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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

  • E. coli

  • Enterobacter aerogenes, cloacae

  • Citrobacter (50%)

  • Klebsiella pneumoniae

  • Vibrio vulnificus

NON-LACTOSE
FERMENTERS

  • Edwardsiella tarda

  • Morganella morganii

  • Pasteurella multocida

  • Proteus mirabilis

  • Providencia

  • Pseudomonas

  • Salmonella typhi

  • Serratia marcescens

  • Shigella dysenteriae

  • Vibrio cholerae, parahaemolyticus

  • Yersinia pestis, enterocolitica, pseudotuberculosis

Urease

UREASE
POSITIVE

  • Citrobacter

  • Klebsiella

  • Morganella

  • Proteus

  • Providencia

  • Y. enterocolitica, pseudotuberculosis

UREASE
NEGATIVE

  • Alcaligenes

  • Eikenella corrodens

  • Pasteurella

  • Salmonella

  • Shigella

  • Y. pestis

H2S Formation

H2S
PRODUCED

  • Citrobacter freundii

  • Edwardsiella tarda

  • P. vulgaris, mirabelis

  • Salmonella

NO
H2S
PRODUCED

  • Citrobacter diversus

  • E. coli

  • Klebsiella

  • Morganella

  • Providencia

  • Serratia marcescens

  • Shigella

  • Y. pseudotuberculosis, enterocolitica

Oxidase

OXIDASE POSITIVE

  • Aeromonas hydrophilia

  • Alcaligenes

  • Burkholderia cepacia

  • Campylobacter jejuni, fetus

  • Flavobacterium

  • H. pylori

  • Kingella kingae

  • Pasteurella multocida

  • Pleisomonas shigelloides

  • Pseudomonas aeruginosa

  • V. cholerae, V. parahaemolyticus

OXIDASE
NEGATIVE

  • Acinetobacter

  • Klebsiella pneumoniae

  • Stenotrophomonas maltophilia

NEGATIVE MacConkey Testing

OXYGEN TOLERANT

  • Bordetella pertussis

  • Brucella sp.

  • Campylobacter jejuni

  • Francisella tularensis

  • Haemophilus influenzae

  • Helicobacter pylori

  • Legionella pneumophila

  • Pasteurella multocida

  • Vibrio cholerae

STRICTLY ANAEROBIC

  • Bacteroides fragilis

  • Fusobacterium sp.

  • Prevotella

  • Porphyromonas

POSITIVE MacConkey Testing

LACTOSE POSITIVE
​

 (PINK colonies)

  • E. coli

  • Klebsiella pneumoniae

  • Enterobacter sp.

LACTOSE NEGATIVE

 (Colorless Colonies)

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

LACTOSE NEGATIVE ORGANISMS:

​

GLUCOSE POSITIVE

  • Proteus sp.

  • Providencia sp.

  • Salmonella sp.​

  • Serratia sp.

  • Shigella sp.

  • Yersinia sp.

​

GLUCOSE NEGATIVE

  • Acinetobacter calcoaceticus

  • Bukholderia cepacia

  • Pseudomonas aeruginosa

  • Stenotrophomonas maltophilia

Summary

ANAEROBES

MICRO & IMMUNO 

INDEX

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