Laboratory Workup: Gram NEG Rods (Bacilli)

Gram negative rods (bacilli) account for the majority of bacterial organisms. It is easier to memorize what is not a gram negative rod rather than try to memorize all of these pink critters. Only a few gram neg rods are anaerobic, thus it is easier to remember them.

 

Many gram neg rods are considered enteric or "gut" bacteria (called enterobacteriaceae) and use aerobic respiration. The non-enterobacteriaceae aerobic gram neg rods (non-gut bugs) can be further categorized by their fermentation patterns.

Review the list below to familiarize yourself with the overall categorization of these bacteria.

Aerobic

ENTEROBACTERIACEAE

  • Citrobacter

  • Edwardsiella

  • Enterobacter

  • Escherichia

  • Hafnia

  • Klebsiella

  • Morganella

  • Proteus

  • Providencia

  • Salmonella

  • Serratia

  • Shigella

  • Yersinia

NON-ENTEROBACTERIACEAE

  • FERMENTATIVE

    • Aeromonas

    • Pasteurella

    • Plesiomonas

    • Vibrio

  • NON-FERMENTATIVE

    • Acinetobacter

    • Alcaligenes

    • Burkholderia

    • Flavobacterium

    • Pseudomonas

    • Stenotrophomonas​​

Anaerobic

  • Bacteroides

  • Fusobacterium

  • Porphyromonas

  • Prevotella

Lab Testing Methods

Several laboratory testing methods can be used for diagnosis:

  • Blood agar plate

  • Selective media to suppress gram positives from growing due to presence of bile salts and bacteriostatic dyes:

    • MacConkey's agar

    • Eosin-methylene blue agar

  • Fermentation patterns & Hydrogen sulphide production:

    • Lactose fermenters form colored colonies.

    • Triple sugar agar (Glucose, Fructose & Sucrose) used to determine fermentation patterns & production of hydrogen sulphide.

  • Urea agar

    • Determine urease production (Urease hydrolyzes urea to ammonia & CO2 causing the pH to turn alkaline)

  •  

    Methlylene blue fecal stain

    • Determines whether PMNs present in fecal sample (presence indicates involvement with invasive organism such as shiegella, salmonella, or campylobacter rather than toxin-producing organisms such as vibrio cholerae, E. coli or Clostridium perfringens.

 

Enterobacteriaceae Family Characteristics

As previously mentioned, most gram neg rods belong to the

enterobacteriaceae family and share several characteristics:

  • Found in colon of animals & humans.

  • Catalase positive.

  • Can ferment glucose.

  • Can reduce nitrates to nitrites

  • Oxidase negative.

 

ENTEROBACTERIACEAE FAMILY

  • Citrobacter

  • Edwardsiella

  • Enterobacter

  • Escherichia

  • Hafnia

  • Klebsiella

  • Morganella

  • Proteus

  • Providencia

  • Salmonella

  • Serratia

  • Shigella

  • Yersinia

Lactose Fermentation

LACTOSE
FERMENTERS
NON-LACTOSE
FERMENTERS

Urease

UREASE
POSITIVE
UREASE
NEGATIVE

H2S Formation

H2S
PRODUCED
NO
H2S
PRODUCED

Oxidase

OXIDASE POSITIVE
OXIDASE
NEGATIVE

NEGATIVE MacConkey Testing

OXYGEN TOLERANT
STRICTLY ANAEROBIC

POSITIVE MacConkey Testing

LACTOSE POSITIVE

 (PINK colonies)

LACTOSE NEGATIVE

 (Colorless Colonies)

 
+ GLUCOSE TESTING
 Oxidase+ => Glucose +
Oxidase- => Glucose-

Summary

ANAEROBES

MICRO & IMMUNO 

INDEX

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