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Microbes can be damaged or killed by elements of their physical environment such as temperature, radiation, or exposure to chemicals; these effects can be exploited in efforts to control pathogens, often for the purpose of food safety.
Irradiation is the use of ionising gamma rays emitted by cobalt-60 and caesium-137, or, high-energy electrons and X-rays to inactivate microbial pathogens, particularly in the food industry. Bacteria such as Deinococcus radiodurans are particularly resistant to radiation, but are not pathogenic. [1] Active microbes, such as Corynebacterium aquaticum, Pseudomonas putida , Comamonas acidovorans, Gluconobacter cerinus, Micrococcus diversus and Rhodococcus rhodochrous , have been retrieved from spent nuclear fuel storage pools at the Idaho National Engineering and Environmental Laboratory (INEEL). These microbes were again exposed to controlled doses of radiation. All the species survived weaker radiation doses with little damage, while only the gram-positive species survived much larger doses. The spores of gram-positive bacteria contain storage proteins that bind tightly to DNA, possibly acting as a protective barrier to radiation damage.
Ionising radiation kills cells indirectly by creating reactive free radicals. These free radicals can chemically alter sensitive macromolecules in the cell leading to their inactivation. Most of the cell's macromolecules are affected by ionising radiation, but damage to the DNA macromolecule is most often the cause of cell death, since DNA often contains only a single copy of its genes; proteins, on the other hand, often have several copies so that damage of one will not lead to cell death, and in any case may always be re-synthesized provided the DNA has remained intact. [2] [3] Ultraviolet radiation has been used as a germicide by both industry and medicine for more than a century (see Ultraviolet germicidal irradiation). Use of ultraviolet leads to both inactivation and the stimulating of mutations. A case study of an irradiated Escherichia coli population found a growing number of bacteriophage-resistant mutants induced by the light. [4]
Carl Nägeli, a Swiss botanist, discovered in 1893 that the ions of various metals and their alloys such as silver and copper, but also mercury, iron, lead, zinc, bismuth, gold, aluminium and others, have a toxic effect on microbial life by denaturing microbial enzymes and thus disrupting their metabolism. This effect is negligible in viruses since they are not metabolically active. [5]
Strong electric field pulses applied to cells cause their membranes to develop pores (electroporation), increasing membrane permeability with a consequent and, for the cell, undesirable migration of chemicals. Pulses of low intensity may result in the increased production of secondary metabolites and a build-up of resistance. PEF treatment is an adequate process for inactivation of microbes in acids and other thermosensitive media, but holds inherent resistance dangers because of incomplete destruction. [6] [7]
A 2004 study found that E. coli is susceptible to pulsed magnetic fields with a survivability figure of 1 in 10 000. As with PEF cell walls are rendered porous with resultant cell death. Enzymes such as lactoperoxidase, lipase and catalase are readily inactivated, though with varying degrees of susceptibility. [8] [9] A 2010 study concentrated on the effects of PMF on Staphylococcus aureus . [10]
Until recently ultrasonic systems were used for cleaning, cutting, [11] the welding of plastics, and in medical therapy. High power ultrasound is a useful tool which is extremely versatile in its applications. Ultrasound generates cavitation bubbles within a liquid or slurry by causing the liquid molecules to vibrate. Temperatures of 5000K and pressures of up to 2000 atmospheres are routinely recorded in these bubbles. Cavitation can be produced using frequencies from the audible range up to 2 MHz, the optimum being at about 20 kHz. Generating ultrasonics requires a liquid medium and a source of ultrasound, usually from either a piezoelectric or magnetostrictive transducer. The process is used for destroying E. coli , Salmonella , Ascaris , Giardia , Cryptosporidium cysts, Cyanobacteria and Poliovirus. It is also capable of breaking down organic pesticides. [12]
The frequencies used in diagnostic ultrasound are typically between 2 and 18 MHz, and uncertainty remains about the extent of cellular damage or long-term effects of fetal scans. (see Medical ultrasonography)
Freezing food to preserve its quality has been used since time immemorial. Freezing temperatures curb the spoiling effect of microorganisms in food, but can also preserve some pathogens unharmed for long periods of time. Freezing kills some microorganisms by physical trauma, others are sublethally injured by freezing, and may recover to become infectious. [13]
Syrup, honey, brine, alcohol and concentrated sugar or salt solutions display an antibacterial action due to osmotic pressure. Syrup and honey have a long history of being used as a topical treatment for superficial and deep wounds. [14] [15]
Wood smoke compounds act as food preservatives. Phenol and phenolic compounds found in wood smoke are antioxidants and antimicrobials, slowing bacterial growth. Other antimicrobials in wood smoke include formaldehyde, acetic acid, and other organic acids, which give wood smoke a low pH—about 2.5. Some of these compounds are toxic to people as well, and may have health effects in the quantities found in cooking applications.
