Yeah, I think people ought to rule infections out if possible, keeping in mind a biopsy may not uncover all types of bacteria and fungi, some over-the-counter antifungals are not to be used on mucous membranes, and that several sufferers reported that a biopsy turned up nothing out of the ordinary.
I can only say that I tried two medicinal anti-fungal treatments for a month each, and natural microbicides for several months each, and my condition was unresolved, leading me to believe there is a different etiolgy for my condition.
There was a time that I had a small amount of white slime between the inner and outer part of the lip upon waking, but never have I had inflammation or redness. I don't get the white line of slime anymore. If you have both inflammation and white slime, plus discoloration of the skin that does not match what you've eaten, I think it's worth investigating the infection hypothesis.
I think these hypotheses have merit:
(a) the white slime is just hyper-moisturized dead skin cells
(b) leaving lip skin to build up over weeks may invite pathogens that feed off dead cells
(c) constantly exfoliating lip skin traumatizes the skin so much that immune response becomes inadequate to deal with new infections
(d) a rare super-pathogen is both undetectable with standard tests and resistant to standard treatments
(e) poor oral hygiene which leads to gingivitis weakens immune system and diverts body's natural healing processes to address chronic infection; bacteria from gingivitis may migrate to the lips
(f) chemicals or allergens in toothpaste and alcohol in mouthwash exacerbate the condition
I wish we could test these hypotheses more scientifically and not be stuck facing multiple Catch-22s. Remember, when we choose the lesser of evils we still wind up with evil.