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Foundations in
Microbiology
Sixth Edition
Talaro
Chapter 20
The Gram-Negative
Bacilli of Medical
Importance
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• Large, diverse group of non-spore-forming
bacteria
• Wide range of habitats – large intestines
(enteric), zoonotic, respiratory, soil, water
• Most are not medically important; some are
true pathogens, some are opportunists.
• All have a lipopolysaccharide outer membrane
of cell wall – endotoxin.
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3
Aerobic Gram-Negative Nonenteric
Bacilli
• Pseudomonas and Burkholderia – an
opportunistic pathogen
• Brucella and Francisella – zoonotic
pathogens
• Bordetella and Legionella – mainly human
pathogens
• Alcaligenes – opportunistic pathogen
4
Pseudomonas: The Pseudomonads
• Small Gram-negative rods with a single polar
flagellum
• Free living
– primarily in soil, sea water, and fresh water; also colonize
plants and animals
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•
•
•
•
Important decomposers and bioremediators
Frequent contaminants in homes and clinical settings
Use aerobic respiration; do not ferment carbohydrates
Produce oxidase and catalase
Many produce water soluble pigments.
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Pseudomonas aeruginosa
• Common inhabitant of soil and water
• Intestinal resident in 10% normal people
• Resistant to soaps, dyes, quaternary ammonium
disinfectants, drugs, drying
• Frequent contaminant of ventilators, IV
solutions, anesthesia equipment
• Opportunistic pathogen
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Pseudomonas aeruginosa
• Common cause of nosocomial infections in hosts with
burns, neoplastic disease, cystic fibrosis
• Complications include pneumonia, UTI, abscesses,
otitis, and corneal disease
• Endocarditis, meningitis, bronchopneumonia
• Grapelike odor
• Greenish-blue pigment (pyocyanin)
• Multidrug resistant
• Cephalosporins, aminoglycosides, carbenicillin,
polymixin, quinolones, and monobactams
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Other Gram-Negative Aerobic Rods
• Genera Burkholderia, Acinetobacter,
Stenotrophomonas
• Similar to pseudomonads
• Wide variety of habitats in soil, water, and related
environments
• Obligate aerobes; do not ferment sugars
• Motile, oxidase positive
• Opportunistic
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Burkholderia
• Burkholderia cepacia – active in biodegradation
of a variety of substances; opportunistic agent in
respiratory tract, urinary tract, and occasionally
skin infections; drug resistant
• B. pseudomallei – generally acquired through
penetrating injury or inhalation from
environmental reservoir; wound infections,
bronchitis and pneumonia, septicemia
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Acinetobacter and Stenotrophomonas
• Acinetobacter baumanii – nosocomial and
community acquired infections; wounds, lungs,
urinary tract, burns, blood; extremely resistant –
treatment with combination antimicrobials
• Stenotrophomonas maltophilia – forms biofilms;
contaminant of disinfectants dialysis equipment,
respiratory equipment, water dispensers, and
catheters; clinical isolate in respiratory soft tissue,
blood ,CSF; high resistance to multidrugs
12
Brucella and Brucellosis
• Tiny Gram-negative coccobacilli
• 2 species:
– Brucella abortus (cattle)
– Brucella suis (pigs)
• Brucellosis, malta fever, undulant fever, and Bang
disease – a zoonosis transmitted to humans from infected
animals
• Fluctuating pattern of fever –weeks to a year
• Combination of tetracycline and rifampin or
streptomycin
• Animal vaccine available
• Potential bioweapon
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Francisella tularensis and Tularemia
• Causes tularemia, a zoonotic disease of mammals
endemic to the northern hemisphere, particularly
rabbits
• Transmitted by contact with infected animals, water
and dust or bites by vectors
• Headache, backache, fever, chills, malaise and
weakness
• 10% death rate in systemic and pulmonic forms
• Intracellular persistence can lead to relapse
• gentamicin or tetracycline
• Attenuated vaccine
• Potential bioterrorism agent
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Bordetella pertussis
• Minute, encapsulated coccobacillus
• Causes pertussis or whooping cough, a communicable
childhood affliction
• Acute respiratory syndrome
• Often severe, life-threatening complications in babies
• Reservoir – apparently healthy carriers
• Transmission by direct contact or inhalation of
aerosols
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Bordetella pertussis
• Virulence factors
– receptors that recognize and bind to ciliated
respiratory epithelial cells
– toxins that destroy and dislodge ciliated cells
• Loss of ciliary mechanism leads to buildup
of mucus and blockage of the airways.