Microorganisms suffer a reduction in viability on contact with ozone which compromises the integrity of their cell walls. Gram-negative bacteria are more vulnerable to ozone than gram-positive organisms. [16] [17]
(see Thermization and Pasteurisation)
Extreme temperatures destroy viruses and vegetative cells that are active and metabolising. Organic molecules such as proteins, carbohydrates, lipid and nucleic acids, as well as cell walls and membranes, all of which play important roles in cell metabolism, are damaged by excessive heat. Food for human consumption is routinely heated by baking, boiling and frying to temperatures which destroy most pathogens. Thermal processes often cause undesirable changes in the texture, appearance and nutritional value of foods. [18] Autoclaves generate steam at higher than boiling point and are used to sterilise laboratory glassware, surgical instruments, and, in a growing industry, medical waste. A danger inherent in using high temperatures to destroy microbes, is their incomplete destruction through inadequate procedures with a consequent risk of producing pathogens resistant to heat.
(see Pascalization)
Water under very high hydrostatic pressure of up to 700 MPa (100,000 psi) inactivates pathogens such as Listeria , E. coli and Salmonella . High pressure processing (HPP) is preferred over heat treatment in the food industry as it eliminates changes in the quality of foods due to thermal degradation, resulting in fresher taste, texture, appearance and nutrition. Processing conveniently takes place at ambient or refrigeration temperatures. [19]
The question whether pressure is an impediment to (microbial) life is surprisingly opposite what has been assumed for a long time. Anurag Sharma, a geochemist, James Scott, a microbiologist, and others at the Carnegie Institution of Washington performed an experiment with Diamond Anvil Cell and utilized "direct observations" on microbial activity to over 1.0 Gigapascal pressures. [20]
Their goal was to test microbes and discover under what level of pressure they can carry out life processes. The experiments were performed up to 1.6 GPa of pressure, which is more than 16,000 times Earth's surface pressure (Earth's surface pressure is 985 hPa). The experiment began by placing a solution of bacteria, specifically Escherichia coli and Shewanella oneidensis , in a film and placing it in the DAC. The pressure was then raised to 1.6 GPa. When raised to this pressure and kept there for 30 hours, at least 1% of the bacteria survived. The experimenters then added a dye to the solution and also monitored formate metabolism using in-situ Raman spectroscopy. If the cells survived the squeezing and were capable of carrying out life processes, specifically breaking down formate, the dye would turn clear. 1.6 GPa is such great pressure that during the experiment the DAC turned the solution into ice-IV, a room-temperature ice. When the bacteria broke down the formate in the ice, liquid pockets would form because of the chemical reaction. The bacteria were also able to cling to the surface of the DAC with their tails. [21]
There was some skepticism recorded with this pioneering experiment. According to Art Yayanos, an oceanographer at the Scripps Institute of Oceanography in La Jolla, California, an organism should only be considered living if it can reproduce. Another issue with the DAC experiment is that when high pressures occur, there are usually high temperatures present as well, but in this experiment there were not. This experiment was performed at room-temperature. However, the intentional lack of high temperature in the experiments isolated the actual effects of pressure on life and results clearly indicated life to be largely pressure insensitive. [21]
Newer results from independent research groups [22] have shown the validity of Sharma et al. (2002) work. [20] This is a significant step that reiterates the need for a new approach to the old problem of studying environmental extremes through experiments. There is practically no debate whether microbial life can survive pressures up to 600 MPa, which has been shown over the last decade or so to be valid through a number of scattered publications. [20] What is significant in this approach of Sharma et al. 2002 work is the elegantly straightforward ability to monitor systems at extreme conditions that have since remained technically inaccessible. While the experiment shows simplicity and elegance, the results are not unexpected and are consistent with most biophysical models. This novel approach lays a foundation for future work on microbiology at non-ambient conditions by not only providing a scientific premise, but also laying the technical feasibility for future work on non-ambient biology and organic systems.
