L14. NONFERMENTATIVE GRAM-NEGATIVE BACILLI

What are Non-Fermentative Gram-Negative Bacilli (NFGNB)?

A diverse group of bacteria that do not ferment carbohydrates and are mostly opportunistic pathogens.

What are the general characteristics of NFGNB?

• Gram-negative rods

• Do not ferment glucose

• Oxidase-positive (except some species)

• Found in soil, water, and hospital environments

Name the four major genera of NFGNB.

Pseudomonas, Burkholderia, Acinetobacter, Stenotrophomonas

Which NFGNB is the most clinically significant?

Pseudomonas aeruginosa

What is the primary virulence factor of Pseudomonas aeruginosa?

Exotoxin A, which inhibits protein synthesis.

What pigment does Pseudomonas aeruginosa produce?

Pyocyanin (blue-green pigment)

Which biochemical test differentiates Pseudomonas from Enterobacteriaceae?

Oxidase test (Pseudomonas is oxidase-positive)

What type of infections does Pseudomonas aeruginosa commonly cause?

Nosocomial infections (pneumonia, UTI, wound infections)

What is the characteristic odor of Pseudomonas aeruginosa?

Grape-like or corn-tortilla odor

How does Pseudomonas aeruginosa appear on MacConkey agar?

Non-lactose fermenting, colorless colonies

What disease does Burkholderia pseudomallei cause?

Melioidosis

What is the major virulence factor of Burkholderia cepacia?

Biofilm formation, leading to antibiotic resistance

Which NFGNB is commonly found in cystic fibrosis patients?

Burkholderia cepacia complex

What is the most resistant NFGNB to antibiotics?

Acinetobacter baumannii

How does Acinetobacter appear on Gram stain?

Gram-negative coccobacilli

What infections are commonly caused by Acinetobacter baumannii?

Ventilator-associated pneumonia, bloodstream infections

What is the oxidase result for Acinetobacter?

Oxidase-negative

Which NFGNB is most associated with hospital-acquired infections?

Stenotrophomonas maltophilia

What is the drug of choice for Stenotrophomonas maltophilia?

Trimethoprim-sulfamethoxazole (TMP-SMX)

Which NFGNB can survive in disinfectants and hospital equipment?

Pseudomonas, Acinetobacter

What are the key tests used for NFGNB identification?

Oxidase test, OF test, Motility, Growth on MacConkey agar

What is the purpose of the Oxidation-Fermentation (OF) test?

To differentiate oxidative vs. fermentative metabolism of glucose

What is the OF test result for Pseudomonas aeruginosa?

Oxidative (yellow in open tube, green in closed tube)

Which agar is best for detecting pigment production in Pseudomonas?

Cetrimide agar

What does Acinetobacter look like on MacConkey agar?

Non-lactose fermenting, pale pink colonies

Which test differentiates Stenotrophomonas from Pseudomonas?

Oxidase test (Pseudomonas is oxidase-positive, Stenotrophomonas is negative)

Which NFGNB is oxidase-positive?

Pseudomonas aeruginosa, Burkholderia spp.

Which NFGNB is oxidase-negative?

Acinetobacter, Stenotrophomonas

What is the motility test result for Pseudomonas?

Motile (polar flagella)

Which test differentiates Burkholderia cepacia from other NFGNB?

B. cepacia is oxidase-positive and grows on Burkholderia cepacia selective agar (BCSA)

Why are NFGNB difficult to treat?

Intrinsic and acquired antibiotic resistance

What is the primary mechanism of resistance in Pseudomonas aeruginosa?

Efflux pumps and beta-lactamase productio

What is the first-line treatment for Pseudomonas aeruginosa?

Piperacillin-tazobactam, ceftazidime, or carbapenems

Which NFGNB is naturally resistant to carbapenems?

Stenotrophomonas maltophilia

Why is Acinetobacter baumannii considered a “superbug”?

Multidrug resistance, including carbapenems and colistin

What is the main resistance mechanism of Stenotrophomonas?

Metallo-beta-lactamases (MBLs)

Which antibiotics are ineffective against Acinetobacter?

Most beta-lactams, aminoglycosides

Which NFGNB can produce biofilms?

Pseudomonas aeruginosa, Burkholderia cepacia

What is the drug of choice for Burkholderia pseudomallei?

Ceftazidime or meropenem

Which NFGNB has colistin as a last-resort treatment?

Acinetobacter baumannii

How can NFGNB infections be prevented in hospitals?

Strict infection control, hand hygiene, disinfecting equipment

Which NFGNB can survive in hospital disinfectants?

Pseudomonas, Acinetobacter

What is the most common NFGNB in burn wound infections?

Pseudomonas aeruginosa

Which NFGNB can cause sepsis in immunocompromised patients?

All major NFGNB (Pseudomonas, Acinetobacter, Burkholderia, Stenotrophomonas)

Which test confirms Burkholderia pseudomallei?

Ashdown’s agar

Which NFGNB is commonly isolated from respiratory samples in CF patients?

Burkholderia cepacia complex

What is the mortality rate of melioidosis if untreated?

Up to 90%

How is melioidosis transmitted?

Inhalation, skin contact with contaminated soil/water

Which NFGNB is commonly isolated from ventilators?

Acinetobacter baumannii

Which NFGNB has a “ground-glass” appearance on agar?

Burkholderia pseudomallei