In water microcosm experiments, the survival times ofCampylobacter isolates differed by up to twofold, as determined by culturing; this difference increased to fourfold when particular combinations of temperature and oxygenation were used. The mean survival times were much longer at 4 and 10°C (202 and 176 h, respectively) than at 22 and 37°C (43 and 22 h, respectively). The influence of anaerobiosis on survival time was less dramatic and differed considerably between isolates. In a two-stage water distribution model preparation containing a biofilm consisting of standardized autochthonous water microflora, Campylobacterisolates continued to differ in survival time. However, the survival times of cultures were considerably longer in the presence of the autochthonous water microflora (strains CH1 and 9752 survived 700 and 360 h, respectively, at 4°C) than in the sterile microcosms (strains CH1 and 9752 survived 230 and 157 h, respectively). Although increased temperature and oxygenation were generally detrimental to culturability, the interaction of these two factors influenced the two strains examined differently. When the organisms were grown aerobically at 30°C, the survival of the two strains was reversed; aerobiosis decreased the survival time of strain CH1 by 30%, but unexpectedly improved the persistence time of strain 9752 by more than threefold. Persistence times within biofilms were much longer when they were determined by detection methods not involving culturing. Immunofluorescent-antibody staining demonstrated that the pathogen persisted up to the termination of the experiments after 28 and 42 days of incubation at 30 and 4°C, respectively. The specificity of detection within intact biofilms was reduced because of high background fluorescence. However, preliminary studies with aCampylobacter-specific rRNA probe revealed the same extended persistence of the pathogen within the biofilms.

A total of 289 Listeria monocytogenes strains isolated from a poultry-processing environment and poultry products over a 6-month period were characterized by random amplification of polymorphic DNA, (RAPD) to pinpoint sources of contamination within the plant and gain some measure of the persistence of individual genotypes within this environment. Eighteen RAPD profiles (A through R) were identified within this group, with 64% (184 of 289) of all strains displaying a single RAPD profile, RAPD type A. This genotype was more prevalent in the raw-poultry-processing environment, where, although its origin within this environment appeared to be the incoming birds, it was also widespread on food contact surfaces, floors, and drains. This was the only genotype which persisted throughout the entire 6-month period, and it and RAPD type B were the only two genotypes found in both the raw-and cooked-poultry-processing environments. L. monocytogenes strains isolated from cooked poultry products and the cooked-poultry-processing environment up to 1 year later (17 strains) contained only RAPD types A and B, highlighting the potential which exists for persistent strains to cross-contaminate foods processed in that environment. The other genotypes (C through R) occurred more sporadically, suggesting varied sources of contamination. These were confined to either the raw-or the cooked-poultry-processing environment and were relatively short-lived. Further characterization of a selection of RAPD type A strains, together with strains of RAPD types B through R, was carried out by multilocus enzyme electrophoresis. Strains of RAPD type A contained two electrophoretic types, one of which was serotype 1/2a and the other was 1/2c. Each electrophoretic type appeared to be equally persistent and widespread, and their isolation from the same samples suggests that they may coexist within this environment. Multilocus enzyme electrophoresis provided no further differentiation within RAPD types B through R, demonstrating the high level of discrimination already achieved by RAPD. The latter method proved to be a valuable typing tool in this study and offers the potential for food processors to gain valuable information on sources of contamination and the persistence of strains within the processing environment.

The incidence ofListeria spp. and Listeria monocytogenes in a poultry processing plant and in raw and cooked poultry products was determined over a 6-month period. Within the raw and cooked poultry processing environments, 46% (36 of 79) and 29% (51 of 173) of the samples contained Listeria spp. while 26% (21 of 79) and 15% (27 of 173) contained L. monocytogenes, respectively. Various sites within the processing environment were found to be consistently positive for L. monocytogenes throughout the entire sampling period. Of the raw and cooked products tested, 91% (53 of 58) and 8% (8 of 96) were found to contain Listeria spp. while 59% (34 of 58) and 0% (0 of 96) contained L. monocytogenes, respectively. Although L. monocytogenes was not detected in the cooked products examined, the presence of other Listeria spp. highlights the potential which exists for postprocessing contamination. Multiplex PCR proved to be a convenient and time-saving technique for rapid confirmation of Listeria spp. and L. monocytogenes in a single reaction.

The 10-mer primer OPM-01 (5'-GTT GGT GGC T-3') was used to generate random amplification of polymorphic DNA (RAPD) profiles by polymerase chain reaction for 91 strains of Listeria monocytogenes from raw milk, food, and veterinary, medical, and food-environmental sources. The profiles obtained contained 1 to 10 bands within the molecular size range of 0.5 to 5.0 kbp. Reproducibility was enhanced by annealing at low stringency and introducing a 1-min ramp time between annealing and extension temperatures. Thirty-three RAPD profiles were observed, with specific profiles being observed for strains from each source. RAPD profiles allowed discrimination within serogroups, although five RAPD profiles which were not confined to one serotype were found. Within food strains, one RAPD profile was more common than others, suggesting this to be a common type among strains from this source.

The European Suspension Test was used to assess the relative resistance of 19 individual Listeria monocytogenes genotypes, isolated from the poultry processing environment, to three commercially used disinfectants employed in the plant at the time of their isolation. To establish the relative resistance between the strains, the concentration of each disinfectant was reduced until inter‐strain variation became apparent. For Darasan 214 and 7058, variation was detected at 0·1% and 0·5% v/v, respectively, while Daraclean 7361 had to be reduced to only 2·5% v/v. At these concentrations, the mean microbiocidal effect (ME) of each disinfectant ranged between 4·3 and 3·1 log10 reduction in cfu ml−1. Significant differences between the strains were obtained with respect to their resistance to the disinfectants employed (P < 0·01), but the overall log10 reduction for genotypes ‘A1’ and ‘A2’, which were found to persist in the poultry processing environment, were not found to be significantly different from the genotypes which had been isolated on a more sporadic basis (P > 0·05). The L. monocytogenes strains fell into four groups with respect to incidence and size of plasmids isolated. The first group contained strains which carried two plasmids (5 and 40 MDa) and the other three (groups 2, 3 and 4) comprised strains which carried a single plasmid (14, 47 and 52 MDa, respectively). There was no correlation between persistent and sporadic strains with respect to incidence and size of plasmids isolated. Moreover, the strains which carried no plasmids were found to be as resistant to the disinfectants as those which did carry plasmids, suggesting that the plasmids isolated did not confer resistance of L. monocytogenes planktonic cells to the disinfectants tested. Therefore, it is unlikely that the strains which had been found to persist in the poultry processing environment did so by means of plasmid‐mediated resistance to the commercial disinfectants used.