27

Minor Bacterial Pathogens

CHAPTER CONTENTS

BACTERIA OF MINOR MEDICAL IMPORTANCE

The bacterial pathogens of lesser medical importance are briefly described in this chapter. Experts may differ on their choice of which organisms to put in this category. Nevertheless, separating the minor from the major pathogens should allow the reader to focus on the more important pathogens while providing at least some information about the less important ones.

These organisms are presented in alphabetical order. Table 27–1 lists the organisms according to their appearance on Gram stain.

Abiotrophia

Abiotrophia species were formerly known as nutritionally deficient streptococci. They are members of the normal flora of the mouth and can cause subacute bacterial endocarditis.

Achromobacter

Achromobacter species are gram-negative coccobacillary rods found chiefly in water supplies. They are opportunistic pathogens and are involved in sepsis, pneumonia, and urinary tract infections.

Acinetobacter

Acinetobacter species are gram-negative coccobacillary rods found commonly in soil and water, but they can be part of the normal flora. They are opportunistic pathogens that readily colonize patients with compromised host defenses. Acinetobacter baumannii, the species usually involved in human infection, causes disease chiefly in a hospital setting usually associated with respiratory therapy equipment (ventilator-associated pneumonia) and indwelling catheters. Sepsis, pneumonia, and urinary tract infections are the most frequent manifestations. A. baumannii is remarkably antibiotic resistant, and some isolates are resistant to all known antibiotics. Imipenem is the drug of choice for infections caused by susceptible strains. Colistin is useful in carbapenem-resistant strains. Previous genus names for this organism include Herellea and Mima.

Actinobacillus

Actinobacillus species are gram-negative coccobacillary rods. Actinobacillus actinomycetemcomitans is found as part of the normal flora in the upper respiratory tract. It is a rare opportunistic pathogen, causing endocarditis on damaged heart valves and sepsis.

Aeromonas

Aeromonas species are gram-negative rods found in water, soil, food, and animal and human feces. Aeromonas hydrophila causes wound infections, diarrhea, and sepsis, especially in immunocompromised patients.

Alcaligenes

Alcaligenes species are gram-negative coccobacillary rods found in soil and water and are associated with water-containing materials such as respirators in hospitals. Alcaligenes faecalis is an opportunistic pathogen, causing sepsis and pneumonia.

Anaplasma

Anaplasma phagocytophilum is a member of the Rickettsia family that causes human granulocytic anaplasmosis (HGA). Disease is endemic in northeastern and north central states (e.g., Connecticut and Wisconsin). Distribution is similar to that of Lyme disease. Ixodes ticks are the main vectors. Rodents and dogs are important reservoirs. In HGA, granulocytes rather than mononuclear cells are infected, but the disease is clinically indistinguishable from that caused by Ehrlichia chaffeensis. The organism forms an inclusion body called a morula in the cytoplasm of infected cells. The morula, which is shaped like a mulberry, is indistinguishable from that formed by Ehrlichia. The diagnosis is made serologically by detecting a rise in antibody titer. Doxycycline is the treatment of choice. This organism was formerly known as Ehrlichia equi, and the disease it caused was formerly known as human granulocytic ehrlichiosis (HGE).

Arachnia

Arachnia species are anaerobic gram-positive rods that form long, branching filaments similar to those of Actinomyces. They are found primarily in the mouth (associated with dental plaque) and in the tonsillar crypts. Arachnia propionica, the major species, causes abscesses similar to those of Actinomyces israelii, including the presence of “sulfur granules” in the lesions.

Arcanobacterium

Arcanobacterium haemolyticum is a club-shaped gram-positive rod that closely resembles corynebacteria. It is a rare cause of pharyngitis and chronic skin ulcers. The pharyngitis can be accompanied by a rash resembling the rash of scarlet fever.

Arizona

Arizona species are gram-negative rods in the family Enterobacteriaceae; they ferment lactose slowly. Arizona hinshawii is found in the feces of chickens and other domestic animals and causes diseases similar to those caused by Salmonella, such as enterocolitis and enteric fevers. The organism is usually transmitted by contaminated food (e.g., dried eggs).

Bartonella quintana & Bartonella bacilliformis

Bartonella quintana is the cause of trench fever and also is implicated as the cause of some cases of bacillary angiomatosis. Trench fever is transmitted by body lice, and humans are the reservoir for the organism. B. bacilliformis

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