Anaerobic Infections

Anaerobic Infections
George C. Mejicano, MD, MS
Department of Medicine
University of Wisconsin Medical School

An Illustrative Case…
• A 24 year old man had been in good
health when he was awakened with
severe perineal pain.
• He sought medical attention and was
found to have massive scrotal edema.
• The following photo and x-ray were
taken three hours after presentation…

Illustrative Case (continued)…
• Soft tissue radiographs confirmed gas
in the tissues of the thigh, buttocks,
scrotum, and anterior abdominal wall.
• Surgical inspection revealed brownish
fluid in the scrotum, with gray, dullcolored,
friable fascia but normal
underlying musculature.
• Gram stain of the fluid revealed…

Illustrative Case (continued)…
• Intra-operative cultures were obtained
and they grew E. coli, Enterococcus
faecalis, Bacteroides fragilis, and
anaerobic streptococci.
• In addition to surgical drainage, the
patient was treated with ampicillin,
gentamicin, and clindamycin for 3
weeks. He had an excellent clinical
response and had a full recovery.

Definitions
• Aerobe
• Facultative anaerobe
• Anaerobe
• Microaerophile
• Aerotolerance

Aerobes
• Bacteria that can replicate only in the
presence of O 2 (which is the terminal
electron acceptor during respiration)
• Examples:
– Mycobacterium tuberculosis
– Pseudomonas species thrive in
oxygen (but they’re not technically
strict aerobes because they can
use nitrate as the final acceptor)

Facultative Anaerobes
• Bacteria that can replicate in
the presence or absence of O 2
• Very versatile organisms can
use respiration or fermentation
• Examples include many
common bacteria:
– Staphylococcus aureus
– E. coli

Anaerobes
• Bacteria that can replicate only in the
absence of O 2
• Fermenting organisms that use organic
molecules (e.g. pyruvate and acetyl-
CoA) as the terminal electron acceptor
• Generally lack cytochromes, catalase,
and superoxide dismutase
• Examples:
– Clostridium perfringens
– Fusobacterium nucleatum

Microaerophile
• Bacteria that require a reduced
level of O 2 , but are not anaerobic
• These organisms have carved out a
rather specialized niche, such as
the placental-uterine interface
• Example:
– Campylobacter jejuni grows well
in “campy gas” that consists of
85% N 2 , 10% CO 2 , and 5% O 2

Aerotolerance
• Term used to denote the resistance
of anaerobes to killing by exposure
to O 2
• Some organisms are very intolerant
of air and will die very quickly once
exposed to O 2
• Others, even some strict anaerobes,
are somewhat aerotolerant

Importance of Aerotolerance
1. Clinical specimens must be handled
optimally because failure to get a
specimen into an anaerobic medium
jeopardizes the validity of lab results
2. Allows organisms to “hang on” until
the environment turns anaerobic
– Example: B. fragilis possesses both
superoxide dismutase and catalase

Although many organisms can
grow in an anaerobic environment,
the discussion today will focus on
the true anaerobes (also known as
the “strict” or “obligate” anaerobes)

Clinically Significant Anaerobes
• Bacteroides fragilis
– Normal flora of the colon
• Prevotella melaninogenica
– Gingival niche
• Fusobacterium species
– Gingival niche
• Clostridium species
– Normal gut flora; some serious pathogens
• Peptostreptococcus species
– Normal flora of mouth, pharynx, & colon

Bacteroides fragilis

Fusobacterium nucleatum

Clostridium perfringens

Aerobic / Anaerobic Sites
• General Conditions
– Skin, muscle, airways, mouth, and
blood are all relatively aerobic sites
– Lumen of both the gastrointestinal
and genitourinary tracts are
relatively anaerobic sites
• Microenvironments
– These are clinically very significant

Concept of Microenvironments
• Aerobic or microaerophilic sites
occur within anaerobic areas
• Examples include the epithelial
surfaces of the GI and GU tracts:
– Intestinal epithelium can support
Pseudomonas aeruginosa
– Placental-uterine interface is ideal
for a microaerophile like C. jejuni

