What are examples of gram-negative bacilli?

Gram-Negative Bacilli: Common Examples

Gram-negative bacilli represent a diverse group of rod-shaped bacteria that are major causes of both community-acquired and nosocomial infections, with the most clinically significant species being Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. 1

Primary Clinically Important Species

Enterobacterales (Enteric Gram-Negative Bacilli)

• Escherichia coli is the most frequently isolated gram-negative bacillus, particularly from urinary tract infections and is a leading cause of bacteremia 1, 2
• Klebsiella pneumoniae causes significant respiratory and urinary tract infections and is increasingly associated with carbapenem resistance 3, 1
• Enterobacter species (including Enterobacter cloacae) are important nosocomial pathogens 3
• Proteus species (including Proteus mirabilis and Proteus vulgaris) are commonly isolated from urinary tract infections 3
• Serratia marcescens is a notable nosocomial pathogen 3
• Citrobacter species (including Citrobacter freundii and Citrobacter koseri) cause various hospital-acquired infections 3
• Morganella morganii is encountered in polymicrobial infections 3

Non-Fermenting Gram-Negative Bacilli

• Pseudomonas aeruginosa is a prominent cause of infection, especially in immunocompromised hosts, patients with cystic fibrosis, and those with nosocomial pneumonia 3, 1
• Acinetobacter baumannii has emerged as an increasingly important nosocomial pathogen with alarmingly high rates of antibiotic resistance, particularly in intensive care units 3, 1
• Stenotrophomonas maltophilia is intrinsically resistant to many antibiotics 4
• Burkholderia cepacia demonstrates resistance to multiple antimicrobial classes 4

Other Clinically Relevant Species

• Haemophilus influenzae is isolated from mechanically ventilated patients, particularly within 48-96 hours after intubation 3
• Salmonella species (including Salmonella typhi and Salmonella enteritidis) cause enteric infections 3
• Shigella species (including Shigella flexneri, Shigella sonnei, Shigella dysenteriae, and Shigella boydii) cause dysentery 3
• Campylobacter jejuni causes gastroenteritis 3
• Legionella pneumophila causes atypical pneumonia 3
• Vibrio species (including Vibrio cholerae, Vibrio parahaemolyticus, and Vibrio vulnificus) cause gastrointestinal and wound infections 3
• Yersinia enterocolitica causes enterocolitis 3
• Pasteurella multocida is associated with animal bites 3

Clinical Distribution Patterns

Hospital-Acquired Infections

• In the National Nosocomial Infections Surveillance (NNIS) System data, Pseudomonas aeruginosa, Enterobacter species, Klebsiella pneumoniae, Escherichia coli, Serratia marcescens, and Proteus species comprised 50% of isolates from respiratory tract specimens in nosocomial pneumonia 3
• Gram-negative bacilli are associated with 71.1% of urinary tract infections, 65.2% of pneumonia episodes, 33.8% of surgical site infections, and 23.8% of bloodstream infections in intensive care units 5
• Acinetobacter species increased from 4% to 7.0% of ICU pneumonia cases between 1986 and 2003, representing a significant emerging threat 5

Polymicrobial Infections

• Nosocomial bacterial pneumonias are frequently polymicrobial, with gram-negative bacilli usually predominating 3
• In mechanically ventilated patients with nosocomial pneumonia, 40% of quantitative cultures were polymicrobial 3

Antimicrobial Resistance Patterns

Carbapenem-Resistant Organisms

• Carbapenem-resistant Enterobacterales (CRE), carbapenem-resistant Acinetobacter baumannii (CRAB), and carbapenem-resistant Pseudomonas aeruginosa (CRPA) are designated by the WHO as pathogens of critical threat 3
• In China, carbapenem-resistant K. pneumoniae increased from 3% in 2005 to 23.1% in 2021, while CRAB increased from 31.0% to 71.5% 3

Multidrug-Resistant Patterns

• Among surgical patients, almost 50% of gram-negative bacilli isolated were multidrug-resistant, with Acinetobacter baumannii showing an 83.9% MDR rate 6
• Extended-spectrum β-lactamase production is common among Enterobacterales, with blaCTX-M being the predominant gene (64.6% to 82.1% of resistant isolates) 7