The fasting process and the gut microbiome, how the process works in detail is explained here.
While fasting definitely impacts the gut, the idea that “bad bacteria die off” and “good bacteria survive” due to sugar dependency is not entirely accurate.

Here is a more detailed breakdown of what the science says about fasting and the gut:

1. Fasting Doesn’t Necessarily “Kill Off” BacteriBacteria Shift, Not Die-Off:

When you fast for 12+ hours, the most significant change isn’t a massive die-off of “bad” bacteria, but rather a shift in the microbial population. The bacteria that thrive on readily available food (like simple sugars from a recent meal) become less dominant.

Adaptation: Bacteria are highly adaptable. Instead of dying, they often go into a quiescent (dormant) state or switch to different, slower-metabolising food sources, such as mucus or shed host cells, until food arrives again.

2. Beneficial Bacteria and Fasting

Butyrate Producers Thrive: Some of the most beneficial gut bacteria, such as those that produce Short-Chain Fatty Acids (SCFAs) like butyrate, can actually benefit from fasting. These bacteria often feed on complex, undigested fibres that move slowly through the gut. More importantly, fasting allows the gut lining to repair, which is critical for their environment.

SCFA Production: SCFAs are a primary energy source for the cells lining the colon, and they are essential for gut barrier integrity and reducing inflammation. Fasting promotes conditions that support the function of these SCFA-producing bacteria.

3. The Role of Sugar and “Bad” Bacteria

Sugar-Feeders are Opportunistic: Many opportunistic or potentially harmful bacteria (like some species of Clostridium or E. coli) are, indeed, efficient at using simple sugars. When those sugars are suddenly unavailable due to fasting, their population size and activity decrease, making them less dominant in the community.

Starvation Resistance: It’s not that “beneficial” bacteria are inherently better at “not eating”; rather, different species have evolved to metabolise different substrates and handle periods of nutrient scarcity in various ways.

Conclusion
It’s more accurate to say that fasting shifts the balance of power in the gut:
It reduces the activity and relative abundance of sugar-dependent microbes.
It may favour beneficial, complex-carbohydrate metabolising microbes and helps heal the gut lining.
The entire microbiome doesn’t just clear out the “bad” guys; it rebalances in response to the lack of fresh nutrient input, which can be beneficial for overall gut health.