Paint and the Constant Threat of Microbial Attack

Gram-positive organisms, endospores. Isolated from H2O-based product.
Paint applied to homes for exterior or interior use can be grouped into two general categories, waterborne and solventborne, and it typically has two basic functions, protection and decoration. Contamination of paint in either the wet state or on dried film can destroy these functions, leading to both aesthetic and physical degradation of the painted surface. For this reason, millions of dollars are spent annually by the paint industry on chemicals to protect their paint from microbial damage.

The protection of paint from attack by microorganisms is an area of the coatings formulation that is often misunderstood. This article provides an understanding of microbial contamination and can assist coatings manufacturers in preparing to do battle with the problems created by microbiological organisms.

The types of microbial attack that are of concern to the paint industry are bacterial, fungal and algal. Bacteria, small, single-celled organisms, grow readily in unpreserved water-based systems. Fungi are unicellular or filamentous organisms that are devoid of chlorophyll and attack the dry paint film, as do algae, which are unicellular or filamentous organisms that require light. While only waterborne coatings are susceptible to bacterial contamination in the can, both water- and solventborne coatings are prone to attack on the dry film by fungi and algae after the paint is applied.

To control these troublesome organisms, coatings formulators use microbicides that fall primarily into two categories: bactericides and fungicides. Bactericides come under the general identification of in-can preservatives and are designed to prevent microbial deterioration that results in spoilage of waterborne paint. Fungicides are predominantly included in a formula to inhibit the growth of fungi and algae on the dry film of both water- and solventborne paint.

Pure culture of Clostridium endospore stain at 1,000X.

In-Can Contamination

A can of waterborne paint is an ideal environment for microorganisms (bacteria/fungi) to grow due to the available nutrient sources (surfactants, thickeners, defoamers, etc.), available water supply, adequate oxygen supply, and suitable pH (most bacteria prefer 3.0–10.0). Bacterial contamination, especially of products “in the can,” may originate from a number of sources and may be influenced by several factors, including the formulation and plant design and overall manufacturing cleanliness.

Aqueous raw materials, notably surfactants, wetting agents, defoamers, polymer emulsions, pigment dispersions and china clay slurries, are themselves susceptible to microbial degradation. Unless these materials are manufactured under relatively clean conditions and with biocidal protection, they can become contaminated, thereby introducing that contamination into the finished product. The largest source of contamination is water, particularly recycled water.

Powdered raw materials, including fillers/extenders, starches, pigments and others, particularly those originating from natural sources, will frequently be contaminated with spores of bacteria and fungi.

Air, specifically in the paint factory, will contain a variety of microorganisms. This is particularly true in the case of dusty environments where starches, cellulosic thickeners, pigment powders and fillers are used. The typical plant environment is not sterile. Storage and mixing tanks may be open to the air or subject to considerable condensation when closed.

Accumulation of dried product, waste raw materials, and general refuse in the production and storage areas will act as a reservoir for microorganisms that may later cause contamination. Similarly, accumulated dried and semi-dried products in mixing vessels, storage tanks, pipework, and filling lines can also become contaminated and, in turn, infect new product with which they come into contact. Coatings manufacturers should be aware of and address all of these potential infection sources to minimize the risk of contamination.

Once a waterborne paint has been contaminated, its physical and chemical properties change. When introduced into paint, microorganisms start to multiply and attack organic components in the paint system. One of the first signs of microbial activity is viscosity loss, which is caused by microorganisms releasing enzymes that can destroy traditional cellulosic thickeners. These enzymes, which are capable of functioning independently of the cells that produce them, are large proteinaceous molecules that attack the polymer chains of the thickener and break them down into smaller compounds that are no longer capable of functioning as a thickener. For example, cellulose used as the thickening agent in a majority of waterborne paints is attacked by cellulytic enzymes and is degraded to cellubiose. The cellubiose is then further degraded to glucose or other sugars that bacteria and fungi ferment, producing acids and carbon dioxide. The production of acid formed by the degradation can significantly lower the pH, possibly resulting in further stability problems. As the breakdown of the thickener continues, the paint separates and suspended solids drop to the bottom of the container. In addition to the loss of viscosity, other effects can also be seen in contaminated paint.

