A 55-year-old man admitted to the ICU with sepsis secondary to aspiration pneumonia develops a secondary infection. Sputum culture grows a gram-negative rod that is mucoid, non-motile, and produces a positive string test (capsule test). The organism ferments glucose, lactose, and sucrose but is negative for urease and indole. It is resistant to ampicillin but susceptible to ceftriaxone. Which organism is most likely responsible for this nosocomial respiratory infection?

Explanation

## Diagnosis: Klebsiella pneumoniae in Nosocomial Pneumonia ### Clinical Context The patient is in the ICU with aspiration pneumonia, a risk factor for secondary gram-negative infection. The isolation of a mucoid gram-negative rod from sputum in a hospitalized patient with sepsis is highly suggestive of *Klebsiella pneumoniae*. ### Microbiological Identification **Key Point:** The **string test (capsule test)** is the single most discriminating feature here. When a mucoid colony is emulsified in saline, the thick polysaccharide capsule causes the suspension to form a viscous "string" when drawn up with a loop — this is pathognomonic for *Klebsiella*. ### Biochemical Profile Comparison | Biochemical Test | Klebsiella pneumoniae | Serratia marcescens | Enterobacter cloacae | Acinetobacter baumannii | |---|---|---|---|---| | **Gram stain** | Gram-negative rod | Gram-negative rod | Gram-negative rod | Gram-negative coccobacillus | | **Motility** | Non-motile | Motile | Motile | Non-motile | | **String test (capsule)** | **Positive** | Negative | Negative | Negative | | **Glucose fermentation** | Positive | Positive | Positive | Non-fermenter | | **Lactose fermentation** | Positive | Negative | Positive | Non-fermenter | | **Sucrose fermentation** | Positive | Positive | Positive | Non-fermenter | | **Urease** | Negative | Negative | Negative | Negative | | **Indole** | Negative | Negative | Negative | Negative | | **Oxidase** | Negative | Negative | Negative | Negative | | **Ampicillin susceptibility** | Usually resistant | Usually resistant | Often resistant (AmpC) | Intrinsically resistant | | **Ceftriaxone susceptibility** | Susceptible (non-ESBL) | Susceptible | Variable (AmpC producers resist) | Resistant | ### Virulence Factors in Respiratory Infection **High-Yield:** *Klebsiella pneumoniae* causes severe nosocomial pneumonia because: 1. **Polysaccharide capsule** — resists phagocytosis and complement-mediated killing 2. **Lipopolysaccharide (LPS)** — triggers robust inflammatory response, leading to tissue damage 3. **Fimbriae** — promote adhesion to respiratory epithelium 4. **Siderophores** — enable iron acquisition in the host environment **Clinical Pearl:** *Klebsiella pneumoniae* is notorious for causing **necrotizing pneumonia** with abscess formation, particularly in alcoholics and diabetics. In the ICU, it is a leading cause of ventilator-associated pneumonia (VAP). ### Antibiotic Resistance Pattern The organism is ampicillin-resistant but ceftriaxone-susceptible, consistent with a non-ESBL *Klebsiella* strain. This resistance pattern is typical for nosocomial isolates due to constitutive expression of chromosomal β-lactamase. **Warning:** ESBL and carbapenem-resistant *Klebsiella* strains are increasingly common in ICU settings; local antibiogram should guide empiric therapy.