Download 27. Minor Bacterial Pathogens

Minor Bacterial Pathogens: Introduction
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 27–1. Minor Bacterial Pathogens.
Type of Bacterium
Genus or Species
Gram-positive cocci
Micrococcus, Peptococcus, Peptostreptococcus,
Sarcina
Gram-positive rods
Arachnia, Arcanobacterium, Bifidobacterium,
Erysipelothrix, Eubacterium, Gardnerella,
Lactobacillus, Mobiluncus, Propionibacterium,
Rhodococcus
Gram-negative cocci
Veillonella
Gram-negative rods
Achromobacter, Acinetobacter, Actinobacillus,
Aeromonas, Alcaligenes, Arizona, Bartonella,
Calymmatobacterium, Capnocytophaga,
Cardiobacterium, Chromobacterium,
Chryseobacterium, Citrobacter, Corynebacterium
jeikeium, Corynebacterium minutissimum,
Edwardsiella, Eikenella, Erwinia,
Fusobacterium, HACEK group, Haemophilus ducreyi,
Hafnia, Kingella, Moraxella, Pleisomonas,
Porphyromonas, Pseudomonas pseudomallei,
Spirillum, Streptobacillus, Yersinia enterocolitica,
Yersinia pseudotuberculosis
Rickettsia
Ehrlichia
Unclassified
Tropheryma
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 calcoaceticus, the species usually involved in human
infection, causes disease chiefly in a hospital setting usually associated with
respiratory therapy equipment and indwelling catheters. Sepsis, pneumonia, and
urinary tract infections are the most frequent manifestations. Previous 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.
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 ofActinomyces
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
Bartonella species
henselae (formerly
are
pleomorphic
called Rochalimaea
gram-negative
henselae)
is
the
rods. Bartonella
cause
of bacillary
angiomatosisand cat-scratch fever. The organism is a member of the oral
flora of many cats. It is transmitted from cat to cat by fleas, but fleas are not
thought to be involved in cat-to-human transmission. Cat scratches or bites,
especially from kittens, are the main mode of transmission to humans.
Bacillary angiomatosis occurs in immunocompromised individuals, especially AIDS
patients. It is characterized by proliferative, vascular lesions resembling Kaposi's
sarcoma in the skin and visceral organs. Pathologic examination of tissue from
the lesion will distinguish bacillary angiomatosis from Kaposi's sarcoma. The
organism can be visualized in the biopsy tissue using the Warthin-Starry silver
stain. An immunofluorescence assay can also be used. The organism can be
cultured on artificial media but takes 5 days or longer to grow. This is not usually
done. Treatment with doxycycline or erythromycin is effective. (Bacillary peliosis
is similar to bacillary angiomatosis except that in peliosis the lesions occur
primarily in the liver and spleen.)
In
immunocompetent
people, B.
henselae causes cat-scratch
fever. This
disease is characterized by localized lymphadenopathy in a person who has had
contact with a cat. The diagnosis is supported by characteristic histopathology on
a lymph node biopsy specimen and an antibody titer of at least 1/64 in an
indirect immunofluorescence assay for Bartonella. The Warthin-Starry silver stain
reveals very few organisms. Cultures are not usually done. The disease is mild
and self-limited, and no antibiotic therapy is recommended.
Bartonella quintana (formerly called Rochalimaea 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. Bartonella bacilliformis causes two rare diseases: Oroya fever and
verruga peruana, both of which are stages of Carrión's disease. The disease
occurs only in certain areas of the Andes Mountains, and an animal reservoir is
suspected.
Bifidobacterium
Bifidobacterium eriksonii is a gram-positive, filamentous, anaerobic rod found as
part of the normal flora in the mouth and gastrointestinal tract. It occurs in mixed
anaerobic infections.
Branhamella
Branhamella
catarrhalis has
been
renamed Moraxella
catarrhalis (see Moraxella, below).
Calymmatobacterium
Calymmatobacterium granulomatis is a gram-negative rod that causes granuloma
inguinale, a sexually transmitted disease characterized by genital ulceration and
soft tissue and bone destruction. The diagnosis is made by visualizing the stained
organisms
(Donovan
bodies)
within
large
macrophages
from
the
lesion.
