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Anaerobic BacteriaAnaerobic Bacteria

Obligate AnaerobesIn the presence of oxygen, anaerobic bacteria produce toxic products such as superoxide and hydrogen peroxide, but they lack superoxide dismutase (SOD), catalase and peroxidase that detoxify these products:

SOD: O2-+2H+ H2O2

Catalase: H2O2 H2O + O2

Peroxidase: H2O2 H2O /NAD to NADH

No oxidative phosphorylationInfection caused by anaerobes usually occurs in the sites with low oxidation-reduction potential such as periodontal pocket, intestinal tract and vagina. Anaerobes are co-infected with other facultative anaerobe which use up oxygen to establish a local anaerobic environment.

Source of anaerobic infection

Endogenous infection: caused by anaerobes of normal flora which are non-spore formers. Exogenous infection: The pathogens are usually anaerobic spore-formers and come from the environment (e.g., soil). Clostridia is the unique genus of anaerobic spore-formers to cause human diseases.

ClostridiaThere are at least 118 species, the clinically important species: Clostridia tetaniClostridia perfringensClostridia botulinumClostridia difficile

Clostridium tetani

sporespore

vegetativevegetative

Gram-positive rods that form terminal spores

Culture: clustering growth with hemolysis occurs on blood agar.

Biochemical activity: does not ferment any carbohydrates.Resistance: spores but not its vegetative form can tolerate boiling for 60 min and stay alive for several ten years in soil.

Clostridia tetani is found in soil. It is occasionally found in intestinal flora of humans and animals.Clostridia tetani is the cause of tetanus when the spores enter wounds.

Spore-contaminated soil or other foreign objects

•Narrow deep wounds with limited blood and oxygen supply•co-infection with facultative anaerobe can use up the local oxygen

Spores germinate and then produce tetanus toxins.

The organism still remains in the local wound.

Process to cause tetanus

Clostridia tetani produces two exotoxins called tetanus toxins:

Tetanolysin: its virulent role remains unknown.Tetanospasmin: it is a neurotoxin with strong toxicity and the most important virulent factor of the bacterium.

one heavy chain (H chain)C end ： binds to the ganglioside receptors of inhibitory neuronesN end ： helps in entrance to the cells

one light chain (L chain)It contains a zinc endopeptidaseBlocks the release of inhibitory neuronal mediators -GABA (- 氨基丁酸 ) and glycin ( 甘氨酸 )Stops inhibitory nerve impulse to skeletal muscles, resulting in persistent muscle contraction.

Pathogenesis of tetanospasmin

Typical symptoms include sardonic smile, lockjaw, neck rigidity, opisthotonos and dyspnea

This baby has tetanus. The infection is usually caused by exposing to Clostridia tetani when cutting umbilical cord.

This the neonatal tetanus patient displays sardonic smile, lockjaw and dyspnea

The adult tetanus patient shows The adult tetanus patient shows opisthotonos

Among all animal species, horses and humans, are most susceptible to tetanospasminIf not treated in time, about 20% of the patients are died of suffocation and respiratory failure

The wound is treated by debridement ( 清创术 ) to destroy anaerobic environment.

Although antibiotics (streptomycin and erythromycin) are used as part of the treatment, tetanus patients must be promptly treated with tetanus antitoxin (TAT) to neutralize free tetanospasmin.

1500 ~ 3000 U for prevention100.000 ~ 200.000 U for therapy

Tetanus toxoid is a component of DPT vaccine (diphtheria toxoid, killed whole cell pertussis, tetanus toxoid).

ClostridiaThere are at least 118 species, the clinically important species: Clostridia tetaniClostridia perfringensClostridia botulinumClostridia difficile

• Gram-positive rods, have capsule, can form terminal

spores

• Form double hemolysis circles on blood agar plates.

• The α-hemolysis is caused byα-toxin while the β-hemolysis byθ-toxin.

Stormy fermentationClostridium perfringens shows “stormy fermentation” in litmus milk. Acid turns the pH indicator litmus from blue to pink. The acid and enzymes coagulate proteins to curd. The gas generated in the milk breaks the coagulated proteins.

Diseases Wound contaminated by soil (main source) and mammalian fecesGas gangrene refers to serious tissue swelling due to release of gas (fermentation product of the bacterium) and tissue necrosis

The death can occur within 2 days if untreated. Treatment includes debridement, antitoxin and antibiotic therapy.

