Model systems allowing quantification of sensitivity to disinfectants and comparison of disinfectant susceptibility of persistent and presumed nonpersistent Listeria monocytogenes

Abstract

Aims: To determine if Listeria monocytogenes persistent strains differ from presumed nonpersistent strains in disinfection susceptibility and to examine the influence of attachment and NaCl on susceptibility.

Methods and results: Two model-systems that allowed quantitative inter-strain comparison of disinfectant sensitivity were developed. Persistent L. monocytogenes were not more tolerant to the disinfectants Incimaxx DES and Triquart SUPER than presumed nonpersistent isolates. When calibrating the systems with respect to presence of biological material and cell density, attached bacteria were as sensitive to disinfectants as were planktonic bacteria. Growth with 5% NaCl increased the tolerance of planktonic cells to Incimaxx DES. All strains of spot inoculated L. monocytogenes survived well 20 h of drying when protected by growth media and 5% NaCl, but were not protected by NaCl against disinfection.

Conclusions: Persistent strains of L. monocytogenes are as susceptible to disinfectants as are presumed nonpersistent strains and attachment does not render the strains more tolerant to disinfectants. Growth with NaCl affected the susceptibility of the planktonic L. monocytogenes to Incimaxx DES and protected spot inoculated cells during drying.

Significance and impact of the study: Attachment to surfaces does not per se offer protection to L. monocytogenes against disinfectants and disinfection tolerances do not appear to influence the ability of a strain to persist.