Apparently, if you use your washing machine for low temperature washes for a long period, and you do not use bleach, slime builds up inside the machine.
“Using mostly low temperature washes, and using washing machine detergent that doesn’t contain bleach causes black mould, which thrive in warm moist places. Bleach in washing machine detergents (plus high temperatures) kills bacteria that can otherwise multiply inside a washing machine. Many “colour-friendly” detergents and liquid detergents do not have any bleach in them which is why they are “friendly” to coloured garments.
To prevent, or try to get rid of, black mould in washing machines, put the washing machine on a monthly boil wash using detergent that contains bleach*, and with no laundry in. If you normally use biological or liquid detergent, buy some normal detergent containing bleach and keep it just for the maintenance wash.
Also, leave the washing machine door open after washing to let it dry out. If the washing machine is in a place where it’s exposed to warm moisture such as in a steamy kitchen then you may need to actually dry out the drum and door seal manually and leave the door closed. At the end of the day mould needs moisture and warmth to grow.”
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* NB Try to avoid chlorine-based bleach. Use low-temperature bleach (made with borax) instead.