Washing machine odor, especially in the new front loading clothes washers, can easily be avoided by using the best detergent to reduce odor. It is necessary for high efficiency washers to use HE detergent. HE stands for high efficiency which means low sudsing.
Suds left by overuse of detergent and softeners will feed bacteria and mold to make your washer smell.
Suds also can change the surface tension of water which allows it into smaller spaces such as the spin bearing in back of the stainless inner tub. Moisture will ruin the sealed bearing and, although it may have a limited warranty, the labor to put it in is expensive.
Powder detergent causes odor much less than liquid. As with any business, detergent makers are in business to make money. The vast majority of store shelves are stocked with liquid detergent because detergent makers push liquid as it is less expensive to manufacture. Everything from drying times to transportation costs to the necessity of exhaust fans in factories to protect the workers affect manufacturing costs.
There are all natural detergent powders that clean very well. Liquid detergent fillers will accumulate deeper inside the washer and studies show some to be carcinogenic. Detergents in powder form are much less prone to cause odor compared to liquid. Powder can be used in liquid dispensers. If the soap is not dissolving too much is being used.
Whatever type of detergent is used, it's necessary to use no more than 1 tablespoon unless washing very soiled clothing. Studies show that in normally soiled laundry there is almost no difference between using recommended amounts and not using detergent at all! Detergent companies are also notorious for advising to use too much detergent in order to increase profit.