Common diseases:•Gas gangrene

•Food poisoning

Other diseases:•Necrotizing Enteritis•Cellulitis ( 蜂窝织炎 )•Septicemia

Diseases

Food poisoning Marked hypersecretion in jejunum and ileum with loss of fluids and electrolytes in diarrhea.

Necrotizing enteritis an acute necrotizing process in the jejunum with symptoms of abdominal pain, bloody diarrhea and peritonitis. The death rate of this disease is as high as approximately 50%.

Virulent factorsClostridium perfringens produces over 10 types of toxins. Some toxins are hemolytic, proteolytic, saccharolytic enzymes. Some are lethal and necrotic.alpha-toxin is the most important, it lyses erythrocytes, platelets, leukocytes and endothelial cells.

According to antigenic differences of 4 major toxins, the bacterial strains can be divided into A~E toxic types.Type A is clinically the most important. Type A can also produce enterotoxin to cause food poisoning.

Morphology Stormy fermentationalpha-toxin (lecithinase)

Debridement (Gas gangrene) A large dose of antibiotics (penicillin)Antitoxin against alpha-toxin and hyperbaric oxygenation ( 高压氧疗法 )No vaccine is available

ClostridiaThere are at least 118 species, the clinically important species: Clostridia tetaniClostridia perfringensClostridia botulinumClostridia difficile

Clostridium botulinum

Clostridium botulinum is a Gram-positive, anaerobic, spore-forming bacillus.

It produces an enterotoxin (botulinum toxin) that causes food poisoning (botulism).

According to the antigenicity of botulinum toxin, The microbe can be divided into A~G types.

Among the 7 types, type A and then type B strains cause most disease.

The foods that are easily contaminated by

the toxins or spores are sausage and canned

foods abroad, and fermented-bean

preparations in China.

In adult, the toxin or spore causes botulism

(death rate 50-70%).

In infant (especially younger than 6

months), the toxin or spore causes infant

botulism (death rate 1-2%) .

Pathogenesis of botulinum toxin

Botulinum toxin is a nuerotoxin. It binds to other non-toxic proteins to form a complex. It is released, when the bacteria are dead and broken.

When the complex enters intestines, the alkaline condition makes botulinum toxin released.

Botulinum toxin binds the cellular receptor of neuromuscular junction and then enters the cells to block the release of acetylcholine ( 乙酰胆碱 ).Acetylcholine is a neurotransmitter, which mediates nerve impulses. Therefore, flaccid paralysis occurs.

The typical symptoms are double vision and squint (eye), dyscatabrosis (throat), and dyspnea. The patients are usually died of respiratory failure. 10 ng of Botulinum toxin can kill an adult.

• Sample collection: foods and patients’ f

eces.

• Identification of botulinum toxin in the s

amples

The patients must be promptly treated with A, B, E multi-valent antitoxin to neutralize the free toxins.Sometimes antibiotic therapy is needed.Vaccination does not protect hosts from botulism.

•Non-spore-forming anaerobes are strict anaerobes and

present in large numbers in intestine (95-99% of the tot

al bacterial mass), mouth and genitourinary tract as nor

mal flora.

•These anaerobes can be divided into anaerobic Gram-p

ositive or negative cocci or rods belonging to 23 genera.

11 genera are associated with human diseases.

Non-Spore-forming Anaerobesof medical Importance  

•Gram-negative rods Bacteroides, Prevotella, Porphyromonas, Fusobacterium•Gram-positive rodsPropionibacterium, Actinomyces, Eubacterium, Bifidobacterium•Gram-positive cocciPeptostreptococcus, Peptococcus•Gram-negative cocci Veillonella

• Most non-spore-forming anaerobes are opportunistic pathogens, and a few of them show relatively stronger pathogenicity.

• Co-infection with facultative bacteria.

• Adhesion to host cells by pili.

• Production of various virulent factors such as enterotoxin, collagenase, hyaluronidas, protease, hemolysin, DNase.

• Oral, genitourinary, abdominal and perineal infections are most common.

Most infections cause chronic pyogenic inflammation, local abscess or tissue necrosis.The secretion or pus in foci are usually colored (black, brown, bloody, pink), putrid and gas-producing.

• Direct microscopy examination: to observe

the bacteria in the smear of secretion.

• Typical bacteriological examination: to

isolate and identify the anaerobes from

samples. In smear of secretion, bacteria can

be seen, whereas the results of common

cultivations are negative.

1. Aminoglycoside antibiotics (e.g., streptomicin) and

some β-lactam antibiotics are ineffective.

2. Antibiotics such as nitrominazole (metronidazole) a

re used for treatment.

2. No vaccines are available.