Since carbon dioxide is also produced, it is common for contaminated paint to have foul odors, bulging cans, and even lids that pop off the containers. Finally, the paint may become discolored and have a stringy appearance.

Occasionally a coating displays gassing or a loss of viscosity that was not instigated by chemical causes, nor are any microorganisms present. The most common reason for this occurrence is that microorganisms were present but eventually died or were killed in the coating environment.

A second explanation is that an enzyme was introduced into the system with a raw material, and there was no bacterial contamination of the paint. Another potential reason is that a microorganism exists in the system but was not found by the normal test methods for aerobic bacteria, which uses specific food media and is carried out in the presence of air. Such microorganisms may have specific nutritional requirements or they may be anaerobic and thus are inhibited, or killed by atmospheric oxygen. This would explain why no bacteria would be found with traditional test methods.

Anaerobic microorganisms in the finished paint, raw materials or wash waters function differently than their air-loving cousins. Anaerobic bacteria oxidize organic matter using electron acceptors other than oxygen for nutrients. In carrying out their metabolic processes, they produce carbon dioxide, water, hydrogen sulfide, methane gas, nitrogen, ammonia, reduced organics and more bacteria. The degradation activities of anaerobic organisms are the result of a hydrolytic or simultaneous oxidation-reduction-type reaction, which most often leaves large acidic fragments in the paint. These acids could react in the paint with any calcium carbonate or a secondary pigment that is present as a filler and produce carbon dioxide gas. Even though anaerobic organisms produce a series of compounds different from aerobic bacteria, in the end, as far as the paint manufacturer is concerned, the results are similar: loss of viscosity, production of offensive odors, color changes, and/or gassing.

The most common bacteria found in paint fall into two main categories: gram-negative and gram-positive. Gram negative means that during the accepted staining procedure the bacterial cells are stained red, where as for gram-positive bacteria, the cells are stained purple. Alone, the staining procedure tells very little, as it is simply a means of differentiating bacteria. However, the gram-negative organisms are prevalent in contaminating waterborne paint, and the few gram-positive organisms that do appear are considered to be just fellow travelers. Except for an occasional situation, gram-positive organisms are not considered the primary culprits in contamination.

The following gram-negative organisms are commonly found in contaminated waterborne paint: Pseudomonas sp., Enterobacter sp., Proteus sp., Micrococcus sp. and Aerobacter sp. Gram-positive organisms of the Bacillus sp. may also be found in contaminated paint.

The following anaerobic species have been isolated from contaminated waterborne paint: Bacteriodes and Desulfvobrio, which are gram-negative, and Clostridium and Lactobacillus, which are gram-positive.

Knowledge of why, where and when microbial contamination can occur would alleviate many problems that coatings manufacturers encounter. A common-sense approach should always prevail. The coating manufacturer should recognize that contamination can be introduced through the raw materials, and that the microbicide should be added early in the production process. Keeping the plant as clean as possible, and paying special attention to the cleanliness of the mixing vessels, pipework, hoses, and storage tanks would stop or curtail any plant-related problems. Clean environments in conjunction with an in-can preservation program can minimize any loss due to contamination.

Pure culture of Aspergillus Wentii at 200X with lactofuchsin stain.

Dry Film Preservation

Surface growth of fungi and algae has been recognized for many years as the major cause of disfigurement and deterioration of the dry film of both water- and solventborne coatings. This defacement is a serious problem that requires expensive and sometimes extensive actions to repair.

Fungal growth can be found in many areas in and around the home: bathrooms, basements, and garages — any location that’s warm, damp, and in proximity of high humidity. On exterior surfaces, fungal growth is often heaviest in locations where shrubbery or shade trees restrict rapid surface drying and on north-facing walls. Paint surfaces protected from direct rainfall accumulate detritus (products of disintegration or wearing away) and fungi.

