Laboratory Workup of Gram POS Rods

  • Actinomyces

  • Arcanobacterium

  • Bacillus

  • Bifidobacterium

  • Clostridium

  • Corynebacterium

  • Erysipelothrix

  • Eubacterium

  • Gardnerella

  • Lactobacillus

  • Listeria

  • Mycobacterium (other than M. tuberculosis)

  • Nocardia

  • Propionibacterium

MOST MEDICALLY RELEVANT:

  • BACILLUS

  • CLOSTRIDIUM

  • CORYNEBACTERIUM

  • LISTERIA

  • +/- MYCOBACTERIUM

There are many gram positive bacilli organisms. However, the four most medically relevant ones encountered are bacillus, clostridium, corynebacterium & listeria. These can be differentiated from one another according to unique chemical and biological properties of the various organisms. We will cover some of the laboratory testing methods used to identify the various gram-positive organisms.

Most Important

To distinguish between the 4 most medically relevant genera, ask these questions:

-Does it form spores?

        Yes vs. No?

-What are its oxygen requirements?

       Aerobic vs. Anaerobic?

-Can it move around?

      Motile vs. Non-motile?

Bacillus          Clostridium     Corynebacterium       Listeria

???

YES

Capsule= B. anthracis

No capsule= B. cereus

???

Aerobe

High Yield Properties of Other Gram POS Rods

 

Spore

Forming

Bacillus

 B. anthracis= Encapsulated & Penicillin sensitive

B. cereus= NON encapsulated & Penicillin resistant

 

Clostridium

Spore

Formers

 

Non-Spore

Forming

A. haemolyticum

Corynebacterium

E. rhusiopathiae

G. vaginalis

Lactobacillus

L. monocytogenes

Mycobacterium

Nocardia

 

Motile

B. cereus

 

L. monocytogenes

Motility

Properties

 

Non-Motile

Corynebacterium

Erysipelothrix

 

Gamma

Hemolysis

B. anthracis

Hemolytic

Properties

 

Beta Hemolysis

A. haemolyticum

B. cereus

L. monocytogenes

 

Catalase POSITIVE

Bacillus (spore forming- see below)

C. diphtheriae (NO-spore)

L. monocytogenes

Catalase

Testing

Catalase

Testing

 

Catalase NEGATIVE

A. haemolyticum

Erysipelothrix

Lactobacillus

 

Starch Hydrolysis

POSITIVE

Bacillus

Corynebacterium

Catalase

Testing

Starch Hydrolysis Test

 

Starch Hydrolysis NEGATIVE

Lactobacillus

Starch Hydrolysis Test

The purpose of the starch hydrolysis test is to determine whether bacteria are able to produce the alpha-amylase enzyme by using starch as a complex hydrocarbon from glucose.

Starch

Amylase enzyme

Blue-black color

NEGATIVE for amylase production

Glucose

No color

POSITIVE for amylase production

Flooding the plate with iodine will turn starch blue-black.

 

If bacteria produce amylase, starch is digested and these areas will remain clear.

Clearly, a lot of bacillus are positive with the starch hydrolysis test. There are several other tests that can be performed to further classify the bacillus species. However, not important for boards.

 

The Starch Hydrolysis test can be combined with other tests to differentiate Corynebacterium (POS) from the Lactobacillus family (NEG).

Summary

anaerobes

MICRO & IMMUNO 

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