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Without testing it is impossible to determine whether any waterborne product will be inhibitory or conducive to microbial growth. This paper focuses on bacteria, the small, single-celled organisms that can grow rapidly in environments that are unpreserved or not preserved properly.

The varied metabolic capabilities of microorganisms enable them to cause many problems in waterborne products from the manufacturing stage to storage and application. These problems can be controlled by effectively utilizing a microbicide.

In-can preservation of waterborne systems must be done to prevent spoilage in the system's container by bacteria, yeast and fungi in much the same manner that milk spoils in the carton if not refrigerated. The utilization of bactericides to ensure the prevention of bacterial contamination is an obvious answer; however environmental considerations and public concern for safer products are also concerns. For this discussion, preventing waterborne systems from bacterial contamination for their entire use period, from manufacture, on the shelf and into the hands of the consumer can be achieved with a better understanding of the biocide, the material being protected and the limitations therein.

The following definitions are to be used to give a better understanding for aspects of this discussion. A biocide, or microbicide, is a substance that kills organisms; a microbistatic agent prevents the growth of microorganisms but does not necessarily kill them. A preservative is a chemical that slows down the biodeterioration of a material. The term toxicity is used when referring to the detrimental effects caused on organisms other than that of the target organism.

Efficacy is the effect of the microbicide on the target organism or group of organisms and could be measured as percent killed versus a control containing no biocide or one with a biocide in study. Efficacy can be expressed as MIC, or minimum inhibitory concentration, against a specific organism. Spectrum refers to the effect a microbicide may have on more than one organism such that a broad-spectrum biocide will be effective against more than one group of target organisms.

It is estimated that product damage due to bacterial contamination costs the manufacturer hundreds of millions of dollars a year in returned, reworked, failed product and consumer claims. When the time comes for both choosing and selecting a biocide there must be a good understanding of their use enabling bactericides to work most efficaciously, as well as recognizing possible limitations that can occur with incompatibilities.

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Bacterial Contamination Sources

Many of the ingredients utilized in waterborne products like coatings, caulks, sealants and adhesives are made with a myriad of materials that bacteria can and will feed upon. Bacterial contaminants are prevalent, and abound not only in the manufacturing environment but also in water, air and many of the raw materials used in the manufacturing process. The air in a manufacturing plant, especially where dust from pigments, thickeners and starches is present, is prone to bacterial contaminants. Waterborne resins, manufacturing pipe lines, vessels and raw materials such as surfactants, dispersants, cellulosic thickeners (HEC), defoamers and pigment slurries are all susceptible to bacterial attack, as they are carbon sources for these organisms.

Bacterial contamination in waterborne products can occur at various points before, during and after production; thus, the inclusion of a biocide is essential, and best preservation occurs when an effective preservative at the proper level is placed as early in the manufacturing process as possible. When conducting a bacterial contamination survey, testing of the waterborne product is typically the first step, next are the raw materials and last, determining if the contamination is coming from the manufacturing facility using a sterile test tube with an environment capable of sustaining the samples taken in the manufacturing facility. This enables the surveyor to mark the locations in the facility onto the tube where the samples were taken. The finished product, raw materials and swoobe tubes are sent overnight to a testing lab to determine the presence of microbes. ASTM D 5588 provides a test method for determining the microbial condition of paint, paint raw materials, and plant areas as well as a procedure to challenge the efficacy of the bactericide being considered.

With a waterborne product being a food source for microbes when not properly protected, fouling, biofilms and biodeposition become additional concerns for bacterial contamination in the final product. Fouling is the deposition of an insoluble material like bacteria. A biofilm is a complex aggregation of microorganisms marked by the excretion of a protective and adhesive matrix and is characterized by surface attachment, structural heterogeneity, genetic diversity, complex community interactions and an extracellular matrix of polymeric substances. Biodeposition is another food source for bacteria and simply refers to the organic deposition on any surface that can then be a food source for bacteria. Thus, bacteria are prevalent everywhere, and they can propagate by utilizing sources for food that may appear innocuous to the human eye.

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Biocide Need and Function

The ideal environment for bacterial growth is one in which there is a food source present, like those previously mentioned, and a pH between 3 and 10. Once a waterborne product has become contaminated, a noticeable change in the physical and chemical make-up will be observed, including a viscosity drop that occurs when the enzymes produced by the bacteria breakdown the cellulosic thickeners to glucose or other sugars. These sugars are then fermented by the bacteria, which produce carbon dioxide and acids leading to foul odors. Proper implementation and use of a bactericide in waterborne products can greatly reduce and even eliminate the problems.

In choosing the proper biocide, the formulator must consider several factors such as cost effectiveness; compatibility with the final product and the other raw materials contained therein; the potential to cause color changes; objectionable odors; and not being detrimental to either the environment, the manufacturing personnel, the end user or disposal areas.

Bacterial contamination can be seen in several ways in the finished product, including gassing when CO2 is released by the bacteria; viscosity loss caused by enzymes that attack the cellulosic thickeners; gelation, where the product becomes thickened to the point it is unusable; odors due to the metabolic products produced by the microorganisms; discoloration caused by the water-soluble and insoluble pigments produced by bacteria propagating in and on materials in the product; and biofilm formation caused by slime-producing bacteria typically seen in pipes and containers.

