Dyes

Dyes

Two classes of dye compounds which have antimicrobial properties are of special interest to microbiologists. These are triphenylmethane and acridine dyes.

Triphenylmethane dyes

Included in this category are malachite green, brilliant green, and crystal violet. As a rule, gram-positive organisms are more susceptible to lower concentrations of these compounds than are gram-negative ones. Crystal violet will inhibit gram-positive cocci at a dilution of 1:200,000 to 1:300,000; 10 times this concentration is required to inhibit Escherichia coli. Staphylococcus aureus is inhibited by malachite green at a concentration of 1:1,000,000; a concentration of about 1:30,000 is required to inhibit E. coli. This general relationship between gram reaction and susceptibility to triphenylmethane dyes has a number of practical applications.

Practical application: certain media can be mad selective by the incorporation of low concentrations (about 1:100,000) of the dyes crystal violet, brilliant green, or malachite green. Gram-positive bacteria will be inhibited. Media of this kind are used extensively in public health microbiology, where detection of E. coli is important. Susceptibility to various dyes can also be used for identification of bacteria. Three species of Brucella can be distinguished by their patterns of resistance to several dyes. Crystal violet has also been used as a fungicide. A concentration of 1:10,000 is lethal for Monilia and Torula, and a concentration of 1:1,000,000 is inhibitory.

Mode of action: The mode of action of triphenylmethane dyes is uncertain, but there is speculation that they exert the inhibitory effect by interfering with cellular oxidation processes.

Acridine dyes

Two examples of dyes derived from acridine are acriflavine and trypaflavine. These compounds exhibit selective inhibition against bacteria, particularly staphylococci and gonococci. Gonococci are inhibited by trypaflavine in dilutions of 1:10,000,000. They possess little, if any, antifungal activity. Presently, they have less application than before the advent of antibiotics and other chemotherapeutic agents. They are used to some extent for the treatment of burns and wounds and for ophthalmic application and bladder irrigation.

Synthetic detergents

Surface-tension depressants, or wetting agents, employed primarily for cleansing surfaces are called detergents. Soap is an example. However, soap is a poor detergent in hard water. For this reason, many new more efficient cleaning agents have been developed, called surfactants or synthetic detergents, many of which are superior to soap. They do not form precipitates in alkaline or acid water, nor do they produce deposits with minerals found in hard water. They are extensively used in laundry and dishwashing powders, shampoos, and other washing preparations. Some are also highly bactericidal.

1. Those which ionize with the detergent property resident in the Anion are referred to as anionic detergents.
2. Those which ionize with the detergent property resident in the Cation are referred to as cationic detergents.
3. The third category of detergents is nonionic; i.e., they do not ionize. However, these substances do not possess significant antimicrobial activity.

Insofar as the reduction of the microbial flora from surfaces such as skin and clothing is concerned, the real value of ordinary soaps lies in the mechanical removal of microorganisms. Soapy water has the ability to emulsify and disperse oils and dirt. The microorganisms become enmeshed in the soap lather and are removed by the rinse water. Various chemicals have been incorporated into soaps to enhance their germicidal activity.

Cationic detergents are regarded as more germicidal than anionic compounds and will be discussed separately here as quaternary ammonium compounds.

Quaternary ammonium compounds

Most compounds of the germicidal cationic-detergent class are quaternary ammonium salts. Their characteristics structure with reference to a common inorganic ammonium salt such as ammonium chloride. The R₁, R₂, R₃, and R₄ groups are carbon groups linked to the nitrogen atom. The R groups may be any one of a large number of different alkyl groups. Accordingly, a very large number of different quaternary ammonium compounds have been synthesized and evaluated for their antimicrobial agents for a variety of uses.

The bactericidal power of the quaternary is exceptionally high against gram-positive bacteria, and they are also quite active against gram-negative organisms. Bactericidal concentrations range from dilutions of one part in a few thousand to one part in several hundred thousand. Another of their characteristics id the ability to manifest bacteriostatic action far beyond their bactericidal concentration. For example, the limit of bactericidal action for a given compound may be at a dilution of 1:30,000; yet it may be bacteriostatic in dilutions as high as 1:200,000. The action of these compounds demonstrates the need to distinguish between static and lethal activity in test procedures for the evaluation of disinfectants. Quaternaries have been shown to be fungicidal as well as destructive to certain of the pathogenic protozoa. Viruses appear to be more resistant than bacteria and fungi.

Practical applications: The combined properties of germicidal activity and detergent action, plus such other features as low toxicity, high solubility in water, stability in solution, and non-corrosiveness, have resulted in many applications of quaternary as disinfectants and sanitizing agents. They are used as skin disinfectants, as a preservative in ophthalmic solutions, and in cosmetic preparations. Quaternaries are widely used for control of microorganisms on floor, walls, and other surfaces in hospitals, nursing homes, and other public places. They are used to sanitize food beverage utensils in restaurants as well as surfaces and certain equipment in food-processing plants. Other applications are to be found in the diary, egg, and fishing industries to control microbial growth on surfaces of equipment and the environment in general.

Mode of action: A variety of damaging effects of quaternary upon microorganisms have been observed. These include denaturation of proteins, interference with glycolysis, and membrane damage. Experimental evidence suggests that the most likely site of the damage to the cell is the cytoplasmic membrane; the quaternary alter the vital permeability features of this cell structure.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.