Tetracycline is the treatment of choice for this disease, which is rare in the United
States but endemic in many developing countries.
Capnocytophaga
Capnocytophaga gingivalis is a gram-negative fusiform rod that is associated with
periodontal disease, but it can also be an opportunistic pathogen, causing sepsis
and mucositis in immunocompromised patients. Capnocytophaga canimorsus is a
member of the oral flora of dogs and causes infections following dog bites. It also
can cause sepsis in immunocompromised patients, especially those without a
spleen.
Cardiobacterium
Cardiobacterium hominis is a gram-negative pleomorphic rod. It is a member of
the normal flora of the human colon, but it can be an opportunistic pathogen,
causing mainly endocarditis.
Chromobacterium
Chromobacterium violaceum is a gram-negative rod that produces a violet
pigment. It is found in soil and water and can cause wound infections, especially
in subtropical parts of the world.
Chryseobacterium
Chryseobacterium species
are
gram-negative
rods
found
in
soil
and
water. Chryseobacterium meningosepticum, the major pathogen in this genus, is
an opportunistic pathogen, causing meningitis and sepsis especially in premature
infants. In adults, it causes outbreaks of nosocomial pneumonia, especially in
intubated patients. It is resistant to most antibiotics but is noteworthy as the only
gram-negative
bacterium
that
is
susceptible
to
vancomycin.
The
genus Chryseobacterium was formerly called Flavobacterium.
Citrobacter
Citrobacter species are gram-negative rods (members of the Enterobacteriaceae)
related to Salmonella and Arizona. They occur in the environment and in the
human colon and can cause sepsis in immunocompromised patients.
Corynebacterium jeikeium
Corynebacterium jeikeium is a small gram-positive rod primarily found on the
skin of hospitalized patients. It causes sepsis in immunocompromised patients,
most often those who are neutropenic. Infections are often associated with
indwelling catheters and prosthetic heart
valves. The drug of choice is
vancomycin. Hospital-acquired strains are resistant to many other antibiotics.
Corynebacterium minutissimum
Corynebacterium
minutissimum is
a
small
gram-positive
rod
that
causes
erythrasma. Erythrasma is characterized by pruritic, scaly, brownish macules on
the skin of the genital region. The diagnosis is usually made by visualizing a
coral-red fluorescence with a Wood's lamp rather than by culturing the organism.
The drug of choice is oral erythromycin.
Edwardsiella
Edwardsiella species
are
gram-negative
rods
(members
of
the
Enterobacteriaceae) resembling Salmonella. They can cause enterocolitis, sepsis,
and wound infections.
Ehrlichia
Ehrlichia chaffeensis is a member of the rickettsia family and causes human
monocytic ehrlichiosis (HME). This disease resembles Rocky Mountain spotted
fever, except that the typical rash usually does not occur. High fever, severe
headache, and myalgias are prominent symptoms. The organism is endemic in
dogs
and
is
transmitted
to
humans
by
ticks,
especially
the
dog
tick, Dermacentor, and the Lone Star tick, Amblyomma. E. chaffeensis primarily
infects
mononuclear
leukocytes
and
forms
characteristic morulae in
the
cytoplasm. (A morula is an inclusion body that resembles a mulberry. It consists
of manyE. chaffeensis cells.) Lymphopenia, thrombocytopenia, and elevated liver
enzyme values are seen. In the United States, the disease occurs primarily in the
southern states, especially Arkansas. The diagnosis is usually made serologically.
Doxycycline is the treatment of choice.
Another form of ehrlichiosis, called human granulocytic ehrlichiosis (HGE), is
caused
by
an
organism
closely
resembling Ehrlichia
equi. Either Dermacentoror Ixodes ticks are the vectors. In HGE, granulocytes
rather than mononuclear cells are infected, but the disease is clinically
indistinguishable from that caused by E. chaffeensis. The diagnostic approach and
the treatment are the same for both forms of ehrlichiosis. E. equi has been
reclassified and is now known as Anaplasma phagocytophilum.
Eikenella
Eikenella corrodens is a gram-negative rod that is a member of the normal flora
in the human mouth and causes skin and bone infections associated withhuman
bites and "clenched fist" injuries. It also causes sepsis and soft tissue infections
of the head and neck, especially in immunocompromised patients and in drug
abusers
who
lick
needles
prior
to
injection. Eik.
corrodens is
also
called Bacteroides ureolyticus.
