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Anaerobic infections
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Common
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May be serious and life-threatening
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Anaerobes are the predominant components of
bacterial flora of normal human skin and mucous membranes
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Difficult to isolate from infectious sites and are
often overlooked. Isolation requires appropriate methods of collection,
transportation and cultivation of specimens
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Treatment is complicated by slow growth of these
organisms, by the commonly polymicrobial nature of the infection and by the
growing resistance of anaerobic bacteria to antimicrobial agents
Risk factors
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Exposure of sterile body sites to large innoculum
of indigenous mucous membrane flora
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Poor blood supply and tissue necrosis
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Trauma
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Foreign body
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Malignancy
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Surgery
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Oedema
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Shock
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Colitis
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Vascular disease
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Diabetes mellitus
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Splenectomy
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Immunocompromise
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Collagen vascular disease
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Previous infection with aerobic or facultative
organisms
Clinical features
Features suggestive of anaerobic infection include:
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Clinical condition predisposing to anaerobic
infection
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Infection after bites, adjacent to a mucosal
surface, related to use of antibacterials that are only effective against
aerobes, related to tumours or other destructive processes
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Infected thrombophlebitis
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Foul smelling discharge
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Necrotic gangrenous tissue and abscess formation
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Free gas in tissue
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Black discolouration of exudates (due to Bacteroides
melaninogenicus)
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“Sulphur granules” in discharge (due to Actinomyces
spp.)
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Bacteraemia or endocarditis with no growth on
aerobic blood cultures
Microbiology
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In order to obtain an accurate microbiological
picture of the infection great care must be taken with the collection,
transport and processing of specimens
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Even minimal contamination of a specimen with
normal flora can result in misleading results due to the large numbers of
anaerobic organisms present on skin and mucous membranes. Specimens that are
appropriate for anaerobic culture are those that can be obtained without
contamination:
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Blood
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Aspirates of body fluids
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Urine collected by suprapubic bladder
aspiration
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Abscess contents (preferably pus rather than a
swab)
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Deep aspirates of wounds
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Transtracheal aspirate, lung aspirate
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Protected specimen brush or bronchoalveolar
lavage specimen combined with quantitative microbiology
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Specimen should be transported to the laboratory
promptly, preferably in an anaerobic transporter
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Organism
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Site of infection
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Antibacterial therapy
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Gram +ve cocci
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Peptostreptococcus spp.
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Respiratory tract, intra-abdominal & soft tissue
infections
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Penicillin
Alternatives: clindamycin, chloramphenicol, cephalosporins
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Microaerophilic streptococci (not true anaerobes)
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Sinusitis, brain abscess
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Gram +ve non spore-forming bacilli
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Actinomyces spp.
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Intracranial abscess, chronic mastoiditis, aspiration
pneumonia, head & neck infections
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Propionibacterium spp.
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Shunt infections, infections associated with foreign body
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Bifidobacterium spp.
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Chronic otitis media, cervical lymphadenitis, intra-abdominal
infection
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Arachnia propionica
Eubacterium lentum
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Gram +ve spore-forming organisms
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Clostridium perfringens
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Soft tissue infection, food poisoning
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Penicillin
Alternatives: metronidazole, cefoxitin, clindamycin,
chloramphenicol
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Clostridium septicum
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Neutropaenic enterocolitis
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Clostridium ramosum
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Soft tissue infection
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Clostridium botulinum
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Botulism
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Clostridium tetani
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Tetanus
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Metronidazole
Alternatives: penicillin
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Clostridium difficile
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Colitis, antibiotic-associated diarrhoea
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Metronidazole
Alternative: vancomycin
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Gram –ve bacilli
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Bacteroides spp.
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Intra-abdominal infection, gynaecological infection, neonatal
infection
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Non-beta lactamase producing organisms
Penicillin.
Alternatives: metronidazole, chloramphenicol, clindamycin
Beta-lactamase producing organisms
Metronidazole, carbapenem, penicillin with beta-lactamase
inhibitor or clindamycin.
Alternative: chloramphenicol
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Prevotella spp
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Orofacial infection, aspiration pneumonia, periodontitis,
intra-abdominal infection, gynaecological infection
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Fusobacterium spp.
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Aspiration pneumonia
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Porphyromonas spp.
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Site of infection
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First-line
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Alternatives
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Intracranial
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Metronidazole (with a penicillin)
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Chloramphenicol
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Dental
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Clindamycin
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Metronidazole (with a penicillin), chloramphenicol
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Upper respiratory tract
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Clindamycin
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Chloramphenicol, metronidazole
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Abdominal
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Clindamycin, cefoxitin or metronidazole (all with an
anti-aerobic bacilli agent)
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Carbapenem, penicillin plus b
lactamase inhibitor
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Pelvic
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Cefoxitin (plus doxycycline), clindamycin (with an
anti-aerobic bacilli agent),
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Penicillin plus b
lactamase inhibitor, metronidazole (plus doxycycline)
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Skin
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Clindamycin or cefoxitin
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Metronidazole (with a penicillin and an anti-Staphyloccal
penicillin)
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Bone and joint
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Clindamycin or a carbapenem
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Chloramphenicol, metronidazole (with a penicillin) or
penicillin plus b
lactamase inhibitor
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Bacteraemia with b
lactamase producing organisms
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Carbapenem or metronidazole
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Cefoxitin or penicillin plus b
lactamase inhibitor
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Bacteraemia with non b
lactamase producing organisms
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Penicillin
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Clindamycin, metronidazole or cefoxitin
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