Quick Review Cards for Medical Laboratory Science

Biochemical Tests for Identification

of Enterobacteriaceae

Clinical Microbiology Review 177

TEST PRINCIPLE INTERPRETATION OTHER

Oxidase

Nitrate reduction

Carbohydrate
fermentation

ONPG

Good test to differentiate Enterobacteriaceae

from nonfermenters. False pos from iron-

containing wire. Use platinum wire or

wooden stick.

If no color develops, zinc dust added.

Reduces nitrates. Red color after zinc dust

indicates presence of residual nitrates, i.e.,

neg rxn. No color after zinc dust means

nitrates reduced to N 2 or NO 2 , i.e., pos rxn.

Frequently tested carbohydrates are glucose,

lactose, sucrose, mannose, sorbitol, manni-

tol, xylose, adonitol, cellobiose, dulcitol,

trehalose. All Enterobacteriaceae ferment

glucose.

Test for slow lactose fermentation. Helpful

in differentiating Citrobacter(pos) from

most Salmonella(neg).

Tetramethyl-para-phenylene-

diamine dihydrochloride reacts

with cytochrome C to produce

blue or purple color.

If organism reduces nitrates to

nitrites, red color develops when

sulfanilic acid & N,N-dimethyl-L-

naphthylamine added.

When carbohydrate fermented,

acidic end products cause color

change in pH indicator.

ONPG is changed to orthonitro-

phenol by beta-galactosidase.

Pos = blue or purple

Pos = red, or no color after

addition of zinc dust

With phenol red indicator,

change from red to yellow

Pos = yellow

continued...