Erwinia
Erwinia species are gram-negative rods (members of the Enterobacteriaceae)
found in soil and water and are rarely involved in human disease.
Erysipelothrix
Erysipelothrix rhusiopathiae is a gram-positive rod that causes erysipeloid, a skin
infection that resembles erysipelas (caused by streptococci). Erysipeloid usually
occurs on the hands of persons who handle meat and fish.
Eubacterium
Eubacterium species are gram-positive, anaerobic, non-spore-forming rods that
are present in large numbers as part of the normal flora of the human colon.
They rarely cause human disease.
Fusobacterium
Fusobacterium species are anaerobic gram-negative rods with pointed ends. They
are part of the human normal flora of the mouth, colon, and female genital tract
and are isolated from brain, pulmonary, intra-abdominal, and pelvic abscesses.
They are frequently found in mixed infections with other anaerobes and
facultative
anaerobes. Fusobacterium
nucleatum occurs,
along
with
various
spirochetes, in cases of Vincent's angina (trench mouth).
Gardnerella
Gardnerella vaginalis is a facultative gram-variable rod associated with bacterial
vaginosis, characterized
by
a
malodorous
vaginal
discharge
and clue
cells, which are vaginal epithelial cells covered with bacteria. The "whiff" test,
which consists of treating the vaginal discharge with 10% KOH and smelling a
pungent, "fishy" odor, is often positive. However, trichomoniasis, which can also
cause a positive whiff test, must be ruled out before a diagnosis of bacterial
vaginosis can be made. The drug of choice is metronidazole. Mobiluncus (see
below), an anaerobic rod, is often found in this disease as well. Women with
bacterial
vaginosis
have
a
higher
incidence
of
preterm
deliveries
and,
consequently, a higher incidence of morbidity and mortality occurs in their
newborn children.
HACEK Group
This is a group of small gram-negative rods that have in common the following:
slow growth in culture, the requirement for high CO2 levels to grow in culture,
and the ability to cause endocarditis. They are members of the human
oropharyngeal flora and can enter the bloodstream from that site. The name
"HACEK" is an acronym of the first letters of the genera of the following
bacteria: Haemophilus
Actinobacillus
aphrophilus and Haemophilus
actinomycetemcomitans,
Cardiobacterium
paraphrophilus,
hominis,
Eikenella
corrodens, and Kingella kingae.
Haemophilus aegyptius
Haemophilus aegyptius (Koch-Weeks bacillus) is a small gram-negative rod that
is an important cause of conjunctivitis in children. Certain strains of H.
aegyptius cause Brazilian purpuric fever, a life-threatening childhood infection
characterized by purpura and shock. This organism is also known asHaemophilus
influenzae biogroup aegypticus.
Haemophilus ducreyi
This
small
gram-negative
rod
causes
the
sexually
transmitted
disease chancroid (soft chancre), which is common in tropical countries but
uncommon in the United States. The disease begins with penile lesions, which are
painful; nonindurated (soft) ulcers; and local lymphadenitis (bubo). The diagnosis
is made by isolating H. ducreyi from the ulcer or from pus aspirated from a lymph
node. The organism requires heated (chocolate) blood agar supplemented with X
factor (heme) but, unlike H. influenzae, does not require V factor (NAD).
Chancroid can be treated with erythromycin, azithromycin, or ceftriaxone.
Because many strains of H. ducreyi produce a plasmid-encoded penicillinase,
penicillins cannot be used.
Hafnia
Hafnia species are gram-negative rods (members of the Enterobacteriaceae)
found in soil and water and are rare opportunistic pathogens.
Kingella
K. kingae is a gram-negative rod in the normal flora of the human oropharynx. It
is a rare cause of opportunistic infection and endocarditis.
Lactobacillus
Lactobacilli are gram-positive non-spore-forming rods found as members of the
normal flora in the mouth, colon, and female genital tract. In the mouth, they
may play a role in the production of dental caries. In the vagina, they are the
main source of lactic acid, which keeps the pH low. Lactobacilli are rare causes of
opportunistic infection.
