Bacteria Health Article

Energy requirements for growth

All living organisms must find in their environment a source of energy to fuel cellular processes. Bacteria are no different. Phototrophs are organisms that use light as an energy source; those that require organic carbon are called heterotrophs. Autotrophs use carbon dioxide. Lithotrophs oxidize inorganic compounds such as hydrogen or ammonia for energy.

Many bacteria have structures and processes that allow them to adapt to hostile environments, and they can exist under an enormous range of conditions. Those that require oxygen for growth are called obligate aerobes. In contrast, obligate anaerobes will not grow in the presence of oxygen. Acidophiles are bacteria that grow optimally under acidic conditions (pH of less than 7.0), while alkaphiles prefer alkaline or basic conditions (pH of greater than 7.0). Organisms that require a temperature near 99°F (37°C) (the body temperature of warm-blooded animals) for growth are called mesophiles; those that grow at temperatures above 113°F (45°C) are called thermophiles; and psychrophiles are able to grow at temperatures near 32°F (0°C). Halophiles require sodium chloride (salt) for growth; osmophiles are able to grow in environments high in sugar; and xerophiles grow under dry conditions.

Binary fission and the growth curve

Bacteria grow and replicate in a process known as binary fission. In this process, one parent cell divides to produce two daughter cells. The process begins with the growth of the parent cell; the chromosome unwinds and replicates, each copy moving to opposite ends of the cell. The cell is then partitioned in half by the production of a dividing wall (called the septum). The cell is cleaved at the septum, and the two daughter cells are freed. The daughter cells then go on to reproduce as parent cells(i.e., if necessary nutrients and energy sources are present).

The dynamics of a population of bacteria change during binary fission. The doubling time, or time required for one parent cell to produce two daughter cells, varies by bacteria species and strain and also by the environmental conditions. All bacteria exhibit a characteristic pattern of growth when introduced to a new medium; this is known as the growth curve. There are four phases of the growth curve:

• During the lag phase, bacteria are adapting to the medium and begin to produce the cellular components necessary for cell division. There is no increase in cell population during the lag phase.
• Cell division occurs at a maximal rate during the log or exponential phase. The doubling time remains constant, so the number of cells increases exponentially.
• Cells stop growing exponentially and therefore remain constant during the stationary phase. This occurs when the medium begins to run out of the nutrients necessary for growth or when toxic products accumulate.
• The number of cells begins to decrease during the death phase as cells begin to die, usually due to toxic conditions or lack of nutrients.

Normal flora

Only a small percentage of the vast population of bacteria is pathogenic (disease-causing) to humans. Many species of bacteria colonize the human body and are called the normal flora. Organisms of the normal flora are normally found on surface tissues (i.e., the skin, mucous membranes, and the gastrointestinal system). It is when bacteria enter normally sterile areas of the body (e.g., the brain, blood, muscle, etc.) that disease may result.

Some organisms of the normal flora neither harm nor provide benefit to the human body; this relationship is called commensalism. Normal commensals are bacteria that can always be found on or in healthy individuals and rarely cause disease. Bacteria that occasionally colonize the human body without causing disease are called occasional commensals. Although a human fetus is sterile in utero, colonization with normal flora bacteria begins with birth when the baby comes into contact with the mother's vaginal bacteria; this continues with breast-feeding and subsequent contact with the environment.

Many other types of bacteria interact with the human body in a relationship called mutualism, from which both organisms benefit. There are a number of ways that bacteria benefit the human host:

• Normal flora bacteria on the skin such as Staphylococcus epidermidis protect against colonization by path ogenic bacteria, through a process called microbial competition.
• Bacteria in the vagina (e.g., Lactobacillus acidophilus) help to establish an acidic environment that inhibits colonization of pathogenic bacteria and yeast.
• The normal flora in the gastrointestinal (GI) tract (e.g., Escherichia coli) secrete vitamins such as K and B₁₂ that are essential for humans. The development of some GI tissues is stimulated by normal flora bacteria.
• Ruminants (animals with a four-chambered stomach) rely on enzymes secreted by bacteria such as Ruminococcus albus to digest cellulose (a major component of plant cell walls).