• Vaccine – DTaP- acellular vaccine contains
toxoid and other Ags
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Alcaligenes
• Live primarily in soil and water
• May become normal flora
• A. faecalis – most common clinical species
– isolated from feces, sputum, and urine
– occasionally associated with opportunistic
infections – pneumonia, septicemia, and
meningitis
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Legionella pneumophila and
Legionellosis
• Widely distributed in water
• Live in close association with amebas
• 1976 epidemic of pneumonia afflicted 200 American
Legion members attending a convention in
Philadelphia and killed 29
• Legionnaires disease and Pontiac fever
• Prevalent in males over 50
• Nosocomial disease in elderly patients
• Fever, cough, diarrhea, abdominal pain, pneumonia
fatality rate of 3-30%
• Azithromycin
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Enterobacteriaceae Family
• Enterics
• Large family of small, non-spore-forming
Gram-negative rods
• Many members inhabit soil, water, decaying
matter, and are common occupants of large
bowel of animals including humans.
• Most frequent cause of diarrhea through
enterotoxins
• Enterics, along with Pseudomonas sp., account
for almost 50% of nosocomial infections.
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• Facultative anaerobes, grow best in air
• All ferment glucose, reduce nitrates to nitrites,
oxidase negative, and catalase positive.
• Divided into coliforms (lactose fermenters) and
non-coliforms (non-lactose fermenters)
• Enrichment, selective and differential media
utilized for screening samples for pathogens
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Antigenic Structures and Virulence
Factors
Complex surface antigens contribute to
pathogenicity and trigger immune response:
• H – flagellar Ag
• K – capsule and/or fimbrial Ag
• O – somatic or cell wall Ag – all have
• Endotoxin
• Exotoxins
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Coliform Organisms and Diseases
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Escherichia coli: The Most Prevalent
Enteric Bacillus
• Most common aerobic and non-fastidious
bacterium in gut
• 150 strains
• Some have developed virulence through
plasmid transfer, others are opportunists.
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Pathogenic Strains of E. coli
• Enterotoxigenic E. coli causes severe diarrhea due to
heat-labile toxin and heat-stable toxin – stimulate
secretion and fluid loss; also has fimbriae
• Enteroinvasive E. coli causes inflammatory disease of
the large intestine.
• Enteropathogenic E. coli linked to wasting form
infantile diarrhea
• Enterohemorrhagic E. coli, O157:H7 strain, causes
hemorrhagic syndrome and kidney damage; ID 100 cells
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Escherichia coli
• Pathogenic strains frequent agents of infantile
diarrhea – greatest cause of mortality among
babies
• Causes ~70% of traveler’s diarrhea
• Causes 50-80% UTI
• Coliform count - indicator of fecal contamination
in water
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Other Coliforms
Clinically important mainly as opportunists
• Klebsiella pneumoniae– normal inhabitant of
respiratory tract, has large capsule, cause of
nosocomial pneumonia, meningitis, bacteremia,
wound infections and UTIs
• Enterobacter sp. – UTIs, surgical wounds
• Serratia marcescens – produces a red pigment;
causes pneumonia, burn and wound infections,
septicemia and meningitis
• Citrobacter sp. – opportunistic UTIs and
bacteremia
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Noncoliform Lactose-Negative
Enterics
• Proteus, Morganella, Providencia
• Salmonella and Shigella
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Opportunists: Proteus and Its
Relatives
Proteus, Morganella, Providencia – ordinarily harmless
saprobes in soil, manure, sewage, polluted water,
commensals of humans and animals
– Proteus sp. - swarm on surface of moist agar in a concentric
pattern
– involved in UTI, wound infections, pneumonia, septicemia,
and infant diarrhea
– Morganella morganii and Providencia sp. involved in similar
infections
• All demonstrate resistance to several antimicrobials.
