How to Kill Infection Causing Organisms ? (7 Physical Agents) | Microbiology

Here is a list of seven effective physical agents that can kill infection causing organisms:- 1. Electricity 2. Sonic and Supersonic Waves 3. Alpha, Beta, and Gamma Rays 4. Electrons 5. Cathode Rays 6. X Rays 7. Distilled Water.

Physical Agent # 1. Electricity:

Beattie and Lewis (1920) employed a current of about 4000 volts and 2 amp. for 4 min. and reported the destruction of over 99.9 per cent of the organisms in milk.

Fabian and Graham (1933) noted the gradual destruction of Escherichia coli by exposure to a high-frequency current of 10 megacycles per second and an intensity of 0.8 amp. However, the application of the current for 10 hr. failed to destroy all of the organisms in a suspension.

A frequency of 10 megacycles corresponds to a wave length of approximately 30 meters. Gale and Miller (1935) were unable to destroy Staphylococcus aureus, Salmonella typhosa, Diplococcus pneumoniae, and streptococci when cultures were exposed to ultra-short waves of 10 meters and 200 watts for 1 hr. on each of three consecutive days.

Ingram and Page (1953) subjected resting suspensions of baker’s yeast, E. coli, tobacco mosaic virus, and a bacteriophage to high-frequency electric fields in cells which kept the temperature below 30°C., very energetic cooling being necessary.

The voltage gradients used were up to 2000 volts per centimeter and were applied for aggregate periods up to 12 min., but no significant kills were observed. They concluded that the lethal effect was too small for any practical value.

Electric current has been employed for the pasteurization of milk and for the destruction of organisms in sewage, water, etc., but the results have been too unreliable to be of practical importance. Its use has been largely abandoned, preference being given to other germicidal agents.

Physical Agent # 2. Sonic and Supersonic Waves:

i. Sonic Waves:

These are waves of audible frequency of about 8900 cycles per second produced by a nickel tube vibrating in an electromagnetic field and in resonance with a 2000-volt oscillating power circuit. Such waves are capable of destroying bacteria if exposed for sufficient time.

Milk has been treated in this manner with a reduction of 99 per cent in the viable count after an exposure period of 40 to 60 min. Williams and Gaines (1930) employed high-frequency audible sound waves of about 8800 cycles per second and reported the destruction of cells of Escherichia coli. They concluded that the lethal effects of the waves were due probably to a violent agitation set up within the cell.

Shropshire (1947a, b) subjected dispersed bacteria to intense sonic energy and reported a decrease in the turbidity of the liquid. He proposed the method for the turbidimetric evaluation of bacterial disruption.

Rotman (1956) treated E. coli and Azotobacter vinelandii to sonic vibrations in a Raytheon 10-kilocycle magnetostrictive oscillator for 10 min. at a temperature below 4°C. and a pH of 6.6, and reported structural damage to the cells.

ii. Supersonic Waves:

These are waves above audible frequency, of 200,000 to 1,500,000 cycles per second, produced by connecting a piezoelectric crystal with a high-frequency oscillator. These waves have also been shown to exert a destructive effect on bacteria and other organisms.

Wood and Loomis (1927) employed sound waves of high frequency and great intensity generated by a piezoelectric oscillator of quartz operated at 50,000 volts and vibrating 300,000 times per second. They noted the fragmentation or the tearing to pieces of organisms such as Spirogyra and Paramecium. Red blood cells suspended in saline were also broken into small fragments. On the other hand, bacteria were able to survive treatment with high-frequency sound waves of great intensity. Harvey and Loomis (1929) found luminescent bacteria to be destroyed in 1. hr. by supersonic energy.

Beckwith and Weaver (1936) treated a yeast and several species of bacteria to supersonic waves and reported destruction of the organisms. They concluded that the organisms were usually killed by sufficient application of supersonic radiation. The presence of protein interfered with the action of the sonic energy.

Hamre (1949) found that Klebsiella pneumoniae and Saccharomyces cerevisiae were destroyed by ultrasonic energy. On the other hand, influenza virus particles were not inactivated by such treatment.

Anderson, Boggs, and Winters (1948) found certain large bacteriophages to be sensitive to ultrasonic treatment, whereas the small, compact viruses were relatively resistant to the shearing forces existing during cavitation of the liquid in which they were suspended.

Galesloot (1955) employed ultrasonic waves with a frequency exceeding 20,000 vibrations per second for the destruction of bacteria in milk. The bactericidal effects were disappointing.

Jacobs and Thornley (1954) exposed seven species of bacteria suspended in milk and in nutrient broth to ultrasonic vibrations of a frequency of 1 megacycle per second. E. coli was killed fairly rapidly. Staphylococcus aureus, Streptococcus faecalis, and other spherical bacteria were considerably more resistant than the rod forms. Under conditions which caused rapid death of E. coli suspended in broth, complete protection was afforded by fresh milk, gelatin, and peptone in solution.

Physical Agent # 3. Alpha, Beta, and Gamma Rays:

Sulfhydryl enzymes such as glycerophosphoric dehydrogenase and urease were shown by Barron and Dickman (1949) to be inactivated when dilute solutions were irradiated with small doses of alpha rays from polonium, beta rays from strontium, and gamma rays from radium.

Partial reactivation of the enzymes, by addition of glutathione, was obtained after inhibition with the alpha rays. This would indicate that the ionizing radiations inhibit such enzymes by oxidation of the sulfhydryl groups (-SH) which are essential for enzyme activity.

Kempe, Graikoski, and Gillies (1954) inoculated sterilized meat with 40,000 spores of Clostridium botulinum and reported that 3,500,000 reps for 24 hr. were required to sterilize the contents.

