonia. This patient has greenish sputum and cystic fibrosis, making Pseudomonas more likely.

Answer 2: Gram-negative, oxidase-negative, non-lactose fermenting bacteria refer to Proteus, Salmonella, and Shigella species. Proteus species can cause urinary tract infections, presenting as dysuria and hematuria. Salmonella and Shigella species cause acute gastroenteritis, presenting as diarrhea. These species do not usually cause pneumonia.

Answer 4: Gram-positive, oxidase-negative, catalase-negative bacteria refer to Streptococcus pneumoniae, which is the most common cause of community-acquired pneumonia in the general population and produces a rusty-colored sputum. This patient has greenish sputum and cystic fibrosis, in which Pseudomonas would be more likely.

Answer 5: Gram-positive, oxidase-negative, catalase-positive bacteria refer to Staphylococcus aureus, which can cause ventilator-or healthcare-associated pneumonia. Methicillin-resistant Staphylococcus aureus (MRSA) commonly causes healthcare-associated infections. Staphylococcus aureus is also often isolated from cystic fibrosis patients, but less frequently than Pseudomonas aeruginosa.

Bullet Summary:
Pseudomonas aeruginosa is a gram-negative, oxidase-positive, non-lactose fermenting rod that is strongly associated with pulmonary infections in patients with cystic fibrosis.