In general, fungi develop more commonly on paint coated over wood surfaces than on metal or masonry. It has been shown that wood substrates can provide nutrients from their water-soluble extractives that diffuse into the paint film and can be a significant moisture and food source. Certain species of wood are more fungal resistant than others (i.e., cedar), while most species of pine exhibit relatively little fungal resistance. Paint disfigurement results primarily from the growth of pigmented fungi and the development of spore clusters on the coated surface. The aesthetic properties of the paint film are often further reduced by the accumulations of dirt and airborne detritus that cling to the sticky exposed mycelia or spore clusters. Optimum fungal growth conditions include a humid environment and a neutral-to-acidic pH with an organic food source.

Alternaria Sp. isolated from a paint chip.
A key variable in the susceptibility of a coating to fungal attack is the formulation itself. Once applied to a surface, emulsion paint loses water by evaporation, and the polymer particles coalesce to form a paint film that contains pigments plus residues of the various components and additives of the paint. The types and amounts of these residues will significantly influence microbial susceptibility of the film.

The organic nutrients required by fungi may also come from an accumulation of organic debris, such as airborne dirt. Given a suitable moist or humid condition, fungal spores present on the surface of the susceptible paint film will germinate to produce a fine network of filaments (hyphae) or colonial growth.

The most common fungal species found on contaminated dry paint film are Aureobasidium, Alternaria, Aspergillus, Cladosporium and Penicillium. However, the dominant fungal species will vary with the climate and condition of the paint film.

The addition of an effective fungicide to the paint formulation during production provides protection during both storage and use. Ideally, fungicides must be highly effective against fungi, nonvolatile, safe in handling and use, environmentally acceptable, compatible with other paint components, have limited solubility, and produce no color changes.

In addition to fungi, alga is the other main organism that disfigures dry paint films. Algae are sometimes confused with fungi, especially if the color of the algae is black, brown or orange instead of the more common green. Algal growth requires high humidity, a neutral-to-alkaline environment, and light to allow for photosynthetic processes. To facilitate growth, algae also need trace minerals, which they can find on masonry surfaces. Protection of the dry film from algal disfigurement has become a growing trend in the paint industry.

For algae to grow on coated surfaces, the criteria previously stated must be met. The growth can be rapid and take place over a wide temperature range. Algal growth is mainly a problem of appearance, as it does not primarily cause damage. Direct deterioration by these organisms is not considered detrimental; however, their capacity to hold water may indirectly result in significant damage. With their chlorophyll and light use, algae can build up organic material from water and carbon dioxide, which can then serve as nutrient material for fungi and other microorganisms. Algal growth in temperate climates tends to be profuse but is much more prolific in the tropics. As with fungi, their initial disfigurement effects result in the discoloration of coatings applied to masonry, metal, timber and other substrates.

The following are representative algal species that have been isolated from contaminated paint surfaces: Chlorella, Chlorococcum, Oscillatoria and Trentepohilia. Since algae obtain their nutrients by photosynthesis, the addition of an algaecide that disrupts this process can prevent growth and development.


Microorganisms are capable of the destruction and disfigurement of both water- and solventborne coatings, costing the coatings manufacturer and consumer millions of dollars per year in contaminated material and damage claims. Under favorable conditions — an ample supply of nutrients, moisture and proper temperature — microbial contamination of unprotected coatings is a virtual certainty. With a minimal amount of effort and some common sense formulating, coatings manufacturers can ensure that their coatings are protected in the can, in use, and on the dry film. Preventing microbial contamination is the most crucial element in the fight against microorganisms. The best line of defense is a regimen that entails the use of microbicides, proper house cleaning, and knowledge of the factors that contribute to microbial attack.

For more information on antimicrobial materials, contact Buckman Laboratories Inc., 1256 N. McLean Blvd., Memphis, TN 38108; phone 800/282.5626 or 800/BUCKMAN; fax 901/276.5343; visit; e-mail or wjconti@buck