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Tech Advisory

The war against invasive bacteria that stick to surfaces

by Mafu Akier Assanta


For the food industry, the world of microbes might be compared to a minuscule battlefield, where its allies and its enemies are engaged in a deadly struggle on a microscopic scale. Pathogens and micro-organisms that cause spoilage are particularly formidable adversaries, as they give rise to many economic problems and threats to human health. One major source of contamination is surfaces. Bacteria tend to stick tightly to surfaces, even surfaces that have been meticulously disinfected. .

Any food, regardless of whether it is fresh or processed, or of plant or animal origin, inevitably harbours various micro-organisms, such as bacteria, viruses, yeasts, parasites and moulds, which may either be vegetating passively or multiplying. In the latter case, if the micro-organisms in question happen to be harmful ones, they may become a major source of contamination. .

Food that has been contaminated with pathogens such as Aeromonas hydrophila, Clostridium botulinum, Eschericha coli, Listeria monocytogenes, Salmonella, Staphylococcus aureus and Yersinia enterocolitica may cause illnesses of various kinds, and this situation is a source of concern both for the food industry and for public health officials. Many epidemiological reports have pointed to the fact that the potential sources of food contamination are many and complex, and that they include, in particular, the surfaces with which the product comes into contact: processing equipment, tanks, counters and so on. Even cleaning with disinfectants does not always eliminate these lurking microscopic intruders. .

STICKING TO A SURFACE IS SO EASY!

Manufacturers can improve the odds in their favour through an understanding of how bacteria stick to surfaces. As a rule, this process comprises four stages: transport, initial adhesion, bioattachment and colonization. .

In response to gravity, convection or diffusion, or by an active movement of its own, a cell in suspension in the environment is carried to the surface, with which it makes contact. At that moment, owing to the interactions that occur upon contact between the two entities (the surface and the bacterium), the cell may adhere to the surface either "reversibly" (i.e. temporarily) or "irreversibly" (i.e. permanently). .

Reversible attachment is a physical phenomenon which as a rule is non-specific and precarious and may come to an end within a few tenths of a second. Various attractive forces may be involved, depending on the type of surface, but as a rule they are either chemical (Van der Waals bonds, hydrogen bonds, hydrophobic interactions and so on) or electrostatic. According to what is known as the DLVO theory, the reversible attachment of a bacterium to a surface, or "adsorption", depends on attractive or repulsive forces between the layer of ions around the bacterium and the charge carried by the surface. In other words, if a surface is positively charged and a bacterium negatively charged, they will attract each other as readily as two polyester socks that have been left too long in the dryer! In addition to DLVO, many other factors must be taken into account, such as the surface tension of the system under consideration (as modulated by the presence or absence of soap or other surface-active agents), the deformation of the bacterial cells, or a non-uniform distribution of electrical charges, where, for example, part of the surface is entirely negatively charged. .

With irreversible attachment, the bacterium is first adsorbed on the surface, and then actively attaches itself to it by its metabolic activity. The cell secretes a substance-a poly-saccharide known as glycocalyx-which subsequently enables it literally to encapsulate itself on the surface. This type of bioattachment occurs much more slowly, and it depends on the type of bacterium involved, the size of the bacterial population in the environment, and the duration of its growth phase. The last-named of these factors also depends on the bacterium's environment-the temperature of the solution, its pH, the electrolyte concentration and the availability of nutrients. In addition, the strength of the attachment will depend on the charge carried by the surface and the duration of the contact. All these factors will greatly affect the effectiveness of disinfection. .

Microcolonies, effectively protected by the glycocalyx, form on the surface. As these grow and run together, they form a coating which is thin and superficial at first, but becomes progressively thicker as the bacteria grow, ultimately attaining a thickness of several millimetres. This is what is known as a biofilm. .

