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Leprosy ( Mycobacterium leprae )

Leprosy, or Hansen disease, is a curable infectious disease caused by Mycobacterium leprae. Widespread implementation of multidrug antibiotic therapy for leprosy in the 1980s dramatically decreased the burden of leprosy worldwide. There is no indication for physical or social isolation of people with leprosy. Sequelae of leprosy, including nerve damage, muscle weakness, and physical deformity, are related to delayed diagnosis and pathologic immune reactions that complicate…

Mycobacterium tuberculosis

Revised March 31, 2020 Revised March 29, 2021 and June 6, 2021 The term tuberculosis describes a broad range of clinical illnesses caused by Mycobacterium tuberculosis (or, less commonly, Mycobacterium bovis ). Tuberculosis (TB) is the leading cause of death due to a single infectious agent worldwide, and ranks ninth among all causes of death. In 2014 the World Health Assembly embraced a resolution to reduce…

Anaerobic Cocci and Anaerobic Gram-Positive Nonsporulating Bacilli

Both anaerobic cocci and anaerobic gram-positive nonsporulating bacilli belong to the commensal microbiota of the digestive tract, and some are members of the microbiota of the urogenital tract and skin. When the environment changes due to trauma, immunosuppression, or antimicrobial therapy, they can cause damage in a susceptible host and result even in life-threatening infections. The major genera of anaerobic cocci and gram-positive anaerobic nonsporulating bacilli…

Bacteroides, Prevotella, Porphyromonas, and Fusobacterium Species (and Other Medically Important Anaerobic Gram-Negative Bacilli)

Overview The genera Bacteroides, Porphyromonas, Prevotella, and Fusobacterium account for most infections caused by gram-negative anaerobic rods (GNARs). Bilophila and Sutterella also cause human infections, although they are less frequently encountered in clinical practice. These obligately anaerobic gram-negative bacteria colonize the oropharynx, gastrointestinal tract, and urogenital tract of humans. Several species from some of these genera are useful symbiotic bacteria, facilitating host metabolism and favorably shaping…

Diseases Caused by Clostridium

The genus Clostridium includes over 200 described species. Members of this genus participate in a variety of invasive and toxigenic infections. They can cause disease that is strictly toxin mediated, such as antibiotic-associated colitis (AAC) and foodborne botulism, or contribute to invasive infections, including bacteremia, clostridial myonecrosis (gas gangrene), and other suppurative infections driven by histotoxins and enzymes that destroy soft tissue. Historically, clostridial infections were…

Botulism (Clostridium botulinum)

Botulism and tetanus result from intoxication with the protein neurotoxins elaborated by two related species of Clostridium. The toxins are very similar in structure and function but differ dramatically in their clinical effects because they target different cells in the nervous system. Botulinum neurotoxins predominantly affect the peripheral neuromuscular junction and autonomic synapses and primarily manifest as weakness. In contrast, although tetanus toxin can affect the…

Tetanus (Clostridium tetani)

History Tetanus was well known to the ancients; descriptions by Egyptian and Greek physicians survive to the present. They recognized the frequent relationship between injuries and the subsequent development of fatal spasms. Gowers provided the quintessential description of tetanus in 1888. Tetanus is a disease of the nervous system characterized by persistent tonic spasm, with violent brief exacerbations. The spasm almost always commences in the muscles…

Clostridioides difficile (Formerly Clostridium difficile ) Infection

Revised June 20, 2022 Reviewed for currency November 8, 2020 The administration of antibiotics can be complicated by a number of unintended consequences, among which gastrointestinal side effects are quite common. Gastrointestinal symptoms occur in up to 25% to 50% of patients depending on the specific antimicrobial agent, patient population, and epidemiology. While the majority of these side effects are mild, consisting of minor antibiotic-associated diarrhea…

Anaerobic Infections: General Concepts

Anaerobic bacteria are a major component of the normal human microbiota (formerly termed the normal flora ) residing on mucous membranes and predominate in many infectious processes, particularly those arising from mucosal sites. These organisms generally cause disease after the breakdown of mucosal barriers and the leakage of indigenous flora into normally sterile sites. The predominance of anaerobes in certain clinical syndromes can be attributed to…

Lyme Disease (Lyme Borreliosis) Due to Borrelia burgdorferi

Revised November, 2019 Lyme disease or Lyme borreliosis, which is caused by the tick-borne spirochete Borrelia burgdorferi (sensu lato), occurs in temperate regions of North America, Europe, and Asia. It is now the most common vector-borne disease in the United States and Europe. The illness usually begins in summer (stage 1) with a characteristic expanding skin lesion, called erythema migrans (EM), which occurs at the site…

