Letters in Applied Microbiology 1997, 24, 261–264 The cross-contamination and survival of Salmonella enteritidis PT4 on sterile and non-sterile foodstuffs M.A. Bradford, T.J. Humphrey 1 and H.M. Lappin-Scott Department of Biological Sciences, University of Exeter, Exeter, and 1 PHLS Food Microbiology Research Unit, Heavitree, Exeter, UK 1270/96: received and accepted 23 August 1996 M.A. BRADFORD, T.J. HUMPHREY AND H.M. LAPPIN-SCOTT. 1997. The ability of two strains of Salmonella enteritidis PT4 to cross-contaminate from inoculated egg droplets on surfaces onto melon or beef (sterile or non-sterile) was investigated. When the foods were placed on these surfaces where egg droplets were still wet, cross- contamination occurred within 1 s onto every piece of food. It took at least 1 min for all the food pieces to be contaminated when egg droplets had been allowed to dry. Both strains were capable of rapid growth on melon and beef (sterile or non-sterile) at 20°C, but growth rates on beef appeared to be slowed by pre-exposure to either 4 or −18°C. INTRODUCTION hydrogen peroxide and to survive well on surfaces, came from a human case (Humphrey et al. 1995). Isolate I, which is Salmonella enteritidis phage type (PT) 4 continues to be of significantly more sensitive to the above conditions (Hum- major importance in human foodborne salmonellosis, par- phrey et al. 1995), was obtained from a chicken carcass. ticularly in Europe (Schmidt 1995). Contaminated chicken meat and eggs have been identified as the most frequently implicated vehicles (Schmidt 1995). As with other sal- Culture conditions monellas (Roberts 1986) cross-contamination in the kitchen may be an important contributory factor in Salm. enteritidis Cultures were maintained on Blood Agar plates (Oxoid Ltd) outbreaks. Previous work at this laboratory (Humphrey et al. at 4°C with subculture onto fresh plates every 2 d. These 1994) demonstrated that, when contaminated egg contents were incubated at 37°C overnight and then stored at 4°C were homogenized, Salm. enteritidis could be recovered from until required. These stock cultures were used in all of the work surfaces over 40 cm away from the mixing bowl. The following experiments, unless otherwise stated. same study also showed prolonged survival of the bacteria in these droplets. Such practices could, therefore, facilitate cross-contamination of salmonellas onto other previously safe Broth cultures foodstuffs. A single colony from the stock culture was inoculated into 9 Contaminated surfaces are unlikely to be a direct hazard ml of Lemco broth (Oxoid) and incubated at 37°C. Cultures and for an outbreak to occur salmonellas must be able to were used 16–18 h after initial incubation and were in station- move onto foodstuffs placed on such a surface and, perhaps, ary phase. Approximately 5×10 8 cells ml −1 were present at be given the opportunity to multiply. This was investigated this stage. in the study reported in this paper using two strains of Salm. enteritidis PT4 with markedly different survival profiles on surfaces (Humphrey et al. 1995). Preparation of melon pieces MATERIALS AND METHODS Honeydew melons were selected firm to the touch, stored at Salmonella isolates 4°C and then used within 2 d of purchase. A methanol-flamed cork-borer (size 18) was used to remove flesh, which was Two Salm. enteritidis PT4 isolates were used. Isolate E, which sliced into discs approximately 3 mm thick, using a sterile has been shown to have enhanced tolerance to heat, acid, scalpel. Seeds and skin were discarded. Melon was prepared Correspondence to: M. A. Bradford, Department of Biological Sciences, just prior to the start of an experiment to prevent drying of Hatherly Laboratories, University of Exeter, Prince of Wales Road, Exeter EX4 4PS, UK the foodstuff. © 1997 The Society for Applied Bacteriology