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There is a wide range of anaerobiosis, from ??
bacteria, even in the human host.

aerotolerant to extremely oxygen sensitive


Anaerobic growth depends upon: Environmental factors:

a. The level of free 02 (less is better),
b. Presence of reducing substances (sulfhydryl groups=cysteine, methionine, glutathione and iron), creates a favorable reducing environment - anaerobes grow better
c. The lower the pH, the better anaerobes grow. Necrotic tissue is acidic and
serves as a favorable nutrient-rich environment for anaerobes to flourish


Anaerobic growth depends upon: the agent

Genetic-encoded ability of the agent to survive at various levels of oxygen.
The ability of the agent to grow anaerobically via fermentation and/or using nitrate as terminal electron acceptor.


For an anaerobic infection in a human to occur, predisposing conditions are needed:
Compromised circulation/peripheral arterial insufficiency which results from: ??

a. Diabetes mellitus.
b. Trauma.
c. Tissue injury, tissue necrosis, surgery, etc.


For an anaerobic infection in a human to occur, predisposing conditions are needed: abscess

abscesses put limitations on effective chemotherapy
Usually, abscesses are areas of low O2 tension, low blood flow/perfusion which results in the following:


in abscesses, Infectious agents multiply more ??, thus minimizing effectiveness of ?

drugs which depend on synthesis of new components such as cell wall/peptidoglycan to work (e.g. penicillins, -lactams, protein synthesis inhibitors, etc.)


in abscesses, Lack of ?? leads to increased replication of anaerobic microorganisms (low O2).
also ??


abx administered may be unable to penetrate due to the abscess’s low blood flow/perfusion.


in abscesses, lack of perfusion can also lead to ??
tissue breakdown products include ??
how does it limit abx?

tissue necrosis -- Tissue breakdown products include abundant folic acid making sulfa drugs ineffective/decrease effectiveness of sulfonamides (inhibitors of folic acid synthesis in bacteria).

Tissue breakdown products/cellular components adhere/bind antibiotics, making them ineffective.
Most phagocytes in purulent lesions are dead, not new cells.


in abscesses, by-products of anaerobes (i.e. ??) inhibit ??

neutrophil migration into tissue


effectiveness of ?? is also depressed in an abscess as a result of the low pH which leads to ??
Their accumulation by bacterial cells requires presence of ?? (which anaerobes lack).


binding of the agent under anaerobic conditions to nucleic acid.

oxidative enzymes


Effective treatment of abscesses requires ??

incision and drainage, and removal of necrotic tissue


The source of the etiologic agent(s) of anaerobic infections are of mostly of
?? origin:

i.e., human body sites containing large numbers of anaerobic NF (polymicrobic flora) so expect polymicrobic infections.
a. NF of mouth/oropharynx
b. NF of gastrointestinal tract:
c. NF of the adult female vagina:
d. NF of skin and cornea.


The source of etiologic agent(s) of anaerobic infections can also be of ??sources are:


a. soil,
b. water,
c. food,

If exogenous origin, monomicrobic infections frequently occur with:
i. Clostridium botulinum,
ii. C. tetani,
iii. C. perfringens, type A (endogenous and exogenous).


The infections are frequently, but not always, ?? which consist of both ??

polymicrobic (mixed infections)
obligate and facultative anaerobes in a synergistic community


order of orgs in polymicrobic infections

1st: Aerobes, facultative anaerobes initiate infection.
2nd: Oxygen tolerant anaerobes begin to grow & produce toxins.
3rd: Oxygen intolerant anaerobes begin to grow and produce toxins.
4th: Effects on host defenses (e.g. inhibition of phagocytosis and killing by neutrophils).


Infections commonly involving anaerobic bacteria:

Bacteremia (rare today)
Ear, nose, throat, and mouth
respiratory tract
intraabdominal infection
OB-GYN infections
wound infections


anaerobic CNS infections

brain parenchyma - brain abscesses (nearly 1⁄2 of all abscess are due to mixed anaerobic infections and tend to be encapsulated [surrounded by intact brain tissue]) and are not meningitis, so anaerobic culture of CSF is not done, only aerobic culture of CFS.


anaerobic Ear, nose, throat, and mouth infections:

otitis media, sinusitis, gingivitis, periodontitis. *Usually subacute or chronic (not acute)* infection.


anaerobic RT infection:

Greatly underestimated in aspiration & necrotizing pneumonia.
*Usually a subacute or chronic infection* (not acute) infection.


anaerobic Intraabdominal infection (intraabdominal sepsis [IAS];

*B. fragilis - Anaerobe – primary agent*
Bilophila wadsworthia – Anaerobe.


