The pH of the small intestine and large intestine mirror each other in that they start out acidic and slowly move towards neutral pH, basic physiology.

pH
It is well established that a pH around neutrality (pH ~6·5) favours hyphal development of C. albicans in vitro, while a low pH (pH <6·5) blocks hyphal formation and stimulates growth of the yeast form (Buffo et al., 1984 ).
Transcription factors in Candida albicans – environmental control of morphogenesis
Joachim F. Ernst1
http://mic.sgmjournals.org/content/146/8/1763.full



Stationary phase cultures of Candida albicans inoculated into fresh medium at 37C synchronously form buds at pH 4.5 and mycelia at pH 6.5
http://www.jstor.org/pss/1610253


When stationary phase cells of the dimorphic yeast Candida albicans are diluted into fresh
medium at pH 4.5 (low pH), they synchronously form ellipsoidal buds, but when diluted into the
same medium at pH 6.7 (high pH), they synchronously form elongate mycelia.
A Comparison of Volume Growth during Bud and Mycelium Formation in
Candida albicans: a Single Cell Analysis
http://mic.sgmjournals.org/content/130/9/2219.full.pdf


Higher oxygen levels would destroy the intestinal flora. It's an anaerobic environment.

OXYGEN:
Most intestinal bacteria are oxygen sensitive, indicating that the conditions in the intestine are extremely anaerobic. If oxygen is able to enter the colon in greater amounts (e.g., during diarrhea), this might be expected to favor oxygen-resistant organisms at the expense of their more fastidious neighbors.
Diet and the Human Intestinal Bacterial Flora
M. J. Hill
http://cancerres.aacrjournals.org/cgi/reprint/41/9_Part_2/3778.pdf


The establishment of the intestinal microbiota commences at birth and new bacteria establish in succession during the first years of life until an adult-type highly complex microbiota has been achieved. The first bacteria to establish in the neonatal gut are usually aerobic or facultatively anaerobic bacteria, like enterobacteria, enterococci and staphylococci. During their growth they consume oxygen and change the intestinal milieu making it suitable for the proliferation of anaerobic bacteria. Bifidobacterium, Clostridium and Bacteroides are among the first anaerobes establishing in the microbiota. As more oxygen-sensitive species establish and the complexity of the microbiota increases, the population sizes of aerobic and facultative bacteria decline. This phenomenon is thought to result from oxygen depletion, substrate competition and the accumulation of toxic metabolites.
Factors influencing the establishment of the intestinal microbiota in infancy.
Nestle Nutr Workshop Ser Pediatr Program. 2008;62:13-29; discussion 29-33.
http://www.ncbi.nlm.nih.gov/pubmed/18626190?ordinalpos=6&itool=EntrezSystem2....


Bacteroides are specialists in the intestinal ecosystem as they are adapted to grow where there is no oxygen. Bacteroides are one of the most numerous of the intestinal microbes and we get to see a great many everyday as about 30 % of what comes out of the intestine consists of Bacteroides! They assist in breaking down food products and supply some vitamins and other nutrients that we cannot make ourselves. Bacteroides are found predominantly in the colon. Through their metabolic activities they participate in the complex physiology of the host. Bacteroides encode glutamine synthetase and glutamate dehydrogenase, which are important for ammonia assimilation but the regulation of these activities is not yet understood. Bacteroides have been found to play a major role in the biotransformation of bile acids, and contain many enzymes required for these reactions, including a hydrolase, dehydrogenase, and dehydroxylase.
http://borg.med.ecu.edu/~webpage/about.html


The results indicate that success of the facultative anaerobes in the intestine depends on their respiratory flexibility.
Respiration of Escherichia coli in the mouse intestine
http:// Infect Immun. 2007 Oct;75(10):4891-9. Epub 2007 Aug http://www.ncbi.nlm.nih.gov/pubmed/17698572?ordinalpos=2&itool=EntrezSystem2....