What are the red streaks in my toilet occasionally?
This article suggests it might be a bacteria known as Serratia marcescen


  if you’ve noticed a pink or red slimy
  substance forming in your toilet bowl,
  you’re not alone.
  
  It is bacteria known as Serratia
  marcescens, according to Roxanne
  Johnson, North Dakota State University
  Extension Service water quality
  associate.
  
  You may find this bacterium in moist
  areas such as toilet bowls, sinks,
  tiles, shower stalls, bathtub
  enclosures and even your pet’s water
  dish. The bacteria will grow in places
  with materials containing phosphorus
  or fatty substances such as feces
  residues in your toilet or soap
  residue in a pet’s dish or the
  bathtub, shower and sink.
  
  Occasionally the pinkish film appears
  during and after new construction or
  remodeling activities. You see it more
  commonly when you have your windows
  open during the summer months.
  
  “I have observed this phenomenon form
  in the toilet bowl along the water
  line and at the openings where the
  water enters the toilet bowl, usually
  when I’m gone for a few days, or in my
  guest bathroom that isn’t used often,”
  Johnson says.
  
  “The bacteria survive there because
  the water sits for a period of time,
  with the chlorine dissipating as it
  stands,” she adds. “If you have an
  activated carbon filter on your water
  line, you are removing the chlorine
  and may be supporting the growth of
  the bacteria.”
  
  While the genus Serratia is not known
  to be a water-borne disease, it has
  been known to be pathogenic to some
  people, causing pneumonia, wound
  infections and urinary tract
  infections in some hospital settings.
  
  The bacteria are difficult to remove
  completely from your home, although
  the stains are removed easily with a
  general-purpose cleanser containing
  chlorine bleach. You also may add ¼
  cup of bleach to the toilet tank, let
  it sit for 15 to 20 minutes and then
  flush the tank a few times to remove
  all of the bleach. Do not leave bleach
  sitting in your tank because it will
  damage rubber valves and seals.
  
  Johnson recommends you scrub with a
  brush and household cleaner to clean
  pet bowls, kitchen surfaces and
  bathroom sinks, bathtubs and showers.
  Follow this up with a strong chlorine
  bleach solution, leaving the bleach in
  contact with the surfaces for 10 to 20
  minutes to disinfect the area. Then
  rinse well with water.
  
  To prevent the bacteria from
  developing, wipe down and dry all
  sinks and bathtubs after using them,
  and use a cleaning solution that
  contains bleach.