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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.

Table 27–1Minor Bacterial Pathogens

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 607.


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 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.

Actinobacillus (Aggregatibacter)

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. A. actinomycetemcomitans has been renamed Aggregatibacter actinomycetemcomitans, but the former genus name Actinobacillus is often used.


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 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.


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” ...

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