Concept of Microenvironments
• Anaerobic microenvironments occur
within aerobic areas
• Often generated by metabolism of
aerobes or facultative organisms
– Gingival crevices
– Tonsils, especially crypts
– Occluded bronchi (allows anaerobic
pneumonia)
– Necrotic area in soft tissues

Anaerobic Colonization
• Anaerobes colonize specific
areas and are generally
innocuous, even protective
–Gut
–Mouth
–Genitourinary tract

Normal Gut Flora
• Bacteroides species
– Rare in stomach,
10 3 -10 7 / g in ileum,
10 10 -10 12 / g in colon
– B. fragilis accounts for
< 10% of gut flora, but
is worst opportunist
• Bifidobacterium and
Peptostreptococcus
– Up to 10 12 / g each
• Clostridium species
– 10 6 -10 11 / g
• All facultative and
aerobic organisms
– 10 10 / g combined
• Bottom line is that
anaerobes comprise
> 99% of gut flora

Crucial Concept
Anaerobes occupy living space
(“lebensraum”) and prevent
overgrowth of gut pathogens
such as Clostridium difficile

Normal Mouth Flora
• Usually these anaerobes are harmless
flora that live in the gingival spaces
• Examples:
– Prevotella melaninogenica, Fusobacterium
species, and Peptostreptococcus species
• They may cause periodontal disease &
gingival abscesses, particularly in AIDS

Acute Necrotizing Gingivitis

Normal Genitourinary Flora
• This primarily consists of vaginal flora
since there are few urethral organisms
• Predominant species include:
– Lactobacillus species and
Streptococcus species
• Strict anaerobes present include:
– Prevotella species and
Peptostreptococcus species

Infection and Disease
• Almost all anaerobic infections are
accidental and due to either:
1. Opportunistic overgrowth of normal
flora, or
2. Accidental contamination of wounds
by environmental organisms
• Occasional “food poisoning”
outbreaks due to ingestion of a
preformed toxin (e.g. botulism)

Major Predisposing Condition
• Although anaerobes in blood
occur commonly after bowel
movements & brushing teeth,
establishing an infection nearly
always requires generation of
an anaerobic site

Predisposing Conditions
• Trauma
– Belly wound or a ruptured appendix leads
to spillage of fecal content into peritoneum
• Malignancy
– Example: lung cancer obstructing bronchi
• Loss of circulation
– Usually due to diabetes or vascular disease
• Occlusion of airways or blood vessels
– Examples: foreign body or emboli

Predisposing Conditions
• Unresolved abscess
– Progressively necrotic if left undrained
• Immune collapse
– Outgrowth of oral anaerobes seen in AIDS
• Antibiotic use
– Normal flora disruption allows anaerobic
opportunists such as C. difficile to flourish
• Prosthetic devices
– Propionibacterium acnes infects spine
hardware at poorly vascularized site

Clinical Syndromes
A. Tetanus
B. Botulism
C. Gas gangrene
D. Pseudomembranous
colitis
E. Peritonitis
F. Oral abscess and
periodontal disease
G. Human bite
wounds
H. Brain abscess
I. Aspiration and
necrotizing
pneumonia
J. Bacterial vaginosis,
endometritis and
septic abortion

Tetanus
• Clostridium tetani is found in soil and
the normal gut flora of some mammals
• Organism produces a potent exotoxin
that causes powerful, sustained muscle
contraction resulting in very painful
muscle spasms (“spastic paralysis”)
• Even tiny stimuli trigger spasms
• Common name of disease: “lock jaw”

Facial spasm and risus sardonicus in
a Filipino patient who has tetanus

Tetanospasmin
• Potent exotoxin, tetanospasmin (TS), is
produced by C. tetani at the site of the wound
• The toxin is internalized at the neuromuscular
junction into the alpha motor neuron
• TS then travels by retrograde axonal flow to to
the cell body and diffuses out into the synapses
and extracellular spaces of the CNS
• Its major effect is to inhibit transmitter release
from the presynaptic inhibitory neuron

Tetanus: Treatment and Prevention
• Treatment:
– Keep patient in dark, quiet room
with minimal stimuli
– Muscle relaxants also useful
• Disease now rare due to vaccine
• Predisposing conditions are very
common; thus, healthcare team
must regularly give Td boosters

