Listeria survives microwave heating

Listeria survives microwave heating

84 Letters to the Editor patients were given a course of cefotaxime and gentamicin and were put on long-term norfloxacin. The other two had persist...

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patients were given a course of cefotaxime and gentamicin and were put on long-term norfloxacin. The other two had persistent fever following operation and were given a succession of various antibiotics including cefotaxime. All the four cases were re-admitted for persistent fever and signs of endocarditis 2-4 months after operation. Blood culture was positive for Aspergillus fumigatus in one case (valvular replacement) whereas repeated blood cultures were sterile in the rest. Femoral embolectomy and subsequent histopathological examination of the embolus in the case with the positive blood culture showed numerous septate fungal hyphae. All four were fatal, autopsy was possible in the three cases with sterile culture and revealed vegetations on the intracardiac prosthesis which on histopathological section showed uniform, septate fungal hyphae. Fungal endocarditis is very rarely seen in our centre and during the preceding 7 years no case of fungal endocarditis had been encountered. No changes in hospital practice, type of patients or operative major environment were evident to explain this cluster of cases except the enthusiastic use of the recently introduced highly effective antibiotics. Investigations are however, still in progress. Antibiotics are known to change the normal flora of the human body and the occurrence of superinfections with the use of broad spectrum antibiotics has been stressed (Newsom, Lee & Rees, 1967; Goodman et al., 1968). Serious infections with opportunistic fungi is a matter of grave concern. It requires the re-evaluation of antibiotic prescribing practices, institution of an antibiotic policy and greater emphasis on reducing environmental contamination rather than a reliance on antibiotics to combat infection. Geeta

Department of Microbiology, Gobind Ballabh Pant Hospital, New Delhi-l 10002, India


References Goodman, J. S., Schaffner, W., Collins, H. A., Battersby, E. J. & Koenig, M. G. (1968). Infection after cardio-vascular surgery. Clinical study including examination of antibiotic prophylaxis. New England Journal of Medicine 278, 117-l 23. iVewsom, S. W. B., Lee, W. R. & Rees, J. R. (1967). Fatal fungal infection following open-heart surgery. British Journal of Surgery 29, 457-460.

Sir, Listeria




Listeriosis is an emerging public health problem (Hall et al., 1988) and while it may not be possible to establish the source of the infection in its primary means of transmission is through specific patients,


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contamination of foodstuffs (World Health Organisation, 1988). Because Listeria monocytogenes is relatively resistant to heat and can grow under refrigerated conditions, it poses a potential danger in food produced by the cook-chill method (Kerr, Dealler & Lacey, 1988). However, if the final reheating of this food could be shown to reliably destroy this pathogen, then such risks would be reduced. Microwave ovens are used widely to reheat such food in institutions, hospitals and in the home. It is important that are not exposed unnecessarily to food vulnerable hospital patients contaminated with listeria. We have studied the ability of microwave reheating to eliminate listeria from cook-chill (“recipe”) food that had been obtained from retail outlets of high repute. In the first series of experiments, one of two cultures of L. monocytogenes was added to these products to provide about lo6 recoverable organisms per g. These were then heated in one of two modern microwave ovens (Creda “‘micro”, Sharp “Carousel 11”) whose energy output was found to be as stated. Immediately after reheating of each item according to package instructions, core temperatures and culture samples were taken. Results for individual microwave ovens and test organisms were similar and therefore combined. Of the 27 dishes tested, 22 (81%) yielded large numbers of viable listeria after heating, whilst from five (19’/,) no organisms were recovered (limit of detection lo*.’ g-l). In the foods where listeria persisted, the mean temperature after reheating was 71°C (range 48-100°C) with mean reduction of only 6.5 x 10’ (range numbers of 6.2 x loo-1 .5 X 10’). These findings raise the question whether other reheating devices for cook-chill food that elevate the temperature to no more than 70°C (as recommended in the DHSS guidelines) will eliminate contaminating listeria. In those samples where listeria was not recovered, the mean temperature was 91°C (range 8497°C). In other experiments a low inoculum (lO*.l g-i) of listeria was allowed to increase in the food to about 106” g- ’ over 7-10 days at 6°C before reheating, after which the organism showed a change in numbers similar to that described above. There is uncertainty as to whether the presence of listeria in dairy products following pasteurization of milk represents subsequent contamination or is due to the survival of the organism during the process (Donnelly et al., 1987; Doyle et al., 1987; Fernandez Garayzabal et al. 1987). Similarly, the presence of listeria in cook-chill food (Kerr et al., 1988) could result from either of these. Of relevance to this, we have found that in each of seven of these products heated to temperatures sufficiently for the inoculated listeria not to be recovered on immediate sampling, the subsequent retention of these products at 6°C for 5-8 days resulted in the “reappearance” of listeria in numbers of 104-105” g-‘. In these experiments, precautions were taken to avoid contamination during refrigeration and sampling, so the likely explanation for the rapid increase in numbers could be the recovery of heat-damaged bacteria during refrigeration (Bunning, Donnelly & Peeler, 1988), although multiplication



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of the residual inoculum might also contribute. Further studies are required in this area. In conclusion, listeria that may be present in cook-chill food will not be reliably eliminated by microwave reheating, and there is an urgent need to assess microbiologically other methods of reheating. M. K. S. P. R.

R. M. Sheeran G. Kerr F. Dealler R. Hayes W. Lacey

Department of Microbiology University of Leeds Leeds LS2 9JT

References Bunning, V. K., Donelly, C. W., Peeler, J. T., Briggs, E. H., Bradshaw, J. G., Crawford, R. G., Beliveau, C. M. & Tierney, J. T. (1988). Thermal inactivation of Listeria monocytogenes with bovine milk phagocytes. Applied and Environmental Microbiology 54, 366370. Donnelly, C. W., Briggs, E. H. & Donnelly, L. S. (1987). Comparison of heat resistance of Listeria monocytogenes in milk as determined by two methods. Journal of Food Protection 50, 14-17. Doyle, M. P., Glass, K. A., Beery, J. T., Garcia, G. A., Pollard, D. J. & Schultz, R. D. (1987). Survival of Listeria monocytopenes in milk during high-temperature, short-time pasteurization. Applied and Envir&&ental Microbiology53,“1433-i438. Fernandez Garayzabal, J. F., Dominguez Rodriguez, L., Vazquez Boland, J. A., Rodriguez Ferri, E. F., Briones Dieste, V., Blanc0 Cancelo, J. L., Suarez Fernandez, G. (1987). Survival of Listeria monocytogenes in raw milk treated in a pilot plant pasteurizer. Journal of Applied Bacteriology 63, 533-537. Hall, S. M., Crofts, N., Gilbert, R. J., Pini, P. N., Taylor, A. G., & McLauchlin, J. (1988). Epidemiology of Listeriosis, England and Wales, Lancet ii, 502-503. Kerr, K., Dealler, S. F. & Lacey, R. W. (1988). Listeria in cook-chill food. Lancet ii, 37-38. World Health Organisation (WHO) (1988). Food borne listeriosis: W.H.O. Working Group recommendations. Weekly Epidemiological Record 63, 62-63.