Identification of Enteric Organisms – Key Vocabulary

A vocabulary set covering culture media, biochemical tests, key pigments, colony morphologies, and clinically relevant facts for identifying enteric organisms.

Eosin Methylene Blue (EMB) agar

Mildly selective, differential medium containing lactose & sucrose; differentiates enterics by lactose fermentation.

Eosin Y & Methylene Blue

Dual pH indicators in EMB that give lactose fermenters colored colonies and non-fermenters colorless colonies.

Lactose fermenter (on EMB)

Bacterium producing colored colonies, often with a green metallic sheen (e.g., E. coli).

Non-lactose fermenter (on EMB)

Bacterium producing colorless colonies that take up the background color of EMB.

Salmonella Shigella Agar (SSA)

Moderately selective, differential medium with lactose & neutral red to separate Salmonella/Shigella from other enterics.

Neutral Red

pH indicator in SSA; lactose fermenters appear pink, non-fermenters colorless.

Bismuth Sulfite Agar (BSA)

Selective medium for Salmonella; uses bismuth sulfite & ferrous sulfate to detect H₂S production.

Jet black colonies

Characteristic appearance of Salmonella typhi on BSA due to heavy H₂S production.

Triple Sugar Iron (TSI) agar

Differential medium detecting glucose, lactose, sucrose fermentation, gas, and H₂S in enterics.

Phenol Red

pH indicator in TSI; yellow = acid, red = alkaline.

A/AG

TSI reaction showing acid slant/acid butt with gas bubbles or cracks.

Alk/Alk

TSI reaction with alkaline slant & butt; indicates no sugar fermentation (e.g., P. aeruginosa).

H₂S production (TSI)

Black precipitate formed when cysteinase generates H₂S, reacting with iron salts.

Lysine Iron Agar (LIA)

Medium assessing lysine decarboxylation in the butt and lysine deamination on the slant.

Lysine deaminase (+)

Red LIA slant (R) indicating removal of an amine group from lysine.

Lysine decarboxylase (+)

Purple LIA butt (P) showing enzyme-mediated removal of carboxyl group, raising pH.

LIA P/P

Purple slant/Purple butt; deaminase (–), decarboxylase (+).

LIA R/Y

Red slant/Yellow butt; deaminase (+), decarboxylase (–).

Motility-Indole-Ornithine (MIO) medium

Semisolid tube testing motility and ornithine decarboxylation; indole read separately.

Motile organism (MIO)

Growth diffuses away from stab line, producing hazy medium.

Ornithine decarboxylation (+)

Purple MIO medium around the growth; alkaline shift from decarboxylation.

Indole test

Assesses tryptophanase activity using tryptone broth and Kovac’s (or Ehrlich’s) reagent.

Kovac’s Reagent

p-dimethylaminobenzaldehyde reagent that forms red ring with indole-positive cultures.

Simmon’s Citrate Agar (SCA)

Medium testing ability to use citrate & ammonium salts; indicator bromothymol blue.

Citrate positive

Deep royal/Russian blue SCA slant due to alkaline metabolites.

Citrate negative

Green, unchanged SCA slant indicating no citrate utilization.

Urease test

Detects hydrolysis of urea to ammonia & CO₂; rapid screen for Proteus spp.

Urea Broth – urease (+)

Bright pink color from pH rise (ammonia).

Urea Broth – urease (–)

Peach or light yellow color (no alkalinization).

Pseudomonas agar P (PAP)

Medium enhancing pyocyanin/pyorubin pigment in P. aeruginosa, suppressing fluorescein.

Pseudomonas agar F (PAF)

Medium promoting pyoverdine (fluorescein) production, reducing pyocyanin.

Pyocyanin

Blue-green pigment produced by P. aeruginosa on PAP.

Pyoverdine (fluorescein)

Yellow-green fluorescent pigment produced by P. aeruginosa on PAF.

Prodigiosin

Red pigment produced by Serratia marcescens.

Fish-eye colony

Enterobacter aerogenes on EMB: dark center with pale edge due to slow lactose fermentation.

Swarming growth

Rapid concentric surface spreading typical of Proteus species on non-inhibitory media.

Enterobacteriaceae

Family of gram-negative rods; facultative anaerobes; normal gut flora & opportunistic pathogens.

Opportunistic pathogen

Microbe causing disease when introduced to normally sterile sites or in immunocompromised hosts.

Water quality indicator – E. coli

Presence signals recent fecal contamination of water supplies.

UTI predominance of E. coli

Attributed to proximity of female urethra to anus and shorter urethral length, easing ascent of bacteria.

R/Y organisms in LIA

Genera Proteus, Providencia, and Morganella give red slant/yellow butt reaction.