Micrococcus
Micrococci are gram-positive cocci that are part of the normal flora of the skin.
They are rare human pathogens.
Mobiluncus
Mobiluncus species are anaerobic gram-positive, curved rods that often stain
gram-variable.
They
are
associated
with bacterial
vaginosis in
women.Gardnerella (see above), a facultative rod, is often found in this disease
as well.
Moraxella
Moraxella species
are
gram-negative
coccobacillary
rods
resembling
neisseriae. M. catarrhalis is the major pathogen in this genus. It causes otitis
media and sinusitis, primarily in children, as well as bronchitis and pneumonia in
older people with chronic obstructive pulmonary disease. It is found only in
humans
and
is
transmitted
by
respiratory
aerosol.
Trimethoprim-
sulfamethoxazole or amoxicillin-clavulanate can be used to treat these infections.
Most clinical isolates produce -lactamase. Moraxella nonliquefaciens is one of the
two common causes of blepharitis (infection of the eyelid); Staphylococcus
aureus is the other. The usual treatment is local application of antibiotic
ointment, such as erythromycin.
Peptococcus
Peptococci are anaerobic gram-positive cocci, resembling staphylococci, found as
members of the normal flora of the mouth and colon. They are also isolated from
abscesses of various organs, usually from mixed anaerobic infections.
Peptostreptococcus
Peptostreptococci are anaerobic gram-positive cocci found as members of the
normal flora of the mouth and colon. They are also isolated from abscesses of
various organs, usually from mixed anaerobic infections.
Pleisomonas
Pleisomonas shigelloides is a gram-negative rod associated with water sources. It
causes self-limited gastroenteritis, primarily in tropical areas, and can cause
invasive disease in immunocompromised individuals.
Porphyromonas
Porphyromonas gingivalis and Porphyromonas endodontalis are anaerobic gramnegative rods found in the mouth. They cause periodontal infections, such as
gingivitis and dental abscesses.
Propionibacterium
Propionibacteria are pleomorphic, anaerobic gram-positive rods found on the skin
and in the gastrointestinal tract. Propionibacterium acnes is part of the normal
flora of the skin and can cause catheter and shunt infections. It is involved in
mixed infections associated with cat and dog bites and in head and neck
abscesses.
P. acnes is also involved in the pathogenesis of acne, a condition that affects
more than 85% of teenagers. The pathogenesis of acne involves impaction of the
sebaceous gland followed by inflammation caused by the presence of P.
acnes. The pustules of acne are composed of sebum, inflammatory cells such as
neutrophils
and
lymphocytes,
and
the
organism.
Antibiotics,
such
as
erythromycin, administered either topically or orally, are effective especially when
coupled with other agents such as benzoyl peroxide or retinoids.
Pseudomonas pseudomallei
Pseudomonas pseudomallei is a gram-negative rod that causes melioidosis, a rare
disease found primarily in Southeast Asia. The organism is found in soil and is
transmitted most often when soil contaminates skin abrasions. This disease has
been seen in the United States, because infections acquired by members of the
armed forces during the Vietnam war have reactivated many years later. The
acute disease is characterized by high fever and bloody, purulent sputum.
Untreated cases can proceed to sepsis and death. In the chronic form, the
disease can appear as pneumonia or lung abscess or may resemble tuberculosis.
Diagnosis is made by culturing the organism from blood or sputum. The
treatment of choice is ceftazidime, which is administered for several weeks. This
organism is also known as Burkholderia pseudomallei.
Rhodococcus
Rhodococcus equi is a gram-positive bacterium whose shape varies from a coccus
to a club-shaped rod. It is a rare cause of pneumonia and cavitary lung disease in
patients whose cell-mediated immunity is compromised. The diagnosis is made
by isolating the organism on laboratory agar and observing salmon-pink colonies
that do not ferment most carbohydrates. The treatment of choice is a
combination
of
rifampin
and
erythromycin.
(R.
equi used
to
be
called Corynebacterium equi.)
Sarcina
Sarcina species are anaerobic gram-positive cocci grouped in clusters of four or
eight. They are minor members of the normal flora of the colon and are rarely
pathogens.
