BY DR TIM SANDLE | 25 JUNE 2023
Introduction
Good contamination control measures include cleaning and disinfection, which play an important function in keeping cleanrooms and equipment sanitary. Such requirements extend to transfer disinfection. The optimal material for the construction of equipment and transfer hatches is electropolished stainless steel due to the minimising of opportunities for bacterial attachment. However, the efficacy of stainless steel decreases when the surface suffers from abrasion, scratching or corrosion. These defects can arise when the surfaces comes into repeated contact with other metal objects or as result of chemical contact, such as the residues from certain disinfectants. This means the care and regular treatment of such surfaces is an important contamination control measure.
Bacterial adhesion
The ability of microorganisms to adhere to surfaces has significant implications for contamination control. As time passes, microorganisms adhere more firmly and become more challenging to remove or inactivate. Hence, without appropriate detergents and disinfectants combined that the correct application techniques, microorganisms attached to surfaces can escape the decontamination process.
The ability to attach to - and subsequently detach from - surfaces is a characteristic of all microorganisms, although some bacteria are more adept than others particularly those with fimbriae or pili which can produce so-termed ‘adhesion proteins’. Bacterial morphologies are also noteworthy. Where surface defects are linear (like scratches), then cocci are more likely to become trapped compared with rod shaped bacteria (1). Of greatest concern are scratch patterns with long linear alternating grooves and ridges. Bacteria will become attached into scratches in the longitudinal orientation when the width of the scratches corresponds to the width of the bacterial cells. This alignment does not appear to be species dependent (2).
Attachment is advantageous and often necessary for survival of microorganisms in the natural environment. The process allows microorganisms to exert some control over their nutritional environment and offers protection from environmental stresses. The adhesion process, the time taken to form an irreversible attachment and the tendency to develop a biofilm, are species dependent (3). These are influenced by the material properties, such as with surface charge, hydrophobicity and hydrophilicity (bacteria are more likely to attach to hydrophobic, positively charged surfaces), and relate to the hydrodynamic conditions of the environment (4). The adsorption of organic molecules, such as proteins, onto surfaces also contributes to bacterial attachment since the conditioning of the surface may alter the physical–chemical properties of the surface. These various factors are interdependent.
Surface materials and types
The type of surface and its relative smoothness is also a factor in bacterial adhesion. Surfaces differ in terms of surface morphology, such as fractal dimensions, Z ranges, and roughness (5). Roughness is irregularities in the material’s surface topography, and it is typically measured by Ra values, with values of <0.8 µm being considered optimal. Of the different materials available, electropolished stainless steel creates a surface of low-level of roughness, allowing fewer bacterial cells to attach (without electropolishing, when metal surfaces are machined, ground or lapped, an amorphous layer forms and this sustains the trapping of bacteria). The quality of the fabrication is also important - corners and sharp angled equipment fabricated of stainless steel often contains welds. Welds need to be smooth, otherwise they can influence bacterial accumulation and colonisation (6).
The grade of stainless steel is an important consideration. 316 Stainless steel is smoother than the type 304 material, and 316 grade has fewer microscopic surface scratches, grooves and associated deformations, making it superior for critical surfaces (7). However, even 316 can become damaged or corroded.
Surface damage and increased increased risk
Damage to surfaces makes the material more challenging to clean and disinfect, in terms of cleaning agents and disinfectants reaching all areas. In addition, the roughened nature of the surface enhances bacterial attachment. For example, one study demonstrated how stainless steel, abraded artificially or impact damaged to a similar degree, was more challenging to decontaminate by one log order of bacteria after cleaning compared with non-altered materials after treatments. By abrasion, this was surfaces characterised by pitting, crevices or jags. The age of a surface may also be a factor.
Corrosion is an important factor for the durability of a metal finish and presents a real concern for many wet processes, including mixing vessels, water distribution pipework and surfaces treated with more aggressive chemicals like sporicides. Smooth surfaces contribute to corrosion resistance (8). Regular inspections of internal surfaces must be made for rouging and the application of sporicidal agents often requires a rinse step to avoid pitting (especially with chlorine-based chemical agents) (9). In general, electropolished surfaces offer greater resistance to corrosion compared with other surfaces.
The reasons for the greater resistance to cleaning and disinfection relate to the abrasion leading to an increased number of attachment sites, the creation of larger bacterial/material surface contact areas, and, applicable to water systems, the forming of topographical areas in which applied cleaning shear forces are reduced (10).
Summary
Bacterial adhesion to surfaces is complex and an interplay between both surface finish and surface chemistry. Materials that most greatly resist surface changes, such as electropolished stainless steel, will remain more hygienic when subjected to natural wear than materials which become more readily damaged. However, stainless steel is not immune to damage – either physical or chemical. It is important that regular inspections are undertaken of critical equipment, especially equipment where contact with the surface is part of the decontamination step, as with transfer hatches. Scratched, damaged or corroded surfaces should be treated and repolished in order to boost the likelihood of contamination control success.