OB-GYN infections (i.e., untreated STD) often progress to an ?

anaerobic infection.


anaerobic Wound infection agents:

S. aureus – facultative anaerobe
Streptococci – *aerotolerant anaerobe*
Eikenella – facultative anaerobe from human mouth, and/or dog and/or cat bite.
P. acnes, diphtheroids – *anaerobes*


skin/ST anaerobic infection agents

S. aureus and S. epidermidis – facultative anaerobe
Streptococci, – *aerotolerant anaerobe*
Enterococci – *aerotolerant anaerobe*
Enterics – *facultative anaerobe*
Peptostreptococcus sp. – *anaerobes*
Bacteroides sp.– *anaerobes*


skin/ST anaerobic infection high-yield complications

*Infected foot ulcer is the #1 cause of hospitalization of diabetics in the US AND infected DM foot ulcer is the #1 cause of amputation in the US*
*Other slow healing wounds are: compression ulcer (bed sores) and vascular statis (venous insufficiency ulcer)*


abscesses can occur in virtually any major organ (e.g., ??)

liver, bones
B. fragilis – Anaerobe.


anaerobic infection dx clue guidelines

If some or all of these clues are present, then you must include anaerobes in your differential and empiric antimicrobial coverage but remember these clues do not exclude facultative anaerobes, so your differential diagnosis and empiric antimicrobial coverage must include facultative anaerobes.


anaerobic dx clues: 1-4

1. contiguous infection with or in proximity to a mucosal surface normally harboring anaerobic NF means anaerobes could be the source of infection.
2. Foul-smelling discharge (may smell sweet). Many facultative anaerobes also produce a foul-smelling discharge. also occurs in many patients with pulm. anaerobic infections and is pathognomonic for anaerobic infection.
3. Severe tissue necrosis with fasciitis, abscess formation or gangrene.
4. Gas production in the tissue, especially the presence of blebs or crepitance in skin infections. However, many facultative anaerobes also produce gas.


anaerobic dx clues: 5-7

5. Gram stain of purulent exudate reveals polymorphic array of organisms, often with unique morphologic characteristics associated with specific anaerobic species.
6. Failure to recover organisms by aerobic bacteriologic techniques, especially when bacteria are seen on the Gram-stained smear.
7. Lack of response to antibiotics that are not active against anaerobes (e.g. aminoglycosides, occasionally penicillin, cephalosporins, tetracycline).


anaerobic infection dx: radiographic studies

(CT scan, MRI, X-rays) reveal abscess, (e.g., gas/liquid
interfaces in lung) and the extent to which gas has penetrated surrounding tissues.


Diagnosis of specimens (anaerobic infection) is dependent upon:

**Collection of clinically useful specimens (from appropriate body sites)**
Use of appropriate collection techniques, special anaerobic transport medium, and culture and Gram-stain within minutes of specimen collection.
Use of appropriate anaerobic culture techniques and media.


anaerobic infection gram stain

Use Modified Gram-stain: A modification specifically developed for staining anaerobes and for weakly staining gram-negative bacilli (Legionella spp., Campylobacter spp., Bacteroides spp. Fusobacterium spp., Brucella spp.) by using a carbol-fuchsin or basic fuchsin as the counter stain.

Antibodies specific for B. fragilis capsular antigen are available.


Specimens that should NOT be cultured anaerobically:

Throat swab
Gingival swab
Gastric contents
Small bowel contents Expectorated sputum Voided urine
Vaginal swabs


Specimens that SHOULD be cultured anaerobically:

Appropriately collected, foul-smelling discharge.
Blood, joint, pleural fluids.
An infection proximal to a mucosal surface.
Tissue fluid from septic thrombophlebitis.
Specimen form an infection following a human bite. Fluids from septic abortions, gastrointestinal surgery, etc. Infections associated with malignancy or other processes that result in tissue destruction.
Material obtained by needle and syringe aspiration of a closed abscess/fluid-filled space; e.g., suprapubic bladder aspirate for suspected anaerobic urinary tract infections; direct lung puncture from a lung abscess.


abscess tx: do all 3 of the following:

1. Drain abscesses surgically or percutaneously.
2. Surgically debride infected necrotic tissue & delay suturing, leave abscess
exposed to air
3. Use efficacious abx, must be functional at low pH and able to penetrate tissues.
-Abscess may contain facultative anaerobes +/- obligate anaerobes; use
antimicrobials that target both


two antibiotics drugs may be required for ??

polymicrobic anaerobic infections, i.e., one effective against facultative organisms and one against anaerobes


Examples of antibiotics used for anaerobic infections:

Meropenem (imipenem – a carbapenem)
Metronidazole (treats all anaerobic infections even by anaerobic eucaryote)


Meropenem (imipenem – a carbapenem): active against?? approved for ??

active against all G+ & G- pathogens, including anaerobes.
Approved for children and adults


all abx for serious anaerobic disease are given ??
Antibiotic ?? is not routinely done -


experimental labs determine resistance and this information is then utilized to determine recommended antibiotics.


B. fragilis anaerobic infections -- All strains produce ?? and some are ??

?? resistance has been reported n Europe and Asia, now in the USA.

β-lactamase and some are MDR.



Metronidazole resistance is due to ??
mechanism ??

due to importation of an infection acquired in Europe.
-Resistance is due to the presence of at least 1 of 5 nim nitroreductase genes,
which are only expressed under anaerobic conitions.


Metronidazole is a ??

prodrug which is activated by reduced ferrodoxin
(ferredoxin serves as the electron carrier by anaerobes). Reduced ferredoxin is generated by pyruvate oxido-reductase.


anaerobic infections with hyperbaric oxygen, useful??

hyperbaric oxygen has been used to treat anaerobic infections but utility is unsure.

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