Chen Chao-Huei, M.D.  asks about persistent nipple cracks - here's my
experience:

I've found that persistent and unhealing nipple cracks, especially at the
base of the nipple, are usually infected with yeast (candida), bacteria
(staph), or both. I've had good results with treating for either or both with
topical antifungals/antibacterials at the advice of a friendly MD. Often
these cracks are hard to culture, and there may be no other visible signs of
infection on the nipple or in baby's mouth. An unhealed crack IS the
presenting symptom.

I've also found that yeast-infected nipple skin is very fragile and cracks
much more easily when a baby wiggles too much while breastfeeding, or the
suck is just a bit imperfect.  Try treating mom and baby aggresively for
yeast first, and if there are no results in 3-4 days, go for antibacterial
medications.

Furthermore, if something cracks the nipple skin accidentally, like the
baby's new tooth, the crack quickly get infected - usually with yeast. Yeast
is so easy for moms and babies to get - I sometimes think they get it simply
because it's Thursday, or the cat needs a bath, or mom had a beer.

Linda Smith, BSE, FACCE, IBCLC - LC in private practice in humid hot Dayton
OH, who suffered through yeast with a still-breastfeeding 2 year old, while I
was pregnant with #3, and still remember the pain 20-plus years later.