The classification of prokaryotic organisms is continually revised as new information is obtained (Buchanan and Gibbons, 1974). Among prokaryotes broadly called the bacteria are distinctive groups closely associated with arthropods: certain true or eubacteria, spirochetes, rickettsias, and mycoplasmas.
The last group was mentioned in the preceding discussion of viruses because, unitl recently,”insect-borne mycoplasmas have been confused with certain persistent plant viruses. The prokaryotes in this section all have cell walls.
Eubacteria are commonly found on the integument, in the gut, and in mycetocytes. Endosymbiotic bacteria occur in Orthoptera, Isoptera, Phthiraptera, Hemiptera, Coleptera, Hymenoptera, and Diptera. Certain larval calliphorid flies, known as “surgical maggots,” are exceptional in that the gut is sterile.
A bactericidal substance, allatonin, kills bacteria in midnight. Napoleon’s surgeon, Larrey, noted that healing was actually enhanced when neglected battle wounds were infected with maggots. Maggots were later used medically because they ingest bacteria and necrotic tissues.
Bacteria pathogenic to insects have been reviewed by Faust (1974). Infection is usually through ingestion. Important examples of bacterial diseases are milky disease of Japanese beetles, caused by Bacillus popilliae and B. lentimorbus, and the honeybee diseases, American foulboard, caused by Bacillus larvae, and European foulbrood, caused by Streptococcus pluton. Bacillus cereus and its variant, B. thuringiensis, produce crystals toxic to more than 182 species of pest insects, mostl” Lepidoptera caterpillars. Commercial preparations of bacteria and crystals are applied with regular spray equipment to trees and crop plants in the control of pests.
Bacterial diseases of plants are mostly transmitted mechanically by insects, but some seem to be endosymbionts of the gut and may be transovarially transmitted. Fire blight of pears, apples, and some 90 kinds of other trees is caused by Erwinia amylovora and mechanically transmitted by many insects, especially bee pollinators.
Human diseases caused by bacteria and mechanically transmitted by insects are dysenteiy bacteria, Shigella, and Salmonella, carried by muscid fifth flies, and tularemia, caused by Francisella tularensis and transmitted by the bite of tabanid deerflies.
The organism causing plague, or Black Death, Yersinia pestis, is transmitted biologically by fleas. Plague bacilli are naturally transmitted among wild rodents by rodent fleas.
While hunting or camping human beings may acquire infections by handling diseased rodents or rabbits. Urban populations are endangered when the pathogen infects domestic rodents such as the black rat, Rattus rattus, and its ectoparasite, the oriental rat flea, Xenopsylla cheopis. Bacteria multiply in the flea’s gut, forming a gelatinuous plug in the proventriculus. A “blocked flea” is unable to pass blood to the midgut and repeatedly attempts to suck.
The result is that plague bacilli are regurgitated into the wounds. As the disease spreads among rats, the susceptible rats die. Fleas leave cold rats and transfer to live rats or humans. Following infection of a human by flea bites, the lymph nodes become inflamed-hence the name “bubonic plague, ” referring to the swollen buboes. In the most deadly phase of an epidemic, bacilli are rapidly spread from human to human by inhalation of infected respiratory droplets.
Plague has devastated the populations of entire countries since ancient times and has played a decisive role in history. In addition to the staggering statistics of human deaths, the impact of this disease is reflected in works of art and literature. In The Decameron, Giovanni Boccaccio (1313-1375) describes plague in Florence in 1348. This was at the beginning of the great epidemic that swept away 25 million Europeans and contributed to social change in the fourteenth century.
The terror of plague is shown in art beginning in 1348 (Brossollet, 1971). Reproduced here is an engraving. “The Pest,” by Pierre Mignard (1610-1695).
The scene was created from Biblical accounts of the plague of David (II Samuel 24 and I Chronicles 21) and Thucydides’s description of the plague in Athens. Accurately illustrated are symptoms of unquenchable thirst (fountains in center background), insanity (delirious patient at left being restrained), and auxiliary buboes ruptured of lanced (armpit of woman in center foreground). The angel in the sky is pouring sulphurous fumes as a disinfectant. Smoke from torch and brazier were also intended to be purifying.
Spirochetes are slender, motile bacteria of spiral configuration. Some are associated with insects. Termites of the advanced Family Termitidae may depend on spirochetes as intestinal endosymbionts.
In the same genus as the spirochetes of syphilis, Treponema, are pathogens causing human skin diseases called pinta and yaws. These are transmitted mechanically from lesions by Hippelates (Chloropidae) eye gnats.
Rickettsiae are small, nonmotile bacteria that are spherical or rod- shaped. All are associated with arthropods some point in their natural cycle. Diptera, Hymenoptera, and ticks possess commensalistic or mutualistic forms.
The best-known rickettsiae are pathogens of vertebrates that are transmitted by bloodsucking fleas, lice, ticks, or mites. Epidemic, or louse-borne, typhus fever is caused by Rickettsia prowazeki and transmitted by human body lice, Pediculus humanus (Pediculidae).
The microbes multiply in the louse’s gut, passing out with feces. Human infection results when infected feces are inhaled or lice are crushed and the pathogens are introduced by fingers into skin abrasions or the eyes. Louseborne typhus spreads rapidly during times of social strife. Crowding and poor sanitation, as in war or famine, favour louse infestations. Typhus was partly responsible for the collapse of Napoleon’s army in 1812.
Murine, or flea-borne, typhus is caused by Rickettsia typhi. The disease is transmitted among rats by rat fleas such as Xenopsylla cheopis (Pulicidae) and others. Humans acquire infection in the same manner as in louse-borne typhus, i.e., through infected flea feces and crushed fleas.
Other important rickettsial diseases are transmitted by acarines: Rocky Mountain spotted fever caused by Rickettsia rickettsii and transmitted by ticks; and scrub typhus, caused by Rickettsia tsutsugamushi and transmitted by chiggers.
Until recently rickettsias have been considered confined to insect or vertebrate hosts. Several diseases of plants, previously thought to be persistent viruses, are now associated with rickettsialike organisms: Pierce’s disease of grapes, phony peach disease, and clover club leaf (Whitcomb, 1973). All are transmitted by leafhoppers. Clover club leaf has been shown to multiply in both the plant and insect.