After a longer term fast, my initial foods are solely fermented ones for the next day or two.

Not exactly what you are looking for, but somewhat pertinent: http://www.ncbi.nlm.nih.gov/pubmed/19700553.

And I agree with Bill, that some of the population 'might' die off. You won't be eating any soluble fiber for them to chow down on. Though bacteria are resourceful little beings that I'm sure could survive for a good amount of time (while you fast) without dying (Source: Microbiology course*). From an adaption standpoint of early human beings, it wouldn't seem right for a good amount of the beneficial gut flora to die during an extended period without food ... would it? Just speculation from me though.

*if you'd like me to find the book title just let me know and I'll try to find it

This old study might apply http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1552988/pdf/gut00700-0056.pdf

I think your best study is this:

Mediterranean diet or extended fasting's influence on changing the intestinal microflora, immunoglobulin A secretion and clinical outcome in patients with rheumatoid arthritis and fibromyalgia: an observational study http://www.biomedcentral.com/content/pdf/1472-6882-5-22.pdf

Keep in mind that...

If you look up 'bacterial sporulation' you will find that many bacteria, when they have run out of food, will 'select' a few of their numbers to undergo sporulation (colony sporulation factor), and these spores are very stable over time and in varying acidities, 'waiting' for the right conditions to come back. Sporulation is used by some probiotics as a way to make sure that the desired strains make it through the stomach acid so that they might better colonize the intestines.

as RG73 points out, only some bacteria do this, but there are many ways for bacteria to survive for extended periods. DIG INTO http://en.wikipedia.org/wiki/Gut_flora for more details on huge variety.

The Arthur Andrew site says it pretty well while promoting their high-end probiotic Syntol:

Spore germination is a dependable means of delivering healthy flora to the intestine in less than ideal conditions. Most bacteria are susceptible to any acidic pH range, mainly thriving in the more alkaline lower GI, but spores are able to withstand a broader range of variables in regards to pH and temperature ranges. This improves the chances that the probiotic strain will properly germinate and begin the process of restoring intestinal balance.

@Adam,

the 'fasting' mentioned in the study was merely a reduced calorie diet consisting of fruit juice and vegetable soup. This would be very unlikely to significantly change the gut flora over an eight day period.

However from personal experience a 10 day extended fast (non-nutritive fluids and vitamins only) almost completely stops feces production. This indicates that bacteria numbers are massively reduced.

I have read that they pretty much stay the same, you might have a little bit of die off in population but the specific species and their proportions generally stay the same and will feed off the glucose you create from your own protein.