Morgan and Reed (1954) found that short heat-treatment prior to irradiation was more efficient in the destruction of spores of CI. botulinum than irradiation alone.

Physical Agent # 4. Electrons:

Trump and Proctor (1951) employed electrons as a sterilizing agent.

Electrons are very small, negatively charged elementary particles that move about protons or positively charged nuclei of atoms. They are readily released by heating a tungsten filament to several thousand degrees centigrade.

When such electrons are acted upon by a high electric field between two metal electrodes, they are accelerated away from the negative electrode or cathode and acquire the energy in volts which produced the field. Electrons may be accelerated to energies of 2, 3, 4 million or more volts.

The penetration of high-energy electrons depends on both the electron voltage and the density of the irradiated material. Electrons accelerated to energies of 2 million volts will penetrate water effectively to a depth of about 2/3 cm. The depth of penetration increases directly with voltage.

Packaged materials containing pharmaceuticals, biological, etc., may be effectively irradiated provided they come within the limitations of penetration of the electrons. The rays apparently produce no reduction in potency or increase in toxicity of the irradiated materials.

The effectiveness of high-energy electrons depends upon the excitation and ionization of the atoms of the organism to produce chemical changes which bring about its death.

Physical Agent # 5. Cathode Rays:

Koh, Morehouse, and Chandler (1956) exposed representative species of non-spore-forming bacteria to cathode rays generated by a 2 million electron volt Van de Graaff accelerator. They reported that all species were susceptible to doses of 0.5 megarep or less.

Gram-negative bacteria were more sensitive to irradiation than Gram-positive bacteria. In a later report, Pepper, Buffa, and Chandler (1956) exposed spores to the same cathode rays and found that the resistance of moist spores was greater than frozen spores and dried spores.

Physical Agent # 6. X Rays:

Haberman (1942) treated Staphylococcus aureus with both soft (>I A.) and hard (<I A.) X rays. Short wave lengths were more effective in killing the organism than long wave lengths of the same intensity.

Ephrati (1948) destroyed tetanus toxin and Staphylococcus hemolysin with X rays. Certain proteins, their split products, reducing amino acids, glutathione, and thioglycollic and ascorbic acids were found to be effective as protective agents, whereas oxidizing substances were ineffective.

All observations supported the theory that the effect of X rays on tetanus toxin and Staphylococcus hemolysin was an indirect one, the radiation causing the formation of active oxidizing radicals which in turn destroyed the toxins by oxidation.

Barron et al. (1949) reported the destruction of a number of respiratory enzymes by small doses of X rays. These observations were interpreted as being due to oxidation of the sulfhydryl (-SH) groups of the enzymes.

Reduction in the concentration of oxygen in bacterial suspensions has been shown to result in a marked decrease in the X-ray sensitivity of Escherichia coli. Stapleton, Billen, and Hollaender (1952) and Burnett et al. (1952) showed that the addition of pyruvate, formate, succinate, serine, alanine, ethanol, and hydrosulfite protected the cells against the lethal action of X rays by virtue of their ability to remove oxygen from the system.

Physical Agent # 7. Distilled Water:

There appears to be a difference of opinion concerning the action of distilled water on the viability of bacteria and spores. Koch (1881) reported that anthrax spores were able to remain alive for more than 3 months in distilled water; vegetative cells were considerably more sensitive.

Some have reported death in a few hours, whereas others have stated that weeks were necessary to destroy all organisms. This discrepancy may be due, in part at least, to the vessel from which the water was distilled. It has already been shown that minute amounts of some metallic ions exert a toxic effect on bacteria. Wilson (1922) reported that water distilled from a copper vessel sterilized a suspension of Salmonella aertrycke in a few hours.

The number of organisms introduced in the inoculum has also been shown to be the cause of considerable discrepancy in results. Ficker (1898) showed that, when distilled water was seeded with 60,000,000 cholera organisms per milliliter, viability was present after several months, but when the number was reduced to 10,000 per milliliter, all bacteria were dead after a period of 2 hr.

He concluded that the inoculation of large numbers of organisms into distilled water resulted in the transfer of sufficient nutrients to prepare a dilute medium. Such a solution no longer possessed the properties of distilled water. Another cause of conflicting results may be due to variations in the pH of the distilled water.

Winslow and Falk (1923a, b) adjusted distilled water to increasing hydrogen-ion concentrations and found that a pH of 6.0 gave the highest percentage of viable organisms after a period of 9 hr. Cohen (1922) showed that the stabilization of distilled water by the addition of buffers gave much more constant results. Spangler and Winslow (1943) reported that washed cells of Bacillus cereus, added to distilled water, died out very rapidly.

However, the addition of NaCl in concentrations from 0.00001 to 0.3 M protected the organisms for a time against the harmful effects of distilled water. Other factors that affect the final results include traces of alkali dissolved from soft glass, dissolved carbon dioxide from the air, percentage of dissolved oxygen, and temperature of incubation.

Whipple and Mayer (1906) found that Salmonella typhosa remained viable in distilled water for 2 months under aerobic conditions but only 4 clays in an anaerobic environment. Houston placed S. typhosa in distilled water kept at different temperatures. At 0°C. the organisms lived for 8 weeks; at 18°C., they lived for 3 weeks; and at 37°C., they lived for only 1 week.

There is no evidence to show that true bacteria, when inoculated into distilled water, are destroyed by the process of plasmoptysis, i.e., the excessive intake of water resulting in the disruption of the cells. Bacterial cells are too resistant to osmotic changes for this to occur.