There are numerous bactericide choices when seeking methods to combat bacterial infestation and contamination. Often the choice for the formulator comes from experience, a certain comfort level and product familiarity. A few of the most commonly used bactericide chemistries are presented in Table 1.

In Table 1, the modes of action are not intended to be all-encompassing and explain every functional aspect, but instead to give a basic guideline to modes of action for the biocides listed.

Biocidal agents that react with acetyl acetone, as when treated with Nash's reagent, will yield formaldehyde while others manufactured through condensation reactions with a starting compound and formaldehyde may not actually function through the release of formaldehyde. Formaldehyde release is also related to pH, and at alkaline pH little or no formaldehyde is actually released.

Formaldehyde will react with protein amine groups under a condensation reaction. Deactivated proteins retard metabolism or even kill the organism by causing leakage of cell contents through the cell wall into the environment. Current public concern with formaldehyde has placed limitations on its use, and few biocide manufacturers prefer to state their product is a formaldehyde releaser. This is the most common mode of action among preservatives and is very effective against bacteria.

Amine groups such as thiols are found in acids, which are the building blocks for proteins found in all living cells. Some of these contain thiols (S-containing groups). Amino acids are found in all cells of all organisms, and biocides that contain electrophilic groups can react with the nucleophiles of the cell, thus killing the organism.

These biocide types can also react with coating components containing nucleophilic groups, resulting in loss of activity proportional to the concentration of each reactant. However, as the biocide reacts, it is used up and thus, unavailable for further activity.

Agents that chelate metals are efficacious in that the trace metallic elements will not be available for the organism to utilize. Metals are often parts of enzymes and/or vitamins needed for metabolism and growth. It is hypothesized that these chemicals disable microorganisms by chelating the metals needed for existence.

Cationic agents react with anionic elements, both inorganic and organic, like those found on the cell wall. The cationic materials cause discharge of materials from the destabilized cell wall, causing cell death.

Coagulation is a term describing any general deactivation of proteins without actually specifying the molecular action. Interference of active transport of food molecules into the cell ‘may' be from deactivation of transport proteins or by adsorption of molecules onto the cell surface, thus, blocking the transport proteins. Also, interference with DNA (deoxyribonucleic acid), RNA (ribonucleic acid) or protein synthesis would prevent cell reproduction and metabolism, killing the organism.

What the Future May Hold

With the various costs involved in creating new and novel anti-microbicidal agents, coupled with various registrations and continued testing, creating new anti-microbial chemistries is an expensive and arduous process. With public awareness and concern for products that have little or no impact on people using products that contain biocidal products or on the environment, there is a hypocrisy of sorts, as no one wants to purchase products that are contaminated or spoiled in the container.

Being overwhelmed by exorbitant developmental costs combined with expensive registrations, testing, toxicology and a myriad of other expenses, the future for new biocides might take a different approach. One approach in creating different microbicides that is less costly is to create biocidal blends or cocktails utilizing two to three bactericides in various ratios. These cocktails can provide a simpler, yet effective and less-costly approach to bacterial containment and control. In fact, this creative method is currently being utilized by biocide companies in which these varied combinations of biocides have shown their efficacy in numerous applications. Blending various biocides enables a novel approach for obtaining new biocides based on older chemistries in a product-driven industry by achieving a possible solution to formulators concerned about bacterial resistance.

A truly unique approach that could find its way into waterborne preservation may be utilizing a phage to control bacterial infection. A phage is a naturally found virus that infects only bacteria. Its capsid head contains the genetic material that codes for the generation of more phage particles. The tail portion of the phage recognizes specific bacterial cell types and injects the genetic material into the bacteria. Once inside a bacterium, the phage genetic material takes over the bacterial cell machinery and codes for the production of more phage. Filling up with phage, the bacterial cell eventually explodes, releasing active phage to infect other cells.

Phage have several advantages over traditional anti-microbials. One advantage is that phage multiply exponentially, just as bacteria. A small initial dose of phage will multiply as it infects cells, diminishing the need for repeated doses. Phage can also mutate during replication, just as bacteria do. Thus, by the same mechanism that may lead to bactericide-resistant bacteria, a new phage can recognize these altered bacteria and change, thereby enabling infection by the phage.

Lastly, the use of enzymes may find their place as anti-microbial agents in waterborne technology. Important characteristics for a chemical antimicrobial substance are their capability of inhibiting bacterial colonization and there are different types of enzymes that show antimicrobial activity in in-vitro tests. One example is dextranase. Certain enzymes are believed to dissolve the linkages, theoretically resulting in a reduced accumulation of bacterial colonies. The gist of the enzymes activity is to control the proliferation of bacteria by augmenting the presence of bacterial-produced materials that aid bacterial resistance.

Conclusion

Manufacturing environments are not sterile, and when bacteria are provided with the proper pH, temperature, food or nutrient sources and moisture, these organisms can and will proliferate. This microbial proliferation and contamination can find its way into finished product, but by instilling good housekeeping and the proper use of bactericides, this potential threat can be greatly reduced and even eliminated. With a basic understanding as to how bacteria prevail and the available resources and techniques that are necessary to control microorganisms, their ability to cause contamination and product destruction can more easily be controlled.