Bacterial cell surfaces may be damaged by the acceleration forces attained in centrifuges. [23] Laboratory centrifuges routinely achieve 5000–15000g, a procedure which often kills a considerable portion of microbes, especially if they are in their exponential growth phase. [24]
An extremophile is an organism that is able to live in extreme environments, i.e. environments with conditions approaching or expanding the limits of what known life can adapt to, such as extreme temperature, radiation, salinity, or pH level.
Food preservation includes processes that make food more resistant to microorganism growth and slow the oxidation of fats. This slows down the decomposition and rancidification process. Food preservation may also include processes that inhibit visual deterioration, such as the enzymatic browning reaction in apples after they are cut during food preparation. By preserving food, food waste can be reduced, which is an important way to decrease production costs and increase the efficiency of food systems, improve food security and nutrition and contribute towards environmental sustainability. For instance, it can reduce the environmental impact of food production.
A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells.
Escherichia coli ( ESH-ə-RIK-ee-ə KOH-ly) is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus Escherichia that is commonly found in the lower intestine of warm-blooded organisms. Most E. coli strains are harmless, but some serotypes such as EPEC, and ETEC are pathogenic and can cause serious food poisoning in their hosts, and are occasionally responsible for food contamination incidents that prompt product recalls. Most strains are part of the normal microbiota of the gut and are harmless or even beneficial to humans (although these strains tend to be less studied than the pathogenic ones). For example, some strains of E. coli benefit their hosts by producing vitamin K2 or by preventing the colonization of the intestine by pathogenic bacteria. These mutually beneficial relationships between E. coli and humans are a type of mutualistic biological relationship — where both the humans and the E. coli are benefitting each other. E. coli is expelled into the environment within fecal matter. The bacterium grows massively in fresh fecal matter under aerobic conditions for three days, but its numbers decline slowly afterwards.
Bacterial growth is proliferation of bacterium into two daughter cells, in a process called binary fission. Providing no event occurs, the resulting daughter cells are genetically identical to the original cell. Hence, bacterial growth occurs. Both daughter cells from the division do not necessarily survive. However, if the surviving number exceeds unity on average, the bacterial population undergoes exponential growth. The measurement of an exponential bacterial growth curve in batch culture was traditionally a part of the training of all microbiologists; the basic means requires bacterial enumeration by direct and individual, direct and bulk (biomass), indirect and individual, or indirect and bulk methods. Models reconcile theory with the measurements.
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular polymeric substances (EPSs). The cells within the biofilm produce the EPS components, which are typically a polymeric conglomeration of extracellular polysaccharides, proteins, lipids and DNA. Because they have three-dimensional structure and represent a community lifestyle for microorganisms, they have been metaphorically described as "cities for microbes".
A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, with an optimum growth range from 20 to 45 °C. The optimum growth temperature for these organisms is 37°C. The term is mainly applied to microorganisms. Organisms that prefer extreme environments are known as extremophiles. Mesophiles have diverse classifications, belonging to two domains: Bacteria, Archaea, and to kingdom Fungi of domain Eucarya. Mesophiles belonging to the domain Bacteria can either be gram-positive or gram-negative. Oxygen requirements for mesophiles can be aerobic or anaerobic. There are three basic shapes of mesophiles: coccus, bacillus, and spiral.
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
Sterilization refers to any process that removes, kills, or deactivates all forms of life and other biological agents such as prions present in or on a specific surface, object, or fluid. Sterilization can be achieved through various means, including heat, chemicals, irradiation, high pressure, and filtration. Sterilization is distinct from disinfection, sanitization, and pasteurization, in that those methods reduce rather than eliminate all forms of life and biological agents present. After sterilization, an object is referred to as being sterile or aseptic.
