Barbara M. Lund

The Occurrence and Prevention of Foodborne Disease in Vulnerable People

In developed countries, such as the United Kingdom and the United States, between 15% and 20% of the population show greater susceptibility than the general population to foodborne disease. This proportion includes people with primary immunodeficiency, patients treated with radiation or with immunosuppressive drugs for cancer and diseases of the immune system, those with acquired immune-deficiency syndrome and diabetics, people suffering from liver or kidney disease or with excessive iron in the blood, pregnant women, infants, and the elderly. Malnutrition and use of antacids, particularly proton-pump inhibitors, also increase susceptibility. We review the occurrence of infection by foodborne pathogens in these groups of people and measures to prevent infection. The nature and use of low microbial diets to reduce the risk of foodborne disease in immunocompromised patients are very variable. Diets for vulnerable people in care should exclude higher-risk foods, and vulnerable people in the community should receive clear advice about food safety, in particular avoidance of higher-risk foods and substitution of safer, nutritious foods.

The effect of recovery medium on the estimated heat-inactivation of spores of non-proteolytic Clostridium botulinum

Heating spores of non‐proteolytic strains of Clostridium botulinum at 85°C, followed by enumeration of survivors on a highly nutrient medium indicated a 5 decimal kill in less than 2 min. The inclusion of lysozyme or egg yolk emulsion in the recovery medium substantially increased apparent spore heat‐resistance, with as little as 0.1 μg lysozyme/ml sufficient to give an increase in the number of survivors. After heating at 85°C for 2 min between 0.1% and 1% of the spores of 11 strains (5 type B, 4 type E, 2 type F) formed colonies on medium containing 10 μg lysozyme/ml. Enumeration of survivors on a medium containing lysozyme showed that heating at 85°C for 5 min resulted in an estimated 2.6 decimal kill of spores of strain 17B (type B). These findings are important in the assessment of heat‐treatments required to ensure the safety with respect to non‐proteolytic Clostridium botulinum of processed (pasteurized) refrigerated foods for extended storage such as sous‐vide foods.

The effect of nisin on Listeria monocytogenes in culture medium and long-life cottage cheese.

M.A.S.S. FERREIRA AND B.M. LUND. 1996. The sensitivity to nisin of 27 strains of Listeria monocytogenes, four of L. innocua and one of L. ivanovii was estimated at pH 6.8 and pH 5.5. Strains of L. monocytogenes showed differences in sensitivity which were not correlated with serotype. Strains of L. innocua were as resistant as the most resistant strains of L. monocytogenes, whereas the strain of L. ivanovii was relatively sensitive. Two of the most resistant strains of L. monocytogenes multiplied in aerated liquid medium adjusted to pH 5.0 with HCl, incubated at 20°C; nisin, 500 IU ml‐1, prevented multiplication and caused death. Following inoculation of a resistant strain into long‐life cottage cheese, pH 4.6–4.7, the number of viable L. monocytogenes decreased approximately 10‐fold during storage at 20°C for 7 d; addition of nisin, 2000 IU g‐1, to the cottage cheese increased the rate of inactivation to approximately a 1000‐fold decrease in 3 d.

Pasteurization of milk and the heat resistance of Mycobacterium avium subsp. paratuberculosis: a critical review of the data

Mycobacterium avium subsp. paratuberculosis (M. paratuberculosis) causes Johnes disease in ruminants (including cattle, sheep and goats) and other animals, and may contribute to Crohns disease in humans. This possibility, and the fact that M. paratuberculosis may be present in raw milk, make it important to ensure that the heat treatment specified for pasteurization of milk will give acceptable inactivation of this bacterium, with an adequate margin of safety. Published studies of the heat resistance of this bacterium in milk have given widely differing results. Possible reasons for these differences, and the technical problems involved in the work, are reviewed. It is concluded that there is a need (i) for the adoption of an agreed Performance Criterion for pasteurization of milk in relation to this bacterium, (ii) a need for definitive laboratory experiments to understand and determine the heat resistance of M. paratuberculosis, and (iii) a need for an assessment of whether the minimum heat treatments specified at present for pasteurization of milk (Process Criteria) will meet the Performance Criterion for M. paratuberculosis. Measures are also required to ensure that commercial processes deliver continually the specified heat treatment, and to ensure that post-pasteurization contamination is avoided.

Microbiological changes in cottage cheese varieties during storage at +7°C

Abstract Studies have been made of microbiological changes in cottage cheese varieties during storage at 7°C. Commercially prepared products obtained directly from three manufacturers were at an initial pH in the range 4·6 to 5·1 and differed significantly in their content of sorbic acid/sorbate and of viable lactic streptococci. In many of the varieties that did not contain sorbic acid/sorbate multiplication occurred of Pseudomonas spp., Enterobacteriaceae and yeasts; spoilage occurred in some batches due to formation of a surface film of Pseudomonas fluorescens or of surface colonies of Sporobolomyces roseus and Trichosporon sp. at times equal to, or slightly greater than the recommended storage life. Where high numbers of viable lactic streptococci were present in varieties immediately after manufacture, no marked decrease in pH or increase in titratable acidity was observed during storage at 7°C. In varieties that contained sorbic acid at a concentration higher than approximately 500 mg kg−1 little increase in numbers of bacteria or yeasts was observed during storage at 7°C. Decrease in quality due to separation of whey occurred in some products from each manufacturer, but no clear correlation was observed between separation of whey and pH or the number of lactic streptococci present. Bacteria of public health significance, Escherichia coli type I, Staphylococcus aureus and Vibrio spp. were not detected in these products. Fifty-nine samples produced by six manufacturers, purchased from retail outlets, were examined on the sell-by date; the results confirmed the conclusions drawn from studies during storage.