Relapsing Fever Caused by Borrelia Species

Relapsing fever is characterized by recurrent fevers with spirochetemia caused by organisms of the Borrelia genus. The spirochetes are broadly divided between endemic tick-borne species (tick-borne relapsing fever [TBRF]) and epidemic louse-borne species (louse-borne relapsing fever [LBRF]). TBRF occurs in North and South America, Europe, Africa, and Asia and can be caused by about 20 different species. In the United States, relapsing fever is endemic in…

Leptospira Species (Leptospirosis)

History A syndrome of severe multisystem disease, presenting with profound jaundice and renal function impairment, was described by Weil in Heidelberg in 1886. Other descriptions of disease that probably represent leptospirosis were made earlier, but the etiology cannot be definitively ascribed to leptospiral infection. Leptospires were first visualized in autopsy specimens from a patient thought to have had yellow fever but were not isolated until several…

Endemic Treponematoses

The endemic treponematoses comprise yaws, endemic syphilis, and pinta and are caused by Treponema pallidum subsp. pertenue, Treponema pallidum subsp. endemicum, and Treponema carateum . Nearly eliminated in the 1960s by an eradication campaign led by the World Health Organization (WHO), these relatively uncommon diseases are now often seen among children living in low-income and middle-income countries. The bacteria that cause the endemic treponematoses are morphologically…

Syphilis (Treponema pallidum)

Revised January 5, 2021 Syphilis is a complex systemic illness caused by the highly invasive, noncultivable spirochete Treponema pallidum. It holds a special place in the history of Western medicine because of its prevalence in modern times before the advent of penicillin, the many historical personages who had or are presumed to have had the disease, and its protean clinical manifestations, for which it came to…

Other Gram-Negative and Gram-Variable Bacilli

A large number of gram-negative aerobic bacilli have been reported to cause human infection. In this chapter, selected gram-negative and gram-variable organisms are discussed that have not been described in other chapters and are important in certain clinical or epidemiologic circumstances, are newly described, or present special problems of diagnosis or therapy. For some of the bacteria considered here, taxonomy is in a state of flux…

Klebsiella granulomatis (Donovanosis, Granuloma Inguinale)

Donovanosis is a chronic, progressive ulcerative disease of the skin and subcutaneous tissues that is caused by the encapsulated, gram-negative bacterium Klebsiella (formerly Calymmatobacterium ) granulomatis. The infection is also commonly referred to as granuloma inguinale, but because this term can easily be confused with lymphogranuloma venereum (caused by the invasive L-serovars of Chlamydia trachomatis ), many experts now recommend donovanosis as the preferred term. McLeod…

Bartonella, Including Cat-Scratch Disease

Revised March 27, 2021 Background and Classification of Bartonella Species The genus Bartonella is a member of the class Alphaproteobacteria and family Bartonellaceae, and it is closely related to the genera Brucella and Agrobacterium ; members of the family Rickettsiaceae are more distantly related. On the basis of genetic similarity, unification of the genera Bartonella and Rochalimaea as a single genus and the removal of the…

Capnocytophaga

The genus Capnocytophaga consists of nine or more gram-negative fastidious species that typically reside as normal flora in the oral cavities of humans and other vertebrates including dogs and cats. Members of this genus can cause serious life-threatening infections in humans including septicemia and meningitis as a direct consequence of inapparent injuries or penetrating trauma resulting from animal contact or bites, most often involving dogs. Persons…

Legionnaires’ Disease and Pontiac Fever

Reviewed for currency January 14, 2021 History Legionnaires’ disease (LD) is an acute pneumonic illness caused by gram-negative bacilli of the genus Legionella , the most common of which is Legionella pneumophila (Lp). Pontiac fever (PF) is a febrile, nonpneumonic, systemic illness closely associated with, if not caused by, Legionella spp. Legionellosis is the term that encompasses all diseases caused by, or presumed to be caused…

Rat-Bite Fever: Streptobacillus moniliformis and Spirillum minus

Rat-bite fever is a rare systemic febrile illness typically transmitted by the bite of a rat or other small rodent. The infection has a worldwide distribution and can be caused by either Streptobacillus moniliformis or Spirillum minus, bacteria commonly found in the oropharyngeal flora of rodents. Streptobacillary disease accounts for the vast majority of cases of rat-bite fever in the United States, whereas S. minus infections…