Overview
This lecture covers three nosocomial (hospital-acquired) bacteria in the Enterobacteriaceae family: Enterobacter, Serratia, and Klebsiella, focusing on their key clinical features, differences, and diagnostic clues.
General Characteristics of Enterobacter, Serratia, and Klebsiella
- All three commonly cause pneumonia and urinary tract infections (UTIs), especially in hospitalized patients.
- They are important nosocomial pathogens, often showing multi-drug resistance.
- Treatment often requires carbapenems due to resistance to many antibiotics.
- All are lactose fermenters, forming pink colonies on MacConkey agar (exception: Serratia may ferment slowly and sometimes appears negative).
- The only other major lactose fermenter in this family to remember is E. coli (not covered here).
Enterobacter
- Enterobacter is motile (capable of movement).
- Mainly distinguished by its motility among these three bacteria.
- No unique clinical features beyond the general family traits.
Serratia
- Serratia is also motile.
- Produces a distinctive red pigment when cultured.
- May be recognized from pink or red discoloration in bathrooms (e.g., pink shower rings).
- Serratia ferments lactose slowly and may sometimes test negative.
Klebsiella
- Klebsiella is non-motile (immotile), unlike the other two.
- Has a thick polysaccharide capsule, contributing to its virulence.
- Commonly associated with alcoholics, abscess formation, and aspiration (Three A's: Alcoholics, Abscesses, Aspiration).
- Pneumonia caused by Klebsiella often presents with "currant jelly" (thick, blood-tinged) sputum.
- Can form lung abscesses and cavitary lesions on chest x-ray, which may mimic tuberculosis.
- Produces urease (raises urine pH), though less important than in Proteus species.
Key Terms & Definitions
- Nosocomial infection — Infection acquired in a hospital setting.
- Multi-drug resistance — Ability of bacteria to resist multiple antibiotics.
- Lactose fermenter — Bacteria that can metabolize lactose, often detected by pink colonies on MacConkey agar.
- Motility — Ability of bacteria to move.
- Capsule — Polysaccharide layer outside some bacteria, contributing to virulence.
- Currant jelly sputum — Thick, red sputum seen in Klebsiella pneumonia.
- Urease — Enzyme that hydrolyzes urea to ammonia, raising urine pH.
Action Items / Next Steps
- Review distinguishing features between Enterobacter, Serratia, and Klebsiella.
- Remember lactose fermentation and MacConkey agar as key diagnostic clues.
- Study Klebsiella’s Three A’s and "currant jelly" sputum association.
- Compare these bacteria with E. coli and Proteus for test preparation.