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Streptococcus pyogenes

Revised December, 2019 Streptococcus pyogenes (group A Streptococcus; [GAS]) is one of the most important bacterial pathogens of humans. This ubiquitous organism is the most frequent bacterial cause of acute pharyngitis, and it also gives rise to a variety of cutaneous and systemic infections. Its unique place in medical microbiology stems from its propensity to initiate two nonsuppurative sequelae: acute rheumatic fever and poststreptococcal glomerulonephritis. The…

Staphylococcus epidermidis and Other Coagulase-Negative Staphylococci

The genus Staphylococcus, with more than 80 recognized species and subspecies ( ) is one of the most abundant microbes inhabiting normal human skin and mucous membranes. They infrequently cause primary invasive disease and are most commonly encountered by clinicians as contaminants of microbiologic cultures. However, because of relatively recent changes in the practice of medicine and changes in underlying host populations, coagulase-negative staphylococci, most…

Staphylococcus aureus (Including Staphylococcal Toxic Shock Syndrome)

Revised July 1, 2020 Staphylococcus aureus is a highly successful opportunistic pathogen. It is a frequent colonizer of the skin and mucosa of humans and animals (it is present in the anterior nares of up to 30% of the healthy human population) and can produce a wide variety of diseases. These diseases encompass relatively benign skin infections, such as folliculitis and furunculosis, and life-threatening conditions, including…

Introduction to Bacteria and Bacterial Diseases

Bacteria, the oldest forms of life on earth, are remarkably diverse and exist in astounding numbers. Diseases caused by bacteria include some of the most common infections in the world and some of the most important human scourges, past, present, and probably future. At the same time, each of us is colonized by as many bacterial cells as we have human cells in our bodies. In…

Ehrlichia chaffeensis (Human Monocytotropic Ehrlichiosis), Anaplasma phagocytophilum (Human Granulocytotropic Anaplasmosis), and Other Anaplasmataceae

Until 1987, infections by members of the family Anaplasmataceae, including the genera Ehrlichia, Anaplasma, and Neorickettsia, were known mainly as veterinary diseases ( Table 192.1 ). Canine ehrlichiosis was first described in 1935 by Donatien and Lestoquard in Algeria. This disease is produced by Ehrlichia canis, which is transmitted to dogs by Rhipicephalus sanguineus ticks. The disease is characterized by fever associated with the presence of…

Orientia tsutsugamushi (Scrub Typhus)

History The earliest clinical reports of possible scrub typhus date back to the Chinese manual Zhouhofang in 313 bc . In 1810 the association of a febrile illness with mite transmission was made in the Niigata Prefecture in Japan, which led to the first clinical definition of tsutsugamushi fever. Historically, the occurrence of scrub typhus was associated with its dominant presence in Asia, in an area…

Rickettsia typhi (Murine Typhus)

In 1922, Hone first described human infections “closely resembling typhus fever.” Since 1926, when Maxcy successfully identified murine typhus as a distinct clinical and epidemiologic entity, and 1931, when Dyer isolated a new typhus group named Rickettsia from rats and fleas, murine typhus has been recognized as a worldwide zoonotic problem. Often underrecognized and believed to be clinically mild, murine typhus may occur in epidemics or…

Rickettsia prowazekii (Epidemic or Louse-Borne Typhus)

Rickettsia prowazekii is the only rickettsial species that can cause devastating, naturally occurring epidemics capable of killing a substantial proportion of human populations infested with body lice. Epidemics are associated with conditions that prevent bathing and washing of clothes in hot water, such as war and poverty, natural disasters such as earthquakes and floods, displacement of populations, jails, and lack of hygiene. Historical accounts of typhus…

Coxiella burnetii (Q Fever)

Q fever remains an intriguing disease despite being studied for more than 80 years. Molecular diagnostics and genomic analysis are adding to our understanding of the disease. The large outbreak in the Netherlands, other outbreaks, and longitudinal studies from France are changing diagnostic and treatment approaches. Although progress has been made, the name Q fever (“Q” for query) remains appropriate, as many questions remain unanswered. Q…

Rickettsia akari (Rickettsialpox)

Rickettsialpox is a worldwide mite-borne rickettsiosis presenting as a febrile and vesicular eruption. It is caused by Rickettsia akari, associated with mice, and transmitted by its ectoparasite, the mite Liponyssoides sanguineus. Etiology Rickettsia akari is classified among spotted fever group rickettsiae based on antigenic and genetic data. Its genome has a size (1.23 Mb) similar to those of other Rickettsia species but its plasmid content varies, possibly…

