[38] Methods for studying microbial colonization of plastics

This chapter presents the basic approaches to counting microorganisms on a plastic surface, which is the material of choice for the fabrication of most medical appliances. Much of the discussion presented in the chapter is also relevant to the study of the colonization of non-plastic materials by environmental organisms and non-plastic medical appliances by pathogenic bacteria. Like the colonization of tissues, the colonization of medical devices is a multiphasic process. Colonization begins with either a directed or a random encounter of the microbe with the substratum followed by the rapid binding of the microbe to the surface. Microbial proliferation, the penultimate phase of colonization, is subject to the concentration of nutrients in the bathing fluid and the presence of other microorganisms on the surface. When colonization takes place in vivo, microbial multiplication may also be subject to the humoral and cellular immune responses of the host as well as the administration of antimicrobial agents. Colonization is concluded by an often-overlooked stage in which there is release of free-floating (planktonic) organisms from the sessile colony and dispersal of the planktonic forms to new colonization sites. Release occurs when daughter cells stop expressing adhesive structures. This may be accomplished either by a cellular response to a change in environmental conditions (phenotypic modulation) or by a (usually reversible) spontaneous change in the genetic mechanisms that govern the expression of microbial adhesive structures (phase variation).