The most familiar type of biofilm, of course, is the plaque that forms on teeth. It is a hazard to dental health, and can be removed only by meticulous brushing. Once a biofilm has formed, it constitutes a microniche that provides bacteria with an environment which is highly conducive to their survival while also protecting them from various types of aggression from outside, such as flows of liquids, changes in pH or temperature, and chemical cleansers or disinfectants expressly designed to exterminate them! .

In food processing, the charged surface of a biofilm constitutes a continuing source of contamination for any food product that comes into contact with it. .

Beware of the biofilm! .

Biofilms have been well known for many years, but they are attracting progressively greater interest in the food industry because of their obvious status as food contaminants. A biofilm is the result of a buildup of microcolonies and their extracellular secretions, and a biofilm on a surface may contaminate all food products with which it comes into contact, causing infections in consumers if it consists of pathogens such as Listeria monocytogenes, E. coli or Salmonella. .

Biofilms cannot readily be eliminated by chemical means, and this greatly complicates the task of cleaning and disinfecting many kinds of surfaces in the industry. Biofilms also directly affect the effectiveness of various procedures. A biofilm on the inner surface of a water supply pipe or plastic tubing carrying maple sap, for example, may cause a partial obstruction, thereby reducing the flow within the system. This can substantially reduce the efficiency of constant flow systems and cause increased corrosion in the case of some materials. Over and above all this, biofilms cause energy losses by acting as thermal insulation in heat exchangers. .

Biofilms are bad news in the food industry: they mean economic losses, but above all, they mean danger! .

IN SEARCH OF THE IDEAL SURFACE

Among conditions that are conducive to biofilm formation in the food industry, the type of surface with which the bacteria come into contact heads the list. Bacteria are especially attracted to porous surfaces such as wood and ceramic materials, owing to their accommodating microcavities. Ranking surfaces by increasing order of porosity, we find that glass is least porous, followed by stainless steel, aluminum, rubber and plastics. Examination with a scanning electron microscope reveals that a stainless steel surface will harbour a much smaller population of Escherichia coli, the cause of hamburger disease, than a plastic surface. .

Elbows in pipes and junctions of any kind on the surface are also factors that are conducive to the accumulation of residues and bacteria. Even in a straight production line, a surface that is cracked, split and fissured by wear will harbour more bacteria than a smooth new surface. Under the microscope, the surface of an old plastic cutting board can be seen to be criss-crossed by veritable "canyons" in which microbes can live undisturbed, busily contaminating any food that is cut up on the board! .

Surface porosity certainly plays a key role, but surface wettability (a property that causes liquids to spread rather than forming drops) seems to be of critical importance as well. Water-repellent surfaces that carry a low charge, such as teflon, polyethylene, polystyrene and the like, appear to facilitate the attachment of bacteria more readily than surfaces made of a hydrophilic material such as glass, regardless of the charge it carries. Furthermore, the presence of some types of organic molecules or ions in the immersion environment can modify a surface's adhesion properties considerably. For example, in most cases, monovalent ions (Na+ and K+), bivalent ions (Ca++ and Mg++) or trivalent ions (Al+++) offer bacteria a good chance of sticking to the surface, whereas in the absence of those ions, not only is adhesion less likely to occur, but any bacterial cells that do become attached are likely to come adrift again in short order. This is a field of research that should be explored in the quest for surfaces that enhance food safety. .

Increasingly, scientists are trying to develop materials with properties that are not conducive to bacterial adhesion: something with self-cleaning properties, for example, characterized by attractive forces too weak to maintain contact with bacteria. Selecting a suitable material is no easy matter, especially in an environment in which various different kinds of foods are handled. Friction between circulating fluids and equipment surfaces may seriously affect the properties of the equipment in question. Accordingly, some scientists are focussing on the possibility of chemical agent (ion) grafting, to make bacterial attachment more difficult. .

To the best of our knowledge at the present time, good cleaning practices at food product manufacturing facilities and more effective control of the micro-organisms that produce biofilms are still essential means of ensuring that surfaces and equipment which are in contact with foods are kept free of contamination. In the next article, we shall look at some possible solutions to the problem of keeping surfaces clean. .