Rickettsia rickettsii and Other Spotted Fever Group Rickettsiae (Rocky Mountain Spotted Fever and Other Spotted Fevers)

The spotted fevers comprise a large group of tick-, mite-, and flea-borne zoonotic infections that are caused by closely related rickettsiae. These include Rocky Mountain spotted fever (RMSF), boutonneuse fever, African tick bite fever, North Asian tick typhus, lymphangitis-associated rickettsiosis, Queensland tick typhus, Flinders Island spotted fever, Japanese spotted fever, tick-borne lymphadenopathy, Far Eastern spotted fever, flea-borne spotted fever, and rickettsialpox. Rickettsiae are emerging or reemerging…

Introduction to Rickettsioses, Ehrlichioses, and Anaplasmoses

Bacteriology Originally, all small gram-negative bacteria, associated (or not) with arthropods and strictly or facultatively intracellular, were considered Rickettsiaceae. The advent of 16S ribosomal RNA gene sequencing and phylogeny has deeply challenged this classification. The controversy has centered on how much difference between strains should constitute a subspecies. Among the agreed-upon changes, Orientia was created from an independent branch of its phylum. The Ehrlichia group has…

Genital Mycoplasmas: Mycoplasma genitalium , Mycoplasma hominis , and Ureaplasma Species

Taxonomy and Microbiology The clinically important genital mycoplasmas— Mycoplasma hominis, Mycoplasma genitalium, Ureaplasma urealyticum, and Ureaplasma parvum —belong to the Mollicutes class and Mycoplasmataceae family of bacteria. Mycoplasma spp. and Ureaplasma spp. are the only two genera in this family. In this chapter the trivial term genital mycoplasmas is used to refer to these genera collectively. Ureaplasma spp. are most prominently distinguished from Mycoplasma spp. by…

Mycoplasma pneumoniae and Atypical Pneumonia

Reviewed for currency January, 2020 The concept of atypical pneumonia antedated the start of the antibiotic era. At least as early as World War I it was recognized that “…in the larger number of cases observed in the [military] camps the pneumonia was of an atypical nature. The onset tended to be slower than that of the lobar pneumonia of civil life; the course more prolonged.…

Chlamydia pneumoniae

Chlamydiae are obligate intracellular bacterial pathogens whose entry into mucosal epithelial cells is necessary for intracellular survival and subsequent growth. Chlamydiae cause a variety of diseases in animal species at virtually all phylogenic levels, from amphibians and reptiles to birds and mammals. In 1999, Everett and coworkers reported a taxonomic analysis involving the 16S and 23S ribosomal RNA (rRNA) genes and found that the order Chlamydiales…

Psittacosis (Due to Chlamydia psittaci)

Reviewed for currency January 14, 2021 Psittacosis is a systemic infection that frequently causes pneumonia. Its relationship to bird exposure has been known for more than 100 years. In 1879, Ritter studied an outbreak in Switzerland and called it pneumotyphus. Morange applied the term psittacosis (from the Greek word for parrot) in 1892 after studying cases associated with sick parrots. In 1930, the organism was identified…

Chlamydia trachomatis (Trachoma and Urogenital Infections)

Revised August 2, 2021 Chlamydia trachomatis imposes a tremendous burden on human health because it is one of the most common causes of bacterial infection. Worldwide in 2011, an estimated 21 million people had active trachoma, which is an ocular C. trachomatis infection that is the leading cause of infectious blindness. The World Health Organization (WHO) estimated that in 2016, 1.9 million individuals were blind or…

Prions and Prion Disease of the Central Nervous System (Transmissible Neurodegenerative Diseases)

Reviewed for currency February, 2020 Prions are the transmissible agents of a class of neurodegenerative diseases of humans and other mammals. Prions differ from other infectious agents in that they contain no information-bearing nucleic acid. Rather, they are composed mainly, perhaps only, of abnormal conformations of a normally produced cell surface glycoprotein called the prion protein (PrP). These abnormal conformations of PrP are able to trigger…

Hepatitis E Virus

Revised March 29, 2022 Hepatitis E virus (HEV) is a ribonucleic acid (RNA) virus that causes a viral hepatitis that is indistinguishable from other forms of viral hepatitis. The World Health Organization (WHO) estimates a global incidence of 20 million cases of HEV infection annually, accounting for 3.3% of mortality from viral hepatitis. HEV infection most often results in an acute, self-limited disease that does not…