KEY TO ALL LABORATORY ORGANISMS

ORGANISM IS GRAM-POSITIVE: GO TO SECTION I

ORGANISM IS GRAM-NEGATIVE: GO TO SECTION II

 

I. ALL GRAM POSITIVE ORGANISMS

A. All Gram-Positive Organisms Studied

1. Organism is a coccus: Go to Section B.

2. Organism is a bacillus: Go to Section E.

 

B. All Gram-Positive Cocci

  1. Catalase positive; some arranged in grape-like clusters and others in regular arrangements of 2,4,6 or 8: Go to Section C.

 

2. Catalase negative; most characteristic arrangement is in chains although chain length can be as small as 2; some members are not true cocci appearing as cocco-bacilli: Go to Section D.

C. Micrococceaceae

1. Random arrangement in grape-like clusters; nutrient agar colonies raised and flat ranging from gray to gold in color; luxurious growth on 6.5% salt media; characteristically commensals or pathogens of vertebrates including man: Go to Section C.3.

2. Regular arrangement in groups of 2,4,6, or 8; nutrient agar colonies moderate to small domed, round, entire colonies; many highly pigmented when cultured at optimum temperature; moderate to light growth on 6.5% salt media; characteristically saprophytic organisms: Go to Section C.4

 

3. Staphylococcus

a. Ferments mannitol: Organism: Staphylococcus aureus

b. Does not ferment mannitol: Organism: Staphylococcus epidermidis

4. Micrococcus

a. Urease positive: Go to Section C.4.c

b. Urease Negative: Go to Section C.4.d

c. Arrangement usually in groups of 4; ferments maltose and mannitol; gelatinase positive.

Organisms: Micrococcus ureae

 

d.      Arrangement most often observed is groups of 4, 6 & 8 from colonies on nutrient media.

Does not ferment mannitol; gelatinase negative.

Organism: Micrococcus luteus

 

D. Streptococceaceae

1. Blood hemolysis

a.       b-hemolytic: Go to D.2

b.      a-hemolytic: Go to D.3

c.       g-hemolytic: Go to D.4

 

2. b-hemolytic Streptococci

a. Ferments mannitol: Streptococcus pyogenes

b. Does not ferment mannitol: Streptococcus agalactiae

 

 

 

3.      a-hemolytic Streptococci

a.       Almost always observed in Gram stains as diplococci; ferments inulin & raffinose: Organism: Streptococcus pneumoniae

 

b. Appears in short to medium chains; does not ferment inulin & raffinose.

Organism: Streptococcus salivarius

 

4.      g-hemolytic streptococci

a. Salt tolerant; ferments mannitol: Organism: Enterococcus faecalis

b.      Salt intolerant; does not ferment mannitol

Organism: Streptococcus mutans

 

E. All Gram-Positive Bacilli Studied

1.      Cellular Morphology

a. Cells regular shaped looking like rectangles: Go To Section E.2

b. Cells irregular in shape: Go To Section E.5

 

2.      Catalase Test

a. Catalase and amylase positive: Go To Section Section E.3

b. Catalase negative: Go to Section E.4

 

3.      Bacillus: All members form endospores that are best observed in Gram stains of older cultures.

a. Ferments mannitol: Organism: Bacillus subtilis

b. Does not ferment mannitol: Organism: Bacillus thuringiensis

 

4.      Lactobacillus

a. Ferments raffinose: Organism: Lactobacillus casei

b. Does not ferment raffinose: Organism: Lactobacillus acidophilus

 

5.      Cellular Morphology

a. Cells observed in a variety of polymorphic forms: X, Y and V shaped very common.

Go To Section E.6

b.      Cells observed as long slender pointed rods in chains resembling long strands like fungal mycelia. Go To Section E.7

 

6.      Corynebacterium

a. Urease positive Organism: Corynbacterium pseudodiphtheriticum

b.      Urease negative Organism: Corynebacterium xerosis

 

7.      Mycobacterium

a. Ferments mannose and rhamnose Organism: Mycobacterium phlei

b.      Does not ferment mannose and rhamnose

Organism: Mycobacterium smegmatis

 

 

II. ALL GRAM NEGATIVE ORGANISMS

A. Organism is a coccus: go to Section B.

B. Organism is a bacillus or coccobacillus: go to Section C

 

 

B. Neisseria

1. Lactose fermentation

a. Does not ferment lactose: Organism: Neisseria gonorrhoeae

b. Ferments lactose: Go To B.2

 

2. Sucrose fermentation

a. Does not ferment sucrose Organism: Neisseria meningitidis

b. Ferments sucrose Organism: Neisseria sicca

 

C. All Gram-negative bacilli

1. Ferments Lactose: Go to Section D

2. Does not ferment Lactose: Go to Section E

D. All Lactose Fermenting bacilli

1. Indole test:

a. Indole positive; citrate negative Organism: Escherichia coli

b.      Indole negative: Go to Section D.2

 

2. H₂S production

a. H₂S produced; citrate positive Organism: Citrobacter freundi

b.      H₂S not produced: Go to Section D.3

 

3. Urease Test

a. Urease postive Organism: Klebsiella pneumoniae

b. Urease negative Organism: Enterobacter aerogenes

E. Lactose Nonfermenters

1. Produce hydrogen sulfide: Go to Section F

2. Do not produce hydrogen sulfide: Go to Section I

 

F. Hydrogen Sulfide Producers

1. Urease positive: Go to Section G.

2. Urease negative: Go to Section H.

 

G. Proteus

1. Indol positive Organism: Proteus vulgaris

2. Indol negative Organism: Proteus mirabilis

 

H. Salmonella

1. Citrate negative: Organism: Salmonella typhi

2. Citrate positive: Organism: Salmonella typhimurium

 

I. Sulfide Negative Organisms

1. Oxidase negative; ferments glucose: Go to Section J.

2. Oxidase positive; does not ferment carbohydrates. Go to Section M.

 

J. Oxidase Negative Glucose Fermenting Organisms

1. Citrate positive: Go to Section K.

2. Citrate negative: Go to Section L.

 

K. Serratiae Organism: Serratia marcescens

 

L. Shigella

1. Indol positive: Organism: Shigella flexneri

2. Indol negative: Organism: Shigella sonnei

 

M. Oxidase Positive Nonfermenting Organisms

1. Gelatinase positive: Organism: Pseudomonas aeruginosa

2. Gelatinase negative: Organism: Alcaligenes faecalis