Spirillum
Spirillum minor is a gram-negative, spiral-shaped rod that causes rat-bite fever
("sodoku"). The disease is characterized by a reddish brown rash spreading from
the bite, accompanied by fever and local lymphadenopathy. The diagnosis is
made by a combination of microscopy and animal inoculation.
Streptobacillus
Streptobacillus moniliformis is a gram-negative rod that causes another type of
rat-bite fever (see Spirillum, above).
Tropheryma
Tropheryma
whipplei is
the
cause
of
Whipple's
disease,
a
rare
disease
characterized by prolonged weight loss, diarrhea, and polyarthritis. Without
antibiotic treatment, it is ultimately fatal. Infiltrates of "foamy" macrophages in
affected tissue are commonly seen. The reservoir of the organism, its mode of
transmission, and pathogenesis are unknown.
The nature of this organism was unknown for many years. In 1992, it was
identified as an actinomycete when ribosomal RNA taken from bacilli seen in
duodenal
lesions
was
compared
to
ribosomal
RNA
of
other
bacteria. Tropheryma is an intracellular organism that has been grown in human
cell culture but that procedure is not used to diagnose the disease. Diagnosis of
the disease is typically made by PAS staining of biopsy specimens of the small
bowel in which inclusions are seen in the macrophages. PCR assays can also be
used for diagnosis. The drug of choice is trimethoprim-sulfa.
Veillonella
Veillonella parvula is an anaerobic gram-negative diplococcus that is part of the
normal flora of the mouth, colon, and vagina. It is a rare opportunistic pathogen
that causes abscesses of the sinuses, tonsils, and brain, usually in mixed
anaerobic infections.
Wolbachia
Wolbachia species are rickettsia-like bacteria found intracellularly within filarial
nematodes
such
as Wuchereria and Oncocerca (see
Chapter
56). Wolbachiarelease endotoxin-like molecules that are thought to play a role in
the pathogenesis of Wuchereria and Oncocerca infections. Wolbachia themselves
are not known to cause human disease but do infect many species of insects
world wide.
Yersinia enterocolitica & Yersinia pseudotuberculosis
Y. enterocolitica and Y. pseudotuberculosis are gram-negative, oval rods that are
larger than Yersinia pestis. The virulence factors produced by Y. pestis are not
made by these species. These organisms are transmitted to humans by
contamination of food with the excreta of domestic animals such as dogs, cats,
and cattle. Yersinia infections are relatively infrequent in the United States, but
the number of documented cases has increased during the past few years,
perhaps as a result of improved laboratory procedures.
Y. enterocolitica causes enterocolitis that is clinically indistinguishable from that
caused
by Salmonella or Shigella. Both Y.
pseudotuberculosis can
cause mesenteric
adenitis that
enterocolitica and Y.
clinically
resembles
acute appendicitis. Mesenteric adenitis is the main finding in appendectomies in
which a normal appendix is found. Rarely, these organisms are involved in
bacteremia or abscesses of the liver or spleen, mainly in persons with underlying
disease.
Yersinia infection is associated with two autoimmune diseases: reactive arthritis
and
Reiter's
syndrome.
Other
enteric
pathogens
such
as Salmonella,
Shigella, and Campylobacter also trigger these diseases. Reactive arthritis and
Reiter's syndrome are described further in Tolerance & Autoimmune Disease.
Y. enterocolitica is usually isolated from stool specimens and forms a lactosenegative colony on MacConkey's agar. It grows better at 25°C than at 37°C;
most biochemical test results are positive at 25°C and negative at 37°C.
Incubation of a stool sample at 4°C for 1 week, a technique called "cold
enrichment,"
increases
the
frequency
of
recovery
of
the
organism. Y.
enterocolitica can be distinguished from Y. pseudotuberculosis by biochemical
reactions.
The
laboratory
is
usually
not
involved
in
the
diagnosis
of Y.
pseudotuberculosis; cultures are rarely performed in cases of mesenteric adenitis,
and the organism is rarely recovered from stool specimens. Serologic tests are
not available in most hospital clinical laboratories.
Enterocolitis and mesenteric adenitis caused by the organisms do not require
treatment.
In
cases
of
bacteremia
or
abscess,
either
trimethoprim-
sulfamethoxazole or ciprofloxacin is usually effective. There are no preventive
measures except to guard against contamination of food by the excreta of
domestic animals.