Bacterial Defense Mechanisms

• Endospores
• Bacterial defense against oxidative damage

Endospores

• An endospore is a dormant, tough, non-reproductive structure produced by certain bacteria from the Firmicute phylum.
• Not a true spore (offspring) - but rather a stripped-down, dormant form of the bacteria
• Endospore-forming bacteria produce some of the most potent toxins known
• Usually occurs in GRAM-POSITIVE bacteria - most types of bacteria cannot change to the endospore form, but the most significant clinically significant examples that can are the Bacillus and Clostridium genera:
• Hospital-borne infections:  Clostridium difficile
• Food contamination:   Bacillus cereus,   Clostridium botulinum
• Wound infestation:  Clostridium perfringens, Clostridium tetani
• Bioterrorism:  Bacillus anthracis

Endospore Formation

• When a bacterium detects environmental conditions are becoming unfavorable it may start the process of endosporulation - which takes about eight hours
• Endospore formation is usually triggered by a lack of nutrients - and endospores can survive without nutrients
•  In endospore formation: The bacterium divides within its cell wall then one side engulfs the other
• The endospore consists of: the bacterium's DNA and part of its cytoplasm, surrounded by a very tough outer coating.

Re-Activation

• Endospores enable bacteria to lie dormant for extended periods, even centuries - revival of spores millions of years old has been claimed
• Upon detecting nutrients, the endospore can convert back to actively growing cells - the most common, initial step in this so-called germination process is the recognition of small molecule germinants by germination receptors in the inner-membrane

Endospores are very resilient

• Endospores are resistant to: ultraviolet radiation, desiccation, high temperature, extreme freezing and chemical disinfectants
• Common anti-bacterial agents (that work by destroying vegetative cell walls) do not affect endospores.

Bacterial defense against oxidative damage

Bacteria need cytochrome c oxidase (oxidase) to survive in presence of oxygen

-   Some bacteria (one-celled organisms) can use aerobic respiratory mechanisms to produce energy from glucose and oxygen, some cannot - the deciding factor is whether or not a bacterium contains a large protein complex cytochrome c oxidase (also called COMPLEX IV or just oxidase) in its membrane (referred to as being OX+ or OX-).

-   Without cytochrome c oxidase in its membrane, a bacterium cannot survive in the presence of oxygen - oxidase is the last (terminal) enzyme in the respiratory electron transport chain (ETC) required to "harvest" electrons from glucose using oxygen for efficient ATP energy production.

Oxidase+ (OX+) or Oxidase- (OX-)? - an oxidase test determines whether a bacterium produces cytochrome c oxidase and thus can use respiratory mechanism to efficiently produce energy from glucose using oxygen. Bacteria are OX+ if they contain cytochrome c oxidase. OX- bacteria can not use aerobic respiratory metabolism and are unable to live in the presence of oxygen.

Some bacteria have protective mechanisms against the effects of oxygen

A bacteria's response to oxygen (or other oxidizers) depends on the presence and distribution of certain enzymes to protect them from damage by oxidation (removal of electrons) - these enzymes react with and neutralize potentially damaging ROS (E.g. Hydrogen peroxide (H₂O₂), Superoxide Radical (O₂.-)) generated by:

(a) Bacterial cells in the presence of oxygen and other oxidizers (E.g. ozone, chlorine, chlorine dioxide)

Or

(b) Aerobic cells due to the incomplete reduction of oxygen

SOD, CAT and GPx detoxify oxygen radicals that are inevitably generated by living systems in the presence of O₂. The distribution of these enzymes in cells determines their ability to exist in the presence of O₂

The main 3 cell-protective enzymes

Superoxide dismutase (SOD)

Superoxide dismutase (SOD) enzyme prevents accumulation of potentially lethal superoxide(O₂.-) in aerobes and Aerotolerant Anaerobes.    All organisms capable of living in the presence ofO₂ necessarily contain SOD (regardless of whether they utilize oxygen in their metabolism)

2H⁺+ 2 O₂^- -  ==>  H₂O₂ +O₂

Catalase (CAT)

Nearly all organisms contain CAT, which catalyzes the decomposition of hydrogen peroxide (H₂O₂) to water

2 H₂O₂-  ==> 2 H₂O+O₂

•   Catalase-positive pathogenic bacteria make catalase to deactivate peroxide radicals.    Thus allowing these bacteria to survive unharmed within the host.

Srinivasa Rao PS, Yamada Y, Leung KY (September 2003). "A major catalase (KatB) that is required for resistance to H₂O₂ and phagocyte-mediated killing in Edwardsiella tarda". Microbiology (Reading, Engl.)149(Pt 9): 2635-2644.

•   Certain aerotolerant bacteria (E.g. lactic acid bacteria such as L. Acidophilus) lack CAT - but can still decompose H₂O₂ by means of GPx enzymes which reduce peroxide to H₂O

Glutathione peroxidase (GPx) (Peroxidase)

Reduces peroxide to H₂O utilizing the body's major in-house antioxidant glutathione

2GSH (Glutathione) + H₂O₂→GS-SG (Glutathione Disulfide) + 2H₂O

Obligate anaerobes usually lack enzymes to remove toxic by-products restricting them to an oxygen-free environment.

Obligate anaerobes lack or have low levelsof SOD, CAT and/or GPxallowing oxygen radicals to persist when exposed to oxygen or other oxidizers.    These radicals inactivate other bacterial enzyme systems leading to lethal oxidations of these bacteria.

Aerobic or facultative anaerobic bacteria contain enzymes which can detoxify oxygen radicals

Aerobic or facultative anaerobic bacteria contain SOD, CAT or GPx,which can detoxify oxygen radicals produced in the presence of oxygen - converting them to harmless oxygen and water