Membrane active microbicides

The ambient medium impairs the effectiveness of microbicides also if its constituents include those capable of interacting with a microbicide in competition with the constituents of the microbe cell. This is true of electrophilically active microbicides in general as far as the ambient medium contains nucleophilically active constituents with which the microbicide can react in competition with the corresponding cell constituents. It is also true, however, of membrane-active microbicides if adsorption of the microbicide on organic matter competes with the adsorptive processes on the cytoplasmic membrane or if such microbicides, e.g. phenol derivatives, become incorporated in micelles that are formed in certain media at levels above the critical micelle concentration with the result that the incorporated active substance molecules are no longer available for the antimicrobial effect (see III. 16, Fig. 34). 

As already mentioned, phenol derivatives are membrane-active microbicides. They adsorptively coat the surface of the microbe cell then, at a higher concentration, they are dissolved more or less rapidly and well by lipoids depending on their chemico-physical properties (see above). They attack the cell wall and penetrate into the cell. There are reactions with the protoplasm and the cellular protein enzymes are also inhibited as a result the oxidoreductases and the enzymes of carbohydrate and protein metabolism react particulary sensitively. Whether the phenol derivatives act microbistatically or microbicidally is purely a question of the application concentration. At low concentrations in ambient medium, there is only reversible adsorption of the phenolic active substance at the cytoplasmic membrane and the related inhibiting effect. As stated above the cell wall is only penetrated and destroyed and the microbe cell killed at higher concentrations. 

PHBA alkyl esters are more effective against moulds and yeasts than bacteria, however, they are also useful for the control of bacterial growth. As membrane-active microbicides, their primary mode of action is based on nutrient transport... 

The phenyl esters of long chain fatty acids (Sections 7.8 and 7.9) are membrane-active microbicides they release phenolic compounds as the active ingredients. The advantage of these esters is that they are easier to handle and to apply than the phenolic compounds they are based on. 

Among other pyridine derivatives 2-hydroxy-pyridine-AT-oxides, 2-mercapto-pyridine- -oxides and 8-hydroxyquinolines are described in this section which may be looked at as membrane-active microbicides with chelating properties. The following pyridine compounds with antimicrobial activity but without significant importance for the protection of materials shall only be mentioned ... 

Guazatine as a membrane-active microbicide exhibits a broad spectrum of effectiveness. It is active by interfering with membrane structures of the microbial cell. Because of its antifungal activity it was developed at first as a fungicide for seed protection. But Guazatine later on also found some usage in wood preservatives for the temporary protection of freshly cut and sawn timber. 

The efficacy of a preservative and the concentration level of a microbicide to be added are very much dependent on the germ content of the material to be protected. This is in particular valid for electrophilic active microbicides which in general react irreversibly with nucleophilic components of the microbial cell, that means that they are used up by being effective. But membrane active microbicides which adsorptively coat microbial cell walls are also withdrawn from action at least temporarily if large numbers of microbial cells are present. The exponential growth profile of microbes (p. 2) always has to be taken into consideration. 

Membrane-active microbicides include alcohols - phenols - acids- saHcylanilides - carbanOides - dibenzamidines -biguanides - quaternary ammonium salts and other active ingredients with cationic character, e.g. azole fungicides which also act as chelate formers. As many of the antimicrobial agents which are able to complex metal cations display membrane-activity, they will be considered here as membrane-active microbicides. 

Due to the fact that OPP is really a broad spectrum microbicide and because of its favourable toxicity and ecotoxicity data it has become one of the most important microbicides which is used in numerous different applications. It is an important active ingredient in disinfectants for hospitals, public buildings, households and animal stables. Preferably OPP is used for heavy duty disinfection where there is a high risk of infection or of the presence of strong impurities, or wherever water with a high hardness is used. As a membrane active microbicide OPP performs best in an acidic, neutral or weakly basic environment, where it is in its undissociated form. Very often OPP is formulated in combination with other phenolic microbicides, e.g. with Chlorophen (7.3.5.) and/or PCMC (7.3.1.). Worthy of note is that such combinations exhibit virucidal efficacy against lipophilic, masked viruses. 

On hydrolytic cleavage of the ester group benzyl alcohol (1.4.) is liberated, an membrane active microbicide. These properties equip benzyl bromoacetate with a broad spectrum of activity which covers bacteria, yeasts and fungi. It may be used as a preservative for the in-can protection of water based functional fluids, e.g. paints. However, due to its properties- irritant, moderate stability-the microbicide has been applied to a limited extent only. 

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