A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than sterilization, which is an extreme physical or chemical process that kills all types of life. Disinfectants are generally distinguished from other antimicrobial agents such as antibiotics, which destroy microorganisms within the body, and antiseptics, which destroy microorganisms on living tissue. Disinfectants are also different from biocides—the latter are intended to destroy all forms of life, not just microorganisms. Disinfectants work by destroying the cell wall of microbes or interfering with their metabolism. It is also a form of decontamination, and can be defined as the process whereby physical or chemical methods are used to reduce the amount of pathogenic microorganisms on a surface.
A diamond anvil cell (DAC) is a high-pressure device used in geology, engineering, and materials science experiments. It enables the compression of a small (sub-millimeter-sized) piece of material to extreme pressures, typically up to around 100–200 gigapascals, although it is possible to achieve pressures up to 770 gigapascals.
An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals are used against fungi. They can also be classified according to their function. The use of antimicrobial medicines to treat infection is known as antimicrobial chemotherapy, while the use of antimicrobial medicines to prevent infection is known as antimicrobial prophylaxis.
Food microbiology is the study of the microorganisms that inhabit, create, or contaminate food. This includes the study of microorganisms causing food spoilage; pathogens that may cause disease ; microbes used to produce fermented foods such as cheese, yogurt, bread, beer, and wine; and microbes with other useful roles, such as producing probiotics.
Microbiology is the scientific study of microorganisms, those being of unicellular (single-celled), multicellular, or acellular. Microbiology encompasses numerous sub-disciplines including virology, bacteriology, protistology, mycology, immunology, and parasitology.
Deinococcus radiodurans is an extremophilic bacterium and one of the most radiation-resistant organisms known. It can survive cold, dehydration, vacuum, and acid, and therefore is known as a polyextremophile. It has been listed as the world's toughest known bacterium in The Guinness Book Of World Records.
Copper and its alloys are natural antimicrobial materials. Ancient civilizations exploited the antimicrobial properties of copper long before the concept of microbes became understood in the nineteenth century. In addition to several copper medicinal preparations, it was also observed centuries ago that water contained in copper vessels or transported in copper conveyance systems was of better quality than water contained or transported in other materials.
Pascalization, bridgmanization, high pressure processing (HPP) or high hydrostatic pressure (HHP) processing is a method of preserving and sterilizing food, in which a product is processed under very high pressure, leading to the inactivation of certain microorganisms and enzymes in the food. HPP has a limited effect on covalent bonds within the food product, thus maintaining both the sensory and nutritional aspects of the product. The technique was named after Blaise Pascal, a 17th century French scientist whose work included detailing the effects of pressure on fluids. During pascalization, more than 50,000 pounds per square inch may be applied for approximately fifteen minutes, leading to the inactivation of yeast, mold, and bacteria. Pascalization is also known as bridgmanization, named for physicist Percy Williams Bridgman.
The host–pathogen interaction is defined as how microbes or viruses sustain themselves within host organisms on a molecular, cellular, organismal or population level. This term is most commonly used to refer to disease-causing microorganisms although they may not cause illness in all hosts. Because of this, the definition has been expanded to how known pathogens survive within their host, whether they cause disease or not.
Astro microbiology, or exo microbiology, is the study of microorganisms in outer space. It stems from an interdisciplinary approach, which incorporates both microbiology and astrobiology. Astrobiology's efforts are aimed at understanding the origins of life and the search for life other than on Earth. Because microorganisms are the most widespread form of life on Earth, and are capable of colonising almost any environment, scientists usually focus on microbial life in the field of astrobiology. Moreover, small and simple cells usually evolve first on a planet rather than larger, multicellular organisms, and have an increased likelihood of being transported from one planet to another via the panspermia theory.
Food and biological process engineering is a discipline concerned with applying principles of engineering to the fields of food production and distribution and biology. It is a broad field, with workers fulfilling a variety of roles ranging from design of food processing equipment to genetic modification of organisms. In some respects it is a combined field, drawing from the disciplines of food science and biological engineering to improve the earth's food supply.
