Passage VI

Bacteria break down sugars by fermentation. To study 2 fermentation pathways, researchers performed 2 experiments using broth that contained either the sugar sucrose or the sugar lactose. One of the fermentation pathways produces CO₂ gas and increases the acidity (lowers the pH) of the solution. The other pathway produces acid but not CO₂.

Experiment 1

Sucrose broth was added to 5 large test tubes. Next, phenol red (a pH indicator that is yellow if pH < 7, red if pH ≥ 7) was added to each large test tube. A Durham tube (a small test tube) was placed, inverted, in each large test tube to collect CO₂ (see Figure 1).

The large test tubes were capped, heated until the solutions were sterile, then cooled. One of 4 bacterial species (Species A-D) was added to each of 4 of the large test tubes. The procedure was repeated using lactose broth instead of sucrose broth. The 10 large test tubes (all containing solutions at a pH of 7) were then incubated at 37°C for 48 hr.

The large test tubes and Durham tubes were examined. If acid was produced, the solution was yellow. If no acid was produced, the solution remained red. If CO₂ was produced, a gas bubble was observed at the top of the Durham tube (see Table 1).

Experiment 2

Synergism occurs when 2 bacterial species act together to ferment a sugar by using a pathway that neither species can use alone. To investigate synergism, Experiment 1 was repeated, except that different pairs of bacterial species were added to each large test tube (see Table 2).