There are great differences in ability of bedding materials to support bacterial growth. The amount and availability of nutrients varies considerably. Paper has slightly fewer nutrients than sawdust, straw or corn stalks. Hardwood sawdust or shavings has more available nutrients than pine sawdust. The pine oil in pine sawdust has some antibacterial properties.
Washed sand has no nutrients to support bacterial growth.
However, this is only when sand is free of organic matter. Bedding particle size is an important factor in the speed of bacterial growth in organic bedding. The finer the chop or grind, the more rapidly the bacteria can grow. Ideally, fresh, dry bedding will have bacteria counts of less than 5,000 bacteria per ml of bedding before it is used in a stall. Regardless of what kind of bedding you are using, it is important that bedding management can keep bedding bacteria counts below 1 million bacteria per ml of bedding.