With antibiotic resistance increasing (Section 12.1),

Here is a tip:
Two species of bacteria that share a common resource engage in interspecific competition.

Explanation
The use of harmless bacteria in place of antibiotics takes advantage of two species interactions:

Competition between the harmful bacteria and harmless bacteria
Mutualism between the cattle and the harmless bacteria

In the first interaction, the bacteria involved share the same resources. Before the harmless bacteria are introduced to the cattle's gut, the harmful bacteria had access to all of the resources so its population could grow rapidly. Once the harmless bacteria are introduced, the available resources will be reduced because the harmless bacteria also consume the same resources. This results in the reduction of the population of the harmful bacteria. If the harmless bacteria are more efficient at taking resources, then they will outcompete the harmful bacteria.

In the second interaction, the harmless bacteria and the cattle benefit from each other. The bacteria are nourished by the cattle, and the cattle are protected from the effects of the harmful bacteria. Both species do not incur any harm in such an interaction.

Verified Answer
Harmless bacteria will compete with harmful bacteria for resources in the gut. If the harmless bacteria are better at taking resources than harmful bacteria, the population of harmful bacteria will be reduced or even eradicated.