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Salmonella and Shigella
• Well-developed virulence factors, primary
pathogens, not normal human flora
• Salmonelloses and Shigelloses
– some gastrointestinal involvement and diarrhea
but often affect other systems
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Typhoid Fever and Other Salmonelloses
• Salmonella typhi – most serious pathogen of
the genus; cause of typhoid fever; human host
• S. cholerae-suis – zoonosis of swine
• S. enteritidis – includes 1,700 different
serotypes based on variation on O, H, and
capsular antigen
• Flagellated; ferments glucose
• Resistant to chemicals –bile and dyes
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Typhoid Fever
• Bacillus enters with ingestion of fecally contaminated
food or water; occasionally spread by close personal
contact; ID 1,000-10,000 cells
• Asymptomatic carriers; some chronic carriers shed
bacilli from gallbladder
• Bacilli adhere to small intestine, cause invasive
diarrhea that leads to septicemia
• Treat chronic infections with chloramphenicol or
sulfa-trimethoprim
• 2 vaccines for temporary protection
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Animal Salmonelloses
• Salmonelloses other than typhoid fever are called
enteric fevers, Salmonella food poisoning, and
gastroenteritis.
• Usually less severe than typhoid fever but more
prevalent
• Caused by one of many serotypes of Salmonella
enteritidis; all zoonotic in origin but humans can
become carriers
– cattle, poultry, rodents, reptiles, animal and dairy
products
– fomites contaminated with animal intestinal flora
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Shigella and Bacillary Dysentery
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•
•
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Shigellosis – incapacitating dysentery
S. dysenteriae, S. sonnei, S. flexneri and S. boydii
Human parasites
Invades villus of large intestine, can perforate
intestine or invade blood
• Enters Peyer’s patches instigate inflammatory
response; endotoxin and exotoxins
• Treatment – fluid replacement and ciprofloxacin and
sulfa-trimethoprim
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The Enteric Yersinia Pathogens
• Yersinia enterocolitica – domestic and wild
animals, fish, fruits, vegetables, and water
– bacteria enter small intestinal mucosa, some enter
lymphatic and survive in phagocytes; inflammation
of ileum can mimic appendicitis
• Y. pseudotuberculosis – infection similar to
Y. enterocolitica, more lymph node
inflammation
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Nonenteric Yersinia pestis and
Plague
• Nonenteric
• Tiny, Gram-negative rod, unusual bipolar
staining and capsules
• Virulence factors – capsular and envelope
proteins protect against phagocytosis and
foster intracellular growth
– coagulase, endotoxin, murine toxin
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Yersinia pestis
• Humans develop plague through contact with wild
animals (sylvatic plague) or domestic or
semidomestic animals (urban plague) or infected
humans.
• Found in 200 species of mammals – rodents,
without causing disease
• Flea vectors – bacteria replicates in gut, coagulase
causes blood clotting that blocks the esophagus;
flea becomes ravenous
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Pathology of Plague
• ID 3-50 bacilli
• Bubonic – bacillus multiplies in flea bite, enters lymph,
causes necrosis and swelling called a bubo in groin or
axilla
• Septicemic – progression to massive bacterial growth;
virulence factors cause intravascular coagulation
subcutaneous hemorrhage and purpura – black plague
• Pneumonic – infection localized to lungs, highly
contagious; fatal without treatment
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• Diagnosis depends on history, symptoms,
and lab findings from aspiration of buboes.
• Treatment: streptomycin, tetracycline or
chloramphenicol
• Killed or attenuated vaccine available
• Prevention by quarantine and control of
rodent population in human habitats
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Oxidase-Positive Nonenteric Pathogens
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Pasteurella multocida
Haemophilus influenzae
H. aegyptius
H. ducreyi
H. parainfluenzae
H. aphrophilus
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Pasteruella multocida
• Zoonotic genus; normal flora in animals
• Opportunistic infections
• Animal bites or scratches cause local
abscess that can spread to joints, bones, and
lymph nodes.
• Immunocompromised are at risk for
septicemia and complications.
• Treatment: penicillin and tetracycline
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Haemophilus
• Tiny Gram-negative pleomorphic rods
• Fastidious, sensitive to drying, temperature
extremes, and disinfectants
• None can grow on blood agar without special
techniques – chocolate agar.
• Require hemin, NAD or NADP
• Some species are normal colonists of upper
respiratory tract or vagina (H. aegyptius, H.
parainfluenzae, H ducreyi).
• Others are virulent species responsible for
conjunctivitis, childhood meningitis, and chancroid.
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Haemophilus
• H. influenzae – acute bacterial meningitis,
epiglottitis, otitis media, sinusitis, pneumonia,
and bronchitis
– subunit vaccine Hib
• H. aegyptius –conjunctivitis, pink eye
• H. ducreyi – chancroid STD
• H. parainfluenzae and H. aphrophilus – normal
oral and nasopharyngeal flora; infective
endocarditis
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