To enhance survival during periods of environmental hostility (such as nutritional deprivation), some gram-positive rods undergo profound structural and metabolic changes. These result in the formation of a dormant cell called an endospore inside the original cell. Endospores can be released from the original cell as free spores (Figure 1). Spores are the most resistant life forms known. They are remarkably resistant to heat (they survive boiling), desiccation, ultraviolet light, and bactericidal chemical agents. In fact, sterilization procedures are assessed by their ability to inactivate spores.

Fig1. Formation of an endospore.

A. Sporulation

Sporulation can be thought of as repackaging a copy of bacterial DNA into a new form that contains very little water, has no metabolic activity, does not divide, and has a restructured, highly impermeable, multilayered envelope. Spore formation begins with the invagination of the parent cell membrane, producing a double membrane that encapsulates and isolates a copy of the bacterial DNA in what will become the core of the spore. The mature spore retains the complete machinery for protein synthesis, and new spore-specific enzymes are synthesized in the core of the spore. The core also has high levels of a unique compound called calcium dipicolinate, which is thought to be important for protection of the spore DNA from environmental damage. Many enzymes of the original vegetative (nondividing) cell are degraded. When the endospore is completed, the parent cell lyses, releasing the spore.

 B. Spore germination

To return to the vegetative state, spores must first be subjected to a treatment that weakens the spore coat (such as heat or extremes of pH), thus allowing germination to occur. If the activated spore is in a nutritious environment, which it senses by monitoring various key metabolites, it begins to germinate. This process involves destruction of the cortex by lytic enzymes, followed by uptake of water, and release of calcium dipicolinate from the cell.

C. Medical significance of sporulation

Some of the most notorious pathogens are spore-formers, including B. anthracis(anthrax), Bacillus cereus(gastroenteritis), Clostridium tetani(tetanus), Clostridium botulinum(botulism), Clostridium perfringens(gas gangrene),and Clostridium difficile. Spores of these organisms can remain viable for many years and are generally not killed by boiling, but they can be killed by autoclaving (that is, subjecting the spores to temperatures above 120oC at elevated pressure). In the absence of an autoclave, spores can be largely eliminated by a primary boiling to activate germination and, after a short period of vegetative growth, a second boiling.

مواضيع ذات صلة

GROWTH AND METABOLISM

Susceptibility Testing

Immunologic Detection of Microorganisms

Identification of Bacteria

Growing Bacteria in Culture

Cultivation of Listeria, Corynebacterium, and Similar Organisms

Pathogenesis and Spectrum of Disease for Listeria, Corynebacterium, and Similar Organisms

The Growth Curve in Batch Culture

The Measurement of Microbial Concentrations

Bacterial Pathogenesis

Distribution of Normal Flora in The Body

Cultivation of Bacillus and Similar Organisms