Surface type influences the mortality rate of bacteria
Wildführ and Seidel reported that bacteria die faster on stainless-steel and glass surfaces than on plastic surfaces. These
comparative studies tested Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans demonstrated that the survival rate on plastic was almost double (50% more) than on stainless-steel or glass coupons (35).
In this study test microorganisms' suspensions were transferred onto stainless-steel, glass, and plastic coupons and then
dried. After 90 minutes, it was evident that only a very small quantity of bacteria was present on the stainless-steel and
glass surfaces, but the quantity of viable bacteria on the plastic was still high up to 120 min later.
Tiller et al. reported that plastic coupons (i.e. polypropylene and Polystyrene) kept bacteria more viable than aluminum,
steel, and glass (34). In this experiment, suspensions (106 cells per mL) of Staphylococcus aureus in distilled water were sprayed over various surface materials, air dried for 2 min, and were incubated in a 0.7% agar bacterial
growth medium overnight, after which the colonies counted. Bacterial adherence in the presence of oral liquid pharmaceuticals
on different coupons showed that rubber and plastic coupons were significantly more accessible to the bacteria than glass
coupons (36, 37).
The literature cited herein supports the argument that porous nonactive carbon polymer surfaces such as plastic and rubber
harbor viable bacteria for longer periods than steel, glass, metals, and metal alloys surfaces. Table I summarizes the interaction
of the different surfaces with microbes.
Table I: Microorganism-substratum interaction for microorganism adherence and survival
As stated previously, validation studies are usually performed using representative surfaces identified on the production
areas but no other justifications are currently included. Because of this, results are subject to variability and low reproducibility
at high costs and efforts. Even though useful information arises during these processes, method suitability must be seen in
terms of the capacity of the method to recover viable microbes. For method suitability purposes, the author highly recommends
selecting surfaces that may potentially harbor and allow the survival of microbes for the longest time possible. Surfaces
that minimize survival may jeopardize method-validation results. Surface(s) must be allowed to verify the method effectiveness
for its intended use without the possibility of harming the microbes housed there.
One porous and one nonporous surface may be chosen to test methods and may support the method suitability for sampling all
other drug-contact surface materials. A porous inert surface such as plastic and a nonporous inert surface such as as polished
stainless steel are represented in most food, medical device, and pharmaceutical production areas. The author highly suggests
performing test method suitability studies using wild-type isolates from production surfaces instead of laboratory-adapted
strains of bacteria. Wild-type strains are a better representation of the organisms encountered on production areas than those
strains that lack wild characteristics. The method will be ready for use in monitoring studies once it is optimized and its
accuracy is established.
Angel L. Salaman-Byron, PhD, is a microbiology consultant at Pharma-Bio Serv, tel. 787.384.7216, email@example.com