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 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. 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. 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.
Metal ions (Oligodynamic effect) 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.
Pulsed electric fields (PEF) 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.
Pulsed magnetic fields (PMF) 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. A 2010 study concentrated on the effects of PMF on Staphylococcus aureus.
High power ultrasound Until recently ultrasonic systems were used for cleaning, cutting, 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. 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)
Low temperatures 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.
High osmotic gradients 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. 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.
Ozone 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.
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