Propionibacterium acnes, P. acnes, is often blamed for acne. I suggest it may help prevent it by digesting sebum and giving it proper flow properties so that it can flow all the way out of the pores, which helps prevent clogging of pores, and by producing propionic acid. Fatty acids plus additional acid, enough to protonate a significant percentage of the fatty acids, is a fairly strong microbiocide in vitro. As the digested, acidified sebum flows out of the pores, it kills germs all along the way. More likely acne is due to improper sebum composition and inadequate secretion of sebum, due to poor diet and stress, with the consequence of inadequate activity of P. acnes, poor sebum flow, with attendant clogging, and the overgrowth of harmful bacterial species, with significant inflammation and pus formation.
Note also: P. acnes lives deep within pores, where it is probably more protected against the damaging effects of the potent combination of UV light + oxygen (a potent free radical generator, more powerful than either by itself, which I believe is also capable of eradicating/nearly eradicating V. zoster virus in the eye without antivirals). By blocking UV light, and possibly also some oxygen absorption, sunscreen is probably contributing to sparing more of the bacterial competitors nearer the surface of the pores. Another thing that must be corrected for acne control to be effective. Adequate exercise in the sun, without sunscreen, kills the germs near the surface of pores, with the potent synergy between UV light and molecular oxygen, reducing the activity of competitors of P. acnes, and the exercise in the hot sun increases the temperature of the skin, reducing sebum viscosity further, thus getting the sebum flowing toward the surface of the skin, protecting the surface of the skin as well as the entire length of the pores. (Combined with a proper diet, could 15 minutes of brisk walking, without sunscreen, around noon, at most latitudes, be enough to constitute an effective acne treatment?)
The same nutritionally adequate diet that Weston Price showed prevents tooth decay, without the use of heroic hygiene measures, likely also prevents acne with the proviso that the skin is exposed, without sunscreen, and with vigorous exercise of the body, to UV light plus oxygen to help P. acnes kill off its competitors who live nearer to the surface of the pores.
The same diet would also normalize an adolescent’s weight, given proper exercise. No need for a separate diet to normalize weight, control acne, and control tooth decay.