The L. monocytogenes strains used in the study were, if not otherwise indicated, derived or obtained from the Special Listeria Culture Collection [SLCC] of H. P. R. Seeliger, Würzburg, Germany, or from the National Collection of Type Cultures [NCTC], Public Health Laboratory Service [PHLS], Central Public Health Laboratory, Colindale, London, UK. ( Table 1). Two auto-agglutinable blood culture and food isolates of L. monocytogenes exhibiting a rough phenotype were obtained from Dr Jim McLauchlin, Food Safety Microbiology Laboratory, PHLS, Colindale, London, UK. The clinical strains PHLRIII and PHLRIV were blood-culture isolates from a 76 and 72 years-old female and male, respectively; both individuals had sepsis and pyrexia. The spontaneously rough variants L. monocytogenes RI, RII and RIII were kindly supplied by Dr Andreas Bubert, Microbiological Analytics, Merck KGaA, 64271 Darmstadt, Germany. Where the rough variants RI and RII were previously derived from L. monocytogenes Mackaness and EGD, respectively ( Kuhn and Goebel 1989). Strain RII (SLCC 5779), was originally obtained from J. Potel (Institute for Medical Microbiology, Medical Academy, Hannover, Germany). L. monocytogenes RIII, another rough mutant derived from a smooth strain of serovar 1/2a, was obtained from J. Potel (Institute for Medical Microbiology, Medical Academy, Hannover, Germany). The rough variants SURI and SURII were derived from parent S1 and S2 strains, respectively, under conditions of heat stress ( Rowan and Anderson 1998). Stored bacteria were kept at −70 °C in phosphate-buffered saline (PBS) with 20% glycerol (v/v) until used.