How to Make a Dichotomous Key From a Table of Bacterial

Characteristics

(for use in running the minimum number of tests in identifying an

unknown bacterium)

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Dichotomous Key to Identifying Eight Bacteria

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight BacteriaTable of Characteristics for Eight Bacteria

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight BacteriaTable of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negativeAlcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

positive

Dichotomous Key to Identifying Eight Bacteria

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negative

positive

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

Galactose fermentation

negativeAlcaligenes faecalis Citrobacter freundii Aeromonas hydrophila Pseudomonas fluorescens

positive

Dichotomous Key to Identifying Eight Bacteria

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negative

positive

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

Galactose fermentation

negativeAlcaligenes faecalis Citrobacter freundii Aeromonas hydrophila Pseudomonas fluorescens

H2S production

positive negativeAlcaligenes faecalis

Aeromonas hydrophila

positive

Dichotomous Key to Identifying Eight Bacteria

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negative

positive

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

Galactose fermentation

negativeAlcaligenes faecalis Citrobacter freundii Aeromonas hydrophila Pseudomonas fluorescens

H2S production

positive negativeAlcaligenes faecalis

Aeromonas hydrophila

positive

Dichotomous Key to Identifying Eight Bacteria

Litmus milk

acid alkalineCitrobacter freundii

Pseudomonas fluorescens

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negative

positive

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

Galactose fermentation

negativeAlcaligenes faecalis Citrobacter freundii Aeromonas hydrophila Pseudomonas fluorescens

H2S production

positive negativeAlcaligenes faecalis

Aeromonas hydrophila

positive

Dichotomous Key to Identifying Eight Bacteria

Litmus milk

acid alkalineCitrobacter freundii

Pseudomonas fluorescens

acid fast

Acid-fast staining

not acid fastMycobacterium smegmatis Corynebacterium xerosis Mycobacterium leprae Streptococcus pneumoniae

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negative

positive

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

Galactose fermentation

negativeAlcaligenes faecalis Citrobacter freundii Aeromonas hydrophila Pseudomonas fluorescens

H2S production

positive negativeAlcaligenes faecalis

Aeromonas hydrophila

positive

Dichotomous Key to Identifying Eight Bacteria

Litmus milk

acid alkalineCitrobacter freundii

Pseudomonas fluorescens

acid fast

Acid-fast staining

not acid fastMycobacterium smegmatis Corynebacterium xerosis Mycobacterium leprae Streptococcus pneumoniae

positive

Catalase produced

negativeMycobacterium smegmatis

Mycobacterium leprae

Organism Characteristics

Alcaligenes faecalis Gram negative bacillus, acid-fast negative, ferments galactose, catalase positive, motile, alkaline with litmus milk, not hemolytic, H2S negative

Citrobacter freundii Gram negative bacillus; acid-fast negative, does not ferment galactose, catalase positive, non-motile, acid produced in litmus milk, not hemolytic, H2S negative

Pseudomonas fluorescens

Gram negative bacillus, acid-fast negative, does not ferment galactose, catalase positive, non-motile, alkaline in litmus milk, hemolytic, H2S negative

Streptococcus pneumoniae

Gram positive oval coccus found as streptococci, acid-fast negative, galactose fermentation negative, catalase negative, non-motile, acid in litmus milk, hemolytic, H2S negative

Mycobacterium smegmatis

Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-negative,, non-motile, alkaline in litmus milk, catalase positive, not hemolytic, H2S negative

Mycobacterium leprae Gram positive club-shaped bacilli, acid-fast positive, galactose fermentation negative, catalase-positive, non-motile, alkaline in litmus milk not hemolytic, H2S negative

Corynebacterium xerosis

Gram positive club-shaped bacilli, acid-fast negative, galactose fermentation positive, non-motile, acid from litmus milk, catalase positive, not hemolytic, H2S negative

Aeromonas hydrophila Gram negative bacilli, acid-fast negative, ferments galactose, catalase positive, motile, acid from litmus milk, not hemolytic, H2S positive

Table of Characteristics for Eight Bacteria

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisStreptococcus pneumoniae Aeromonas hydrophila

Gram stain

negative

positive

Alcaligenes faecalis Mycobacterium smegmatisCitrobacter freundii Mycobacterium lepraePseudomonas fluorescens Corynebacterium xerosisAeromonas hydrophila Streptococcus pneumoniae

Galactose fermentation

negativeAlcaligenes faecalis Citrobacter freundii Aeromonas hydrophila Pseudomonas fluorescens

H2S production

positive negativeAlcaligenes faecalis

Aeromonas hydrophila

positive

Dichotomous Key to Identifying Eight Bacteria

Litmus milk

acid alkalineCitrobacter freundii

Pseudomonas fluorescens

acid fast

Acid-fast staining

not acid fastMycobacterium smegmatis Corynebacterium xerosis Mycobacterium leprae Streptococcus pneumoniae

positive

Catalase produced

negativeMycobacterium smegmatis

Mycobacterium leprae

positive

Hemolysis

negativeCorynebacterium

xerosisStreptococcus pneumoniae