Specialized Microbial Growth

Biofilms are complex communities of one or more types of microorganisms (microbes). In nature, biofilms are composed of dozens of different types of microbes, including bacteria, fungi, algae, and protozoa. The microbes secrete a sticky matrix of sugars called the extracellular polymeric substance (EPS), which holds the microbes to a flat surface. As more microbes arrive, more EPS is generated, increasing the size of the biofilm. Biofilms protect the microbes from desiccation, predation, and removal from the surface. Inside the biofilm, the gene expression within the microbes changes to support further biofilm development. Additionally, cell-to-cell communication happens between the microbes in the biofilm, and nutrients are shared. Antibiotics and other treatments are often ineffective against biofilms because of the sticky EPS holding the microbes together. Because of their ability to protect bacteria from many sterilization techniques, biofilms on or within medical tools, such as catheters, are a frequent cause of nosocomial (hospital-acquired) infections.

One common biofilm is dental plaque. Bacteria, typically Streptococcus, bind to sugar present on teeth to begin colonization. More bacteria arrive and bind to the EPS being excreted by the bacteria within the biofilm. Over 300 different bacterial species have been identified within human dental plaque. As more bacteria join the biofilm, the outer layer can harden, forming tartar. The microbes closest to the teeth grow anaerobically, fermenting the sugar and producing acid, which damages the tooth. Left untreated, plaque is one of the leading causes of dental cavities.

One way that microbes protect their genetic material is through the formation of spores, or sporulation. Sporulation is one mechanism through which some microbes are protected from harsh environmental conditions, and ensure that the population does not die out during conditions unsuitable for growth. Protection of genetic material is critical to ensuring that once environmental conditions become suitable for survival, the next generation can again begin to growth.

When environmental conditions become unfavorable, some bacteria begin the process of sporulation. Spores are formed by several groups of unrelated bacteria and the process of sporulation has evolved independently in the different groups. In some spore-forming species, an exospore, or cyst, begins to form within the bacterial cell and the final encapsulation step occurs by budding, which also releases the spore from the cell. A spore that is formed completely within the bacterial cell and is released when the cell dies is called an endospore. Endospores and exospores contain the genetic material of the mother cell but are metabolically dormant. When environmental conditions change back to being hospitable to microbial life, the spores can germinate into living vegetative bacterial cells and continue to grow and divide into a new population of bacteria that is identical to the bacteria from which the spores were derived.

The sporulation process is best understood in groups that form endospores. To begin endospore formation, the normal bacterial cell, or vegetative cell, undergoes a partial, asymmetric division. A copy of the chromosomes is separated from the mother cell by a membrane, or septum, and then surrounded the plasma membrane of the mother cell. The resulting structure is called the forespore. A peptidoglycan wall embedded with a calcium compound is synthesized between the membrane layers of the forespore. This wall is called the spore cortex. A protein spore coat is the final layer wrapped around the forespore before release.