Wound Classification
Clinical Features
Age of Wound
Configuration
Depth
Mechanism of
Injury
Contaminants
(dirt, saliva, etc.)
Devitalized Tissue
Tetanus Prone
> 6 hours
Stellate, avulsion
> 1 cm
Missile, crush,
burn, or frostbite
Present
Present
Non-Tetanus
Prone
< 6 hours
Linear
< 1 cm
Sharp surface
(glass or knife)
Absent
Absent

Tetanus Prophylaxis
History of
Tetanus Toxoid
Immunization
Clean, Non-
Tetanus Prone
Wounds
Td TIG
Dirty, Tetanus
Prone Wounds
Td TIG
Unknown or
< 3 doses
Yes No
Yes Yes
> 3 doses
No, unless No
10 years
since last
dose
No, unless No
5 years
since last
dose

Botulism
• Due to environmental bacteria: C. botulinum
– Types A, B, E cause majority of human illness
• Organism produces a potent exotoxin,
botulinin, that produces a loss of muscle
tone (descending “flaccid paralysis”)
• Causes distinct illnesses
– Food poisoning leading to flaccid paralysis
– Infant colonization leading to flaccid paralysis
– Infection due to wound botulism is very rare
• Trivalent A, B, E anti-toxin rarely helpful

59 Cases of Botulism: Interval Between
Exposure and Onset of First Neurologic
Symptoms (Michigan, 1977)

Flaccid Paralysis due to Mild Botulism:
Note Disconjugate Gaze, Asymmetric
Smile, and Minimal Smile Creases

Patient Position for Botulism

Gas Gangrene
• Due to Clostridium perfringens and other
Clostridium species
• Pathogenesis factors: most strains produce
α and σ toxins, but only selected strains
produce the enterotoxin of food poisoning
– Lethicinase (α toxin)
– Hemolysin (σ toxin)
• Predisposition: deep, necrotic wounds seen
in war injuries, compound fractures, MVA’s

Gas Gangrene
• Clinical picture: gas crepitation
(crackling sounds over soft tissue)
• Treatment:
– Immediate wound debridement
that may require amputation
– Antibiotics, including clindamycin in
order to decrease toxin production

Case One: Gas Gangrene
• A 35 year old man sustained a knife wound to
the forearm but did not seek immediate care;
36 hours later he experienced severe pain in
the upper arm and presented to the ER.
• The arm was extremely tender and crepitus
was easily demonstrated. A radiograph also
demonstrated gas in the soft tissues.
• Surgical debridement and antibiotics were
instituted, but later amputation at the level of
the shoulder was necessary.

Gram variable rods were seen on
tissue stains; specimens grew a
pure culture of C. perfringens

C. perfringens growing on blood agar in anaerobic environment. Sigma
toxin causes sharp zone of hemolysis nearest the colonies while alpha toxin
(that has lethicinase activity) causes larger zone of partial hemolysis.

Electron Micrograph of C. perfringens
[Note the Endospores]

Case Two: Gas Gangrene
• A 62 year old man suddenly developed
severe pain and swelling in his arm.
• He immediately sought medical attention
and was found to have crepitus over the
affected site. Radiographs confirmed
soft tissue gas.
• Immediate surgical debridement revealed
necrotizing fasciitis, sparing the muscle.

Case Two (continued):
• Cultures grew Clostridium septicum
• Because of the microbiology results, a
colonoscopy was done and this revealed
a mass in the colon. Surgical resection
revealed adenocarcinoma of the colon.
• The malignancy served as the portal of
entry and gas gangrene occurred after
the arm was seeded via the bloodstream.

