Rope is the most common type of bacterial bread spoilage, caused by bacteria of the genus Bacillus. It is becoming less common with the proliferation of food preservatives.
Rope is named after the soft, mushy texture of infected bread, which will form little 'ropes' of dough when pressed together and pulled apart. Ropey bread smells sweet, often being described as smelling like pineapple or melon, sometimes with overtones of old cheese. It causes the bread to appear molted, and become soft and sticky.
Rope is most commonly caused by the strains Bacillus subtilis and Bacillus mesentericus, but it can also be caused by B. licheniformis, B. megaterium, B. cereus, and perhaps other strains. The Bacillus bacteria occur in many places in nature; B. subtilis lives on grains, while B. mesentericus lives on potatoes and other starchy plants. These and other strains are also commonly present in compost, manure, and rotting gunk of all types. B. subtilis and B. licheniformis may sometimes produce enough toxins to cause food poisoning, although this has not been shown with other strains of Bacillus.
Bacillus is a tough bacterium, and it is not killed in the baking process; while the surface temperature of baking bread is enough to kill off almost anything (around to 400 degrees Fahrenheit), the center of bread hovers near the boiling point of water, and does not quite get to the point of sterilization. The rope germinates when the bread cools from baking, and starts to feed on the protein and starches in the bread. Within a few days the rope infection will become apparent, and eventually the bread will be rendered inedible.
Rope enters a bakery in shipments flour, and may spread to stores of sugar or yeast. Because it is not killed by baking, it can be very hard to eliminate once it gets a foothold. An infected bakery may need to be steam cleaned or treated with special chemicals to prevent the bacteria from spreading. Fortunately modern food management strategies and copious preservatives have gotten rope infection under control. Calcium propionate in particular is effective at preventing rope.
Rope is traditionally a summer problem, as the bacteria grows best in warm, wet conditions. Of course, bakeries tend to be warm, wet places, so it could often become a year-round problem. Traditionally the growth of rope has been inhibited by the addition of vinegar to bread (at a ratio of about one pound of vinegar per 100 pounds of flour), or by baking drier loafs, which denies rope of the moisture it needs to thrive. These days more subtle preservatives are preferred.
Infected food supplies were once a serious problem; during World War I America's food supply was threatened by a rope outbreak in the summer of 1917. These days it is still a problem in equatorial countries, particularly poorer countries, although even developed countries like South Africa have to battle rope outbreaks.
Rope was first described by Belgian botanist Emile Laurent in 1884; no doubt it existed before then, but we have no record of it. In 1906 E.J. Watkins identified Bacillus as the causative agent. In places and times where bread was eaten quickly or preserved by being baked in dry loaves (such as hard tack), rope may not have had a chance to form. It is possible that rope is largely a result of our decadent modern lifestyle, a problem only to people who bake large, moist loaves.