Question

What are the possible explanations why some biochemical tests resulted in different results than expected.

(1) lactose fermentation

(2) sucrose fermentation

(3) motility test

Answer 1

1. In lactose fermentation , lactose production is indicated by low PH. But temperature may also decrease PH. Carbondioxide level also decrease PH, which wrongly indicate the result value.

2.Over incubation will help the bacteria to degrade proteins and will result give false positive results. Sometimes , microbes cannot ferment sucrose but their some metabolic product may decrease PH and result in wrong end point .Do not overheat the Phenol red Carbohydrate fermentation broth. The overheating will result in breaking down of the molecules and form compounds with a characteristic color and flavour. The process is known as caramelization of sugar (the browning of sugar).

3 . If bacterial flagella are damaged due to heating, shaking, or other trauma. Such environmental shock will render the organism non-motile . This makes motility test result in different results than expected .