Pseudomembranous Colitis
• Due to C. difficile, an organism considered
to be a minor member of the normal flora
• Implicated in a huge number of hospital
acquired (nosocomial) infections
• Predisposition: antibiotic disruption of
bowel flora allows organism to flourish
• Pathogenesis
– Toxin A: enterotoxin ordinary diarrhea
– Toxin B: cytotoxin pseudomembranous
colitis

Pseudomembranous Colitis
• Clinical appearance
– Semi-solid diarrhea, fever, abdominal
pain following antibiotic administration
– Pseudomembranes in colon comprised of
fibrin, WBC’s, and necrotic enterocytes
• Treatment
– Withdraw predisposing antibiotic
– Use metronidazole or oral vancomycin

Peritonitis
• Due to a mixture of endogenous
aerobes & anaerobes from the gut
– Anaerobe is commonly B. fragilis and
sometimes Clostridium species but
many others implicated
• Predispositions:
– Penetrating wounds from knives,
bullets, and shrapnel
– Ruptured appendix
– Gut surgery or trauma

Peritonitis
• Clinical appearance
– Inflamed peritoneum causes intense
pain and rebound tenderness
– Life-threatening problem
• Treatment
– Surgical debridement, drainage, and
irrigation, followed by repairing injury
– Broad spectrum antibiotics

Oral Abscess / Peridontal Disease
• Due to mixed aerobic, facultative,
and anaerobic flora
• Anaerobes typically seen include
Fusobacterium, Prevotella, and
Peptostreptococcus species
• Due to low virulence organisms that
act as opportunists as a result of
– Anatomic conditions
– Decreased immunity

Oral Abscess / Periodontal Disease
• Treatment
– Surgical debridement, drainage,
and irrigation
– Broad spectrum antibiotics,
typically penicillin together with
metronidazole

Vestibular
Abscess

Submandibular
Abscess from
Left 2 nd Molar

Bite Wounds
• Human bite wounds are often worse
than animal bite wounds
• Typically caused by Prevotella
melaninogenica, but polymicrobial
infections are common
• Clinical picture
– Very serious, ugly injuries that present
very late due to embarrassment factor

Bite Wounds
• Treatment
– Surgical debridement, drainage,
and irrigation
– Don’t close the wound!
– Broad spectrum antibiotics,
typically amoxicillin-clavulanate
– Consider tetanus prophylaxis

Brain Abscess
• Commonly caused by oral anaerobes
such as Prevotella, Fusobacterium,
and Peptostreptococcus species
• Usually occurs when a local infection
(sinusitis, otitis media, mastoiditis, or
a periorbital abscess) is not contained
• Life threatening infection that
requires prolonged IV antibiotics

Aspiration/Necrotizing Pneumonia
• Commonly caused by oral anaerobes
such as Prevotella, Fusobacterium,
and Peptostreptococcus species
• Typically due to aspiration of saliva
or other oral contents into lung
• Pathogenesis due to occlusion of the
airway followed by generation of an
anaerobic site in lung parenchyma
• Treat with long course of clindamycin

Lung
Abscess

Bacterial Vaginosis
• Common condition in women of child
bearing age
• Associated with Gardnerella vaginalis &
Mobiluncus species, as well as pH > 4.5
• Clinical picture:
– Vaginal discharge that has a fishy odor
– Presence of vaginal epithelial cells (“clue
cells”) confirms the diagnosis
• Rx: clindamycin or metronidazole

Endometritis and Septic Abortion
• Endometritis is essentially the
same thing as gangrene of the
uterus caused by C. perfringens
• Can occur after spontaneous
abortion as well as procedures
• Treat with surgical debridement
(D & C = dilation and curettage)
and antibiotics

Diagnosis of Anaerobic Infections
• Key is obtaining a good specimen
collected into anaerobic medium
• Gram stain sometimes helpful
• Identification may require
specialized procedures
– Gas chromatography in micro lab
– Toxin identification

Glove Box Used to Grow Anaerobes

Prevention of Anaerobic Infections
• Cleanliness helps!
– Aseptic technique, wash hands, and
appropriate antiseptics
• Early and aggressive debridement and
irrigation will save lives
• Use antibiotics appropriately
• Contact isolation for C. difficile
• Tetanus immunization

Treatment of Anaerobic Infections
• Source removal/containment
– Aggressive surgical debridement of
devitalized tissue and infected areas
– Drainage of abscess pockets
• Antisera (immunoglobulin)
– Tetanus immune globulin is important
– Possibly helpful for botulism
• Antibiotics with anaerobic activity

Thank You!!