Heat-resistance of spores of non-proteolytic Clostridium botulinum estimated on medium containing lysozyme

Heat treatment of spores of non‐proteolytic strains of Clostridium botulinum at 75–90°C, and enumeration of survivors on a nutrient medium containing lysozyme gave biphasic survival curves. A majority of spores were inactivated rapidly by heating, and the apparent heat‐resistance of these spores was similar to that observed by enumeration on medium without lysozyme. A minority of spores showed much greater heat‐resistance, due to the fact that the spore coat was permeable to lysozyme, which diffused into the spore from the medium and replaced the heat‐inactivated germination system. The proportion of heated spores permeable to lysozyme was between 0.2 and 1.4% for spores of strains 17B (type B) and Beluga (type E), but was about 20% for spores of strain Foster B96 (type E). After treatment of heated spores with alkaline thioglycolate, all were permeable to lysozyme. D‐values for heated spores that were permeable to lysozyme (naturally and after treatment with thioglycolate) were: for strain 17B, D85°C, 100 min; D90°C, 18.7 min; D95°C, 4.4 min; for strain Beluga, D85°C, 46 min; D90°C, 11.8 min; D95°C, 2.8 min. The z‐values for these spores of strains 17B and Beluga were 7.6°C and 8.3°C.

Factors influencing the soft-rotting of potato tubers by bacteria

SummaryThe production of extensive soft rot in potato tubers which were wounded, inoculated withErwinia carotovora var.atroseptica and stored inc. 100% RH at 20° was greatly increased by replacing air in containers with N2. Accumulation of CO2 due to tuber respiration did not significantly affect the production of rots in these conditions. When tubers were inoculated with sterile water in place of theErwinia and held under anaerobic conditions, spreading soft rots were also produced andClostridia, but not soft rotErwinia, were isolated from the rotted tissue. Some of theseClostridia were shown to be capable of breaking down potato tissue, and may be a significant cause of potato soft rots developing in store.

The effect of oxygen and carbon dioxide concentrations on bacterial soft rot of potatoes. I. King Edward potatoes inoculated withErwinia carotovora var.atroseptica

SummaryMatureKing Edward potatoes were wounded, inoculated withErwinia carotovora var.atroseptica and stored at +10°C and 100% r.h. in controlled concentrations of oxygen and carbon dioxide for up to 18 days. After incubation in air or in 5% O2 in N2 rots were brown, dry and restricted; more extensive, soft spreading rots were produced in potatoes stored in 5% O2+16% CO2, in 1% O2+20% CO2, in 1% O2 and in N2, the most extensive rots occurring in the anaerobic conditions. In addition toE. carotovora var.atroseptica, pectolytic clostridia could also be recovered from the spreading rots. The experiments demonstrate the extent to which the concentration of oxygen and carbon dioxide in the environment may effect the sensitivity of tubers to bacterial attack.

The effect of redox potential, and its interaction with sodium chloride concentration, on the probability of growth of Clostridium botulinum type E from spore inocula

Abstract A most probable number (mpn) count has been used to assess the effect of redox potential and of redox potential combined with increasing concentrations of sodium chloride on the probability of growth of spores of C. botulinum type E strain Beluga. In the majority of cases the maximum count developed within five days. In medium at pH 6·8 – 7·0, containing 0·1% w/v NaCl, incubated at 20°C, the probability of growth of spores in five days was not significantly lower at an E h of approx +60 mV, adjusted by the introduction of air, than in strictly anaerobic conditions at E h −400 mV. Increasing the E h above +60 mV resulted in a decrease in the probability of growth of spores, and at a redox potential between approx +122 mV and +164 mV the probability of growth was decreased by a factor of over 10 5 compared with that at the lowest E h . The inclusion of 3·25% w/v and 4% w/v NaCl in medium at a redox potential of −400 mV decreased the probability of growth by factors of 10 2 and 10 4 respectively, while the combination of 3·25% w/v NaCl and an E h between +62 mV and +122 mV decreased the probability of growth by a factor of 10 6 .

Factors affecting growth from heat‐treated spores of non‐proteolytic Clostridium botulinum

Heat treatment of spores of non‐proteolytic Clostridium botulinum at 85°C for 120 min followed by enumeration of survivors on a medium containing lysozyme resulted in a 4.1 and 4.8 decimal reduction in numbers of spores of strains 17B (type B) and Beluga (type E), respectively. Only a small proportion of heated spores formed colonies on medium containing lysozyme; this proportion could be increased by treatments designed to increase the permeability of heated spores. The results indicate that the germination system in spores of non‐proteolytic Cl. botulinum was destroyed by heating, that lysozyme could replace this germination system, and that treatments that increased the permeability of the spore coat could increase the proportion of heated spores that germinated on medium containing lysozyme. These results are important in relation to the assessment of heat‐treatments required to reduce the risk of survival and growth of non‐proteolytic Clostridium botulinum in processed (pasteurized) refrigerated foods for extended storage.