Microbial Growth and Culturing

Physical and Chemical Factors Affecting Microbial Growth

Physical Factors Affecting Microbial Growth

Obligate microbes have strict requirements for survival, while facultative microbes can adjust to tolerate other environmental conditions, such as pH, temperature, oxygen, salinity, and hydrostatic pressure.

Different microorganisms (microbes) need different factors in order to grow and survive. These factors are physical or chemical properties that define the environment of the microbe. An organism that has a strict requirements for specific factors in order to survive is said to be obligate for them. Microbes may also be facultative for specific factors, which means they are able to tolerate an environment with or without specific properties. There are a variety of environmental factors that affect microbial growth. The most important physical factors are pH, temperature, oxygen, pressure, and salinity.

pH measures how acidic or basic (alkaline) a solution is, and microbes may grow in either acidic, basic, or neutral pH conditions. An organism that grows in acidic (low pH of usually 2 or below) conditions is called an acidophile. An organism that grows in basic (high pH of usually 8.5–11) conditions is called an alkaliphile. An organism that grows in neutral pH (between 6.5 and 7.5) conditions is called a neutrophile. Most microbes grow in neutral pH conditions of around pH 7.0, but a number of bacteria can grow in acidic or basic conditions. Helicobacter pylori is an obligate acidophile bacteria found in the human stomach, which is a highly acidic environment. The facultative alkaliphile, Bacillus halodurans, is a soil bacterium whose morphology is dictated by the soil pH; at neutral pH the bacterium takes a coiled shape, and at basic pH it assumes a rod shape. Many human pathogens are neutrophilic, such as the bacteria E. coli and Streptococcus pyogenes and the protozoa Naegleria fowleri.
Different microorganisms have evolved to live in different types of environments. Some are able to live in acidic environments with a pH of 2 or below-these are called acidophiles. H. pylori is able to withstand the acidic environment of the human stomach. Other organisms are able to live in very basic environments of pH 8.5-11 (alkaliphiles), while yet others require a more neutral environment of around pH 7. E. coli is an example of this type of microorganism and is called a neutrophile.
Credit: David Gregory and Debbie MarshallLicense: CC BY-SA
Temperature is another factor that affects microbial growth. Microbes live in all sorts of environments around the world and have adapted to survival in harsh environments. A mesophile is an organism that grows at temperatures between 20°C and 45°C. A psychrophile is an organism that grows best at temperatures below 20°C. An organism that lives in extreme heat is a thermophile, which grows best at temperatures between 41°C and 80°C, while hyperthermophiles grow at temperatures above 80°C.
Different microorganisms require different environments for survival. Mesophiles such as Naegleria fowleri require environments of about 20-45°C. Thermophiles, such as Synechococcus, require temperatures of 41-80°C.
Credit: Toby KurkLicense: CC BY-SA
Thermophiles can be found growing in environments such as the inside of a geyser, while psychrophiles can be found growing at the North and South Poles. Mesophiles include all of the human pathogens, such as the bacteria E. coli, Staphylococcus aureus, and Streptococcus pyogenes, and the protozoans Naegleria fowleri and Plasmodium falciparum. Many psychrophilic microbes are found in Antarctic soil and surrounding waters; there have been species from every group of microbes found on this continent. Most thermophiles are prokaryotic (Archaea and Bacteria), but a few yeasts have also been described. An example of a thermophile is the bacteria Synechococcus, which is found in the hot springs of Yellowstone National Park, where temperatures range between 54°C and 75°C.
Psychrophiles such as Carnobacterium inhibens grow best at temperatures below 20°C.
Credit: Nicholson et al.License: CC BY
The presence or absence of molecular oxygen (O2) is a significant factor for microbial growth and survival. An obligate aerobe is an organism that requires oxygen for survival. Mycobacterium tuberculosis, the causative agent of tuberculosis, is an obligate aerobe. Most aerobes can tolerate normal atmospheric conditions, but a microaerophile, which requires oxygen, can only survive in low-oxygen conditions. Helicobacter pylori is a microaerophile that causes ulcers in human stomach linings. A facultative anaerobe is an organism capable of aerobic and anaerobic respiration. Many bacteria are facultative anaerobes, including Staphylococcus aureus. Another example is yeast, which is used to ferment beer. An aerotolerant anaerobe is an organism that thrives in the absence of oxygen but is not harmed by the presence of oxygen. Lactobacillus acidophilus is an aerotolerant bacterium that is used to make yogurt. An obligate anaerobe dies in the presence of oxygen. Examples of obligate anaerobes include the bacteria Clostridium difficile, the causative agent of C. diff, a common hospital-acquired infection, and the fungi Neocallimastix, which is found in the rumen of cows. A microbes relative sensitivity or requirement for oxygen can be determined based on the locations where it grows in test tube culture. Microbes with a greater affinity for oxygen will grow nearer the top of the medium while species that are unable to tolerate oxygen grow at the bottom.

Microorganisms and Oxygen

A microorganism's requirement or tolerance of oxygen, or it's sensitivity to oxygen is determined by where it grows in a test tube.
The physical and osmotic pressure that water exerts on cellular organisms is a crucial factor for microbes that live in aquatic environments. Because of its density, water exerts more pressure than does air on organisms living in it. This pressure is called hydrostatic pressure, or the pressure a fluid exerts on points below its surface. This pressure is due to the force of gravity. Microbes must be able to withstand high hydrostatic pressure in order to live deep in the ocean, for example. A barophile is a microbe that lives in conditions of high hydrostatic pressure.

