Resistance of Escherichia coli growing as biofilms to disinfectants

The bactericidal activity of various disinfectants (cationic or amphoteric surfactants, oxidizing agents, phenolic derivatives) was determined against Escherichia coli CIP 54127 obtained by culture on tryptic soy agar (in-suspension or on-germ-carrier test) or in the form of biofilms produced in a continuous culture system. The bacteria tested on germ-carriers or included in biofilms were more resistant than the same strain in suspension. The extent of the reduction in activity depended on the nature of the disinfectant. In the two cases, the greatest reduction was observed with benzalkonium chloride and hexadecyl trimethylammonium bromide, the agents with the lowest hydrophile-lipophile balance. The activity of the oxidizing agents (sodium hypochlorite, peracetic acid/H2O2) and alkyl trimethylammonium derivatives (C12 and C14) was somewhat reduced, while that of the phenolic derivatives (o-cresol, phenol) was either slightly attenuated or unaffected. The reduction in sensitivity was attributed to a reduced accessibility of the bacterial cells to the disinfectants, due to the fact that the former adhered to a support. Furthermore, the interfering action of the substances in contact with the bacteria (milk in the germ-carrier test and exopolymers in the biofilms) could play a role. The reduced sensitivity of the bacteria in the biofilms was not due to any alteration in the metabolic state of the bacteria (mostly in a quiescent state) since this resistance was lost after the mechanical resuspension of the cells before the contact with the disinfectants.