Product Protection - Aqueous products such as emulsion paints, lacquers and a number of plasters partly consist of organic components that can be used as sources of nutrient by micro-organisms such as bacteria, fungi and yeasts. They are therefore susceptible to microbial attack and require optimum protection against biocorrosion. The use of microbicides is often the only way to provide the customer with a long-term stable and usable product.

For a wall paint that is to be used on a customer's property, preservation firstly prevents the growth of organisms that might be harmful to health, such as molds, and secondly the property remains presentable for longer, which means that expensive redecoration does not have to be carried out so often. Apart from avoiding risks to health, appropriate preservation therefore makes a substantial contribution to prolonging the service life of a coating, thereby saving natural resources. This shows that sustainability and economic interests are not necessarily incompatible, but can also complement each other.

Methods of Identifying Microorganisms

The selection of the right microbicide is based on the type of destructive micro-organisms in the product. It is usually not a single genus of bacteria, fungi or yeasts, but rather complex microbial communities that are responsible for degradation of emulsion paints. To effectively control the spectrum of destructive microorganisms, the composition of these communities must be known.
Up to now, conventional methods of identification have been used that necessitate cultivation of the affected product. There are two general methods of identifying microorganisms:

The first method characterizes bacteria by phenotyping. Here, the colony that has grown is examined, for example, for size, shape and color. This examination is normally followed by biochemical methods, such as Gram staining and oxidize reaction, which allow the search to be narrowed down to species and genus.

Another method was only recently established in routine practice that allows characterization at genetic level. This characterization is based on evolutionary changes. During the course of development, because of changed environmental conditions, different bacterial species have developed from just a few antecedents. These adaptations took place at DNA level in the information-carrying regions, the genome. Molecular-biological identification is based on differences in the genetic code.

An area of the DNA that is used in practice for this is, for example, the 16S rRNA gene. The sequence of a segment of this gene can be assigned precisely to a specific species of bacteria.
With the above-mentioned method, the bacteria have first of all to be cultured in the products, and the grown colonies have to be separated. Genomic DNA is isolated from each colony and the 16S rRNA gene segment is amplified by polymerase chain reaction (PCR). By sequencing the gene segment and making a comparison with published sequences in a database, the individual organisms can be identified by family, genus and ultimately species.

The disadvantage of the methods described here is that they need a cultivation step. With this, however, because of the chosen experimental parameters such as nutrient supply, temperature and oxygen level, the result can be a preselection of certain microorganisms. Organisms which are unable to reproduce under the selected culture conditions cannot be identified by the methods described above.
This problem could be overcome using a method which is not culture-dependent. We have therefore developed a method which allows DNA-based identification of bacteria directly from an infested product, e.g. an emulsion paint.

Culture-Independent Identification of Bacterial Communities

This newly-developed method is based on the sequence-dependent separation of gene segments of the same size by denaturing gradient gel electrophoresis (DGGE).
In this case, the DNA of all the bacteria contained in the product is isolated and the respective 16S rRNA gene segments are then amplified. If there is infestation by a bacterial community, a mixture is obtained of 16S rRNA gene segments of the same size, which only differ from one another in their sequence. This mixture is then separated by DGGE according to sequence.

This separation is possible because gene segments with different DNA sequences have different melting points, i.e. the type and frequency of the building blocks of a DNA sequence determine the temperature at which the DNA double strands separate from one another. This characteristic is utilized in electrophoresis as a partly separated strand of DNA is less mobile in the gel than a double strand. A mixture of 16S rRNA gene segments can therefore be separated using this method.

The fragments that are now separated from each other are sequenced and compared with known DNA segments in a database. Ideally, using this method, a specific number of identification results is obtained which reflects the number of contaminating bacterial strains in the product.

Direct DNA Isolation from Contaminated Products

To be able to use this method in practice, bacterial DNA have to be isolated from emulsion paints and the gene fragments have to be optimally separated from each other in the DGGE. This was tested in a test system with three strains of bacteria, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus in the same bacterial concentration in three different paint systems. Typical external emulsion paints with a base of pure acrylic, styrene acrylic and polyvinyl acetate binders were used.
This simple method of DGGE should allow individual species to be identified from a mixture of bacteria or directly from the paint. For this, isolated DNA was amplified and the corresponding gene fragments separated by DGGE.

As expected, all the organisms from a mixture of bacteria were identified with a high degree of conformity. Somewhat surprisingly, from a contaminated emulsion paint, only the Pseudomonas aeruginosa species was prominently represented.
A comparison of the methods shows that culturing gives a varying number of identification results (table 1), with the non-culture-dependent DGGE method allowing a rather more accurate identification as regards the Pseudomonas species found.

The fact that relatively fewer bacterial species were found directly from a coating product compared to an artificial mixture of bacteria could be an indication of the fact that not all bacterial strains that are present are equally involved in the destruction of the product. Separation using DGGE correlates to some degree with the concentration of bacterial DNA in the paint. To this extent, DGGE can be regarded as a semi-quantitative method. Thus, the bacteria detected by DGGE could be the dominant species in the coating product which are mainly responsible for destruction of the product.

With the conventional method the growth of these bacteria, which may be insignificant in terms of destruction of the paint, is also promoted. This means that the presence of these strains of bacteria would be substantially overvalued.

‘As Much As Necessary, As Little As Possible'

The results show that it is possible in principle for a bacterial community to be identified by DGGE without culturing. Regarding contaminated products such as emulsion paints, at present fewer strains are identified than with the conventional method. It seems possible, however, that optimization of the DGGE method would close this gap, thus providing new insights into the processes involved in the destruction of paint in the container which would allow the user, in line with the motto "As much as necessary, as little as possible," to protect products not only effectively but also in a sustainable way.