Non-Fermentative Gram-Negative Bacilli & Related Microbiology Concepts

Vocabulary flashcards covering key terms, organisms, pigments, media, and biochemical tests related to non-fermentative Gram-negative bacilli.

Opportunistic pathogen

A microorganism that normally does not cause disease but can produce infections in immunocompromised or hospitalized patients.

Non-fermentative Gram-negative bacilli (NFGNB)

Environmental Gram-negative rods that oxidize but do not ferment carbohydrates; common opportunists in hospitals.

Oxidative-Fermentative (OF) test

Biochemical assay that differentiates oxidizers from fermenters by detecting acid production in open (aerobic) and oil-sealed (anaerobic) tubes.

Hugh and Leifson OF medium

Semisolid medium containing carbohydrates (e.g., glucose) and bromthymol blue pH indicator for the OF test.

Bromthymol blue

pH indicator in OF medium that turns yellow in acid and blue in alkaline conditions.

Oxidizer

Bacterium that produces acid only in the open tube of the OF test, indicating aerobic carbohydrate metabolism.

Fermenter

Bacterium that produces acid in both open and closed OF tubes, indicating anaerobic and aerobic carbohydrate utilization.

Triple Sugar Iron Agar (TSIA)

Differential slant medium with glucose, lactose, sucrose (1:10:10) plus phenol red to detect sugar fermentation, H₂S and gas.

Slant / Butt (TSIA)

Upper aerobic portion (slant) and lower anaerobic portion (butt) of TSIA used to interpret metabolic reactions.

Phenol red

pH indicator in TSIA that turns yellow in acid and remains/reddened in alkaline conditions.

A/A (Acid/Acid)

TSIA result showing yellow slant and butt, indicating fermentation of glucose plus lactose and/or sucrose.

K/A (Alkaline/Acid)

TSIA result with red slant, yellow butt; glucose fermented but lactose/sucrose not fermented.

K/K (Alkaline/Alkaline)

TSIA reaction with red slant and butt, indicating no sugar fermentation (typical of non-fermenters).

H₂S production

Black precipitate in TSIA butt caused by hydrogen sulfide reacting with iron salts.

Gas production (TSIA)

Bubbles, cracks, or media lift indicating CO₂ / H₂ gas formation during carbohydrate fermentation.

Oxidase test

Rapid assay detecting cytochrome c oxidase with tetramethyl-p-phenylenediamine; positive colonies turn purple within 10 s.

Cytochrome c oxidase

Terminal electron-transport enzyme detected by the oxidase test; present in many oxidizers like Pseudomonas.

Pseudomonas aeruginosa

Most common NFGNB; produces pyocyanin, grape-like odor, β-hemolytic colonies, and causes ‘blue pus’ infections.

Pyocyanin

Blue, water-soluble pigment unique to P. aeruginosa responsible for its green/blue colony coloration.

Pyoverdine

Yellow-green siderophore pigment of many Pseudomonas spp.; combines with pyocyanin to give green sheen.

Blue pus

Purulent exudate colored blue by pyocyanin from P. aeruginosa infection.

Swimmer’s ear (Acute otitis externa)

Painful external ear infection frequently caused by Pseudomonas aeruginosa in contaminated water.

Exotoxin A

Major P. aeruginosa virulence factor that inhibits protein synthesis by ADP-ribosylating EF-2.

Grape / tortilla odor

Characteristic sweet smell of P. aeruginosa colonies due to 2-aminoacetophenone.

Cetrimide agar

Selective medium that enhances pigment production and growth of Pseudomonas aeruginosa.

Acinetobacter baumannii

Second most common NFGNB; oxidase-negative, non-motile, produces purple colonies on MacConkey; notable nosocomial pathogen.

Stenotrophomonas maltophilia

Oxidase-negative NFGNB producing lavender-green colonies with ammoniacal odor; maltose-loving and frequent blood culture contaminant.

Burkholderia cepacia complex

Group of motile oxidase-weak NFGNB that form yellow colonies; important in cystic fibrosis pneumonia and IV-drug endocarditis.

Burkholderia mallei

Non-motile species causing Glanders (Farsi) primarily in equines; potential bioterrorism agent.

Glanders (Farsi disease)

Zoonotic infection by Burkholderia mallei characterized by ulcerating nodules and septicemia.

Burkholderia pseudomallei

Agent of Melioidosis (‘Vietnamese time-bomb’); produces dry wrinkled colonies with earthy odor; found in muddy soil.

Melioidosis

Severe infection mimicking many illnesses, acquired from Burkholderia pseudomallei in soil or water aerosols.

Tetramethyl-p-phenylenediamine dihydrochloride

Reagent used in oxidase test; reduced form turns purple when oxidized by cytochrome c oxidase.

Monotrichous flagellation

Single polar flagellum pattern seen in Pseudomonas aeruginosa providing motility.

Nosocomial infection

Infection acquired in a hospital setting, often caused by opportunistic NFGNB.

Lavender-green pigment

Distinct colony color of Stenotrophomonas maltophilia on blood agar.

Ammoniacal odor

Sharp smell produced by colonies of Stenotrophomonas maltophilia.

Maltose utilization

Metabolic trait of S. maltophilia producing acid from maltose but not glucose.

Wrinkled colony morphology

Dry, folded surface appearance typical of Burkholderia pseudomallei on culture media.

Strict aerobe

Organism that requires oxygen for growth; includes many non-fermenters like Pseudomonas and Stenotrophomonas.

Moraxella catarrhalis

Oxidase-positive, β-lactamase-producing coccobacillus causing respiratory and ear infections; member of family Moraxellaceae.

Mineral oil overlay (OF test)

Layer providing anaerobic environment in one OF tube to differentiate oxidative vs fermentative metabolism.

1:10:10 sugar ratio (TSIA)

Proportion of glucose:lactose:sucrose in TSIA allowing early depletion of glucose to reveal lactose/sucrose fermentation.