A large number of medical procedures aim to control or destroy microbial pathogens by the use of drugs or chemicals, whether allopathic or homeopathic. Some methods such as irradiation, or sterilising and pasteurising by high temperatures, use physical effects. Both physical and chemical approaches need to address the problem of causing microbial resistance, and to explore new avenues of achieving a stable resolution. ^[1]

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.^[2] 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. ^[3][4]

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 movement of chemicals. Pulses of low intensity may result in the increased production of secondary metabolites and a build-up of resistance.

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. ^[5][6] A 2010 study concentrated on the effects of PMF on Staphylococcus aureus. ^[7]

High power ultrasound

Until recently ultrasonic systems were used for cleaning, 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 2MHz, the optimum being at about 20kHz. 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. ^[8]

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. ^[9]

High temperatures

(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.

High pressures

(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. ^[10]

High acceleration

Bacterial cell surfaces may be damaged by the acceleration forces attained in centrifuges. ^[11] In particular, high acceleration may damage cellular surface macromolecules necessary for cellular adhesion. ^[12] 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. ^[13]

Related articles

• Food preservation

External links

• Scientific sources & legislation - Physical intervention equipment

References