The photos below present the findings in the MAC of a wound culture. The culture

Question

The photos below present the findings in the MAC of a wound culture. The culture shows profuse growth of grayish large convex shiny colonies on BA. No growth in 48hrs was in CNA agar. The photo at the left was the 24 hr MAC agar findings showing red pigmented colonies on the first and second quadrants of the streaked plate and some of the isolated colonies on the third and fourth cuadrant showed as non lactose fermenters (NLF) (3 colonies enclosed in circle). These NLF colonies on next day (48hrs old) also showed the red pigmentation. The oxidase test for the MAC NLF colonies was negative The OF glucose showed growth in both tubes: tube with overlay of mineral oil and tube exposed to oxygen The IMViC test results were as follows: (-*) Gram: gnb (Indole:neg; Methyl Red:neg; Voges-Proskauer: pos; Citrate:pos) Other tests Urea:neg ISI: K/A no gas Motility: positive Ornithine Decarboxylase:pos Lysine Decarboxylase: pos Arginine Dihydrolase: neg

Explanation / Answer

1.The organism is non lactose fermenter but ferments glucose produces both acid and gas.

2. Based on the cultural characteristics and biochemical tests, this organism may be Hafnia alvei, belongs to the family Enterobacteriaceae. These are gram negative, motile rods.

3. For selective isolation and identification of Hafnia alvei, following media are recommended. They are EMB, MacConkey Agar, Hektoen Enteric Agar, SS or  red or pink colonies on XLD Agar.

a. The assay for glutamate decarboxylase (GAD) activity can be performed in a single-tube format. A change in color from blue to yellow is considered to be a positive test. Escherichia species, including E. coli and E. albertii, are glutamate decarboxylase (GAD) positive, while hafniae are glutamate decarboxylase (GAD) negative.

b. L-prolineaminopeptidase (LPA) activit: Among enteric bacteria, strong L-prolineaminopeptidase (LPA) activity is detectable only in H. alvei, Serratia species. The assay can be performed in tube or microtiter format or using commercially available kits. LPA is useful in distinguishing hafniae from E. cloacae and related groups and from some Serratia species.

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