Microbes must also be able to adjust to the concentration of salt in their environment. Osmosis causes water molecules to diffuse across membranes from areas of high water concentration (i.e., low salt concentration) to areas of low water concentration (i.e., high salt concentration). Regulation of cellular salt concentration is especially important for microbes in aquatic environments, since they will gain or lose water through osmosis. To avoid swelling or shrinking in response to osmosis, microbes must maintain a cellular salt concentration similar to their external environment. A microbe that can tolerate living in high salt concentrations is called a halophile. Very few bacteria are halophiles; instead, most halophilic microbes are archaea. Staphylococcus aureus is a facultative halophile capable growing in environments of 15% salt. Tetragenococcus halophilus is a halophilic bacterium that is used to ferment salty foods such as soy sauce. The halophilic Archaea Haloferax volcanii is found in the Dead Sea.

Chemical Requirements for Growth

Microbes require certain elements, such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, to survive, which they can gain from their host or from inorganic sources.

Different microbes have different requirements for nutrients, as well as the amount of each nutrient, that they must acquire. A macronutrient is a nutrient that organisms need in higher concentrations, while a micronutrient is a substance required in small amounts for an organism's survival. A trace element is a nutrient that is needed in very small amounts. Some trace elements are also present at extremely low concentrations in the environment, while macronutrients and micronutrients are more abundant and easier for the microbes to obtain. Some elements are necessary for the growth of all microbes. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are among the macronutrients that a microbe must obtain from its environment. The micronutrients and trace elements needed for microbial growth include manganese, zinc, cobalt, molybdenum, nickel, and copper. Some microbes have additional nutrient requirements, depending on the environment in which they live. For example, a subset of bacteria requires silicic acid as a component of the cell wall, while halophilic microbes depend on sodium in addition to other elements for survival.

Every microbe can be categorized based upon its method of acquiring nutrition. A heterotroph obtains all of its nutrients from its environment, while an autotroph synthesizes some nutrients using compounds it uptakes. A chemoautotroph is an organism that obtains energy through chemical reactions with certain organic or inorganic compounds, such as those containing sulfur. A photoautotroph uses light energy to synthesize sugar from carbon dioxide. Most pathogens are heterotrophs; some are entirely dependent on the host, unable to survive outside of it.

There are many different types of relationships that microbes may have with their host, such as mutualism, commensalism, and parasitism. Some of the bacteria that live in mammalian intestines have a mutualistic relationship between the bacteria and the mammal, because both the bacteria and the mammal benefit. The bacteria rely on the nutrients that the animal ingests for their survival, and the animal relies on these bacteria for digestion. Some of the bacteria that live on mammalian skin, on the other hand, are commensal bacteria, or neither harmful nor helpful to the host, as the bacteria require the skin environment for their survival, while the mammal does not need the bacteria to serve any particular function. Finally, some relationships between microbes and other living organisms are harmful to the organism in which the microbe is living, and these are termed parasites. In humans, parasitic microbes can cause intestinal diseases such as toxoplasma. There are parasites that are not microbes, as well, such as parasitic worms, ticks, and lice. Any organism, including a microbe, that can cause disease is called a pathogen.

Nutrients Can Cross the Plasma Membrane

Nutrient transport utilized by microbes can be passive, requiring no energy, or active, requiring energy.

The two general categories of nutrient transport that microbes can use are passive transport and active transport. Passive transport does not require energy. Diffusion is the random movement of molecules along a concentration gradient from an area of higher concentration to an area of lower concentration. Molecules that are abundant in the environment but at a low concentration within the microbial cell are able to move into the cell by this process. Diffusion may either involve molecules moving through the semipermeable plasma membrane directly (simple diffusion) or by passing through a carrier protein channel (facilitated diffusion). Oxygen, carbon dioxide, and water are the most common molecules that enter or exit microbial cells by simple diffusion. Osmosis passively moves water across membranes in response to a concentration gradient.

Sugar molecules, amino acids, and iron are transported into microbe cells via facilitated diffusion. In facilitated diffusion, the carrier protein channels are specific to a certain molecule or group of molecules. Passive transport avoids the use of energy because molecules are moving from an area of high concentration toward an area of low concentration.

A cell may need to import more of a particular molecule that is already present in higher levels within the cell than in the environment. Active transport is the movement of a material across the cell membrane against its concentration gradient, requiring the cell to expend energy, often in the form of adenosine triphosphate (ATP) or with electron carriers. This frequently happens via pumps that span the cell membrane; chemical reactions allow the pump to open and for molecules to pass through. Microbes may also uptake nutrients from their environment using endocytosis, a form of bulk transport that moves material into a cell by an infolding of the cell membrane around the material, forming a vesicle (small sac) that moves into the cell. Examples of active transport are proton pumps used in photosynthesis and cellular respiration. The energy for these proton pumps comes from the electron carriers in these metabolic processes. Without these proton pumps actively moving hydrogen ions from the interior to the exterior of the cell (or organelle for eukaryotes), the microbe would be unable to produce ATP.

Passive and Active Transport

Nutrients and gases can move across a cell membrane via passive transport (diffusion or facilitated diffusion), which moves materials with their concentration gradient and does not require energy, or via active transport, which moves materials against their concentration gradient and requires energy.