Illness associated with Bacillus cereus may be underreported as very few of those affected seek medical attention owing to the mild nature and short duration of ...
FOODBORNE PATHOGENS AND DISEASE Volume 7, Number 7, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089=fpd.2009.0469
The Prevalence and Concentration of Bacillus cereus in Retail Food Products in Brisbane, Australia Sofroni Eglezos,1 Bixing Huang,2 Gary A. Dykes,3 and Narelle Fegan3
Abstract
Illness associated with Bacillus cereus may be underreported as very few of those affected seek medical attention owing to the mild nature and short duration of symptoms. For this reason there is little information on the prevalence and concentration of this pathogen in retail food products. A total of 1263 retail food samples were examined for B. cereus using the Australian Standard 1766.2.6 (1991): spread plate technique on polymyxin pyruvate egg yolk mannitol bromothymol blue agar, of which the limit of detection was log10 2.0 cfu=g. Bacillus cereus was not detected in samples of skim milk powder, sandwiches, sushi, fresh beef mince, tortillas, or shelf stable stir-fry sauces. Bacillus cereus was detected in the following food samples: uncooked pizza bases (1 of 63 samples, log10 count of 2.0 cfu=g), cooked pizzas (8 of 175, mean log10 3.4 cfu=g), cooked meat pies (7 of 157, mean log10 2.2 cfu=g), cooked sausage rolls (5 of 153, mean log10 2.6 cfu=g), processed meats (1 of 350, log10 3.3 cfu=g), and raw diced chicken (3 of 55, mean log10 4.3 cfu=g). It appears that composite food products have more positive detection samples because the numerous ingredients may introduce spores into the foods. This study provides valuable data on the distribution, prevalence, and concentration of B. cereus in selected retail products.
Introduction
T
he food-poisoning organism Bacillus cereus is ubiquitous in nature. Its primary ecological niche is the soil (Andersson et al., 1995) and it is from here that spores are introduced into cereal crops, vegetation, animal hair, fresh water, and sediments (Melling and Capel, 1978; ICMSF, 1996). Raw plant foods, especially rice, potatoes, peas, beans, and spices, are the most common source of B. cereus. OzFoodNet, Australia’s network for enhanced foodborne disease surveillance, reported that 18 of the 741 outbreaks of foodborne or suspected foodborne disease that occurred in Australia between January 2001 and September 2007 were associated with B. cereus (OzFoodNet, 2008, unpublished data, Australian Government Department of Health and Ageing). The most commonly implicated food vehicles in Australian outbreaks of B. cereus were cooked foods with a chicken component (8 out of 18 outbreaks) and cooked foods with a rice component (6 out of 18 outbreaks) (OzFoodNet, 2008, unpublished data, Australian Government Department of Health and Ageing). There was also a single outbreak
associated with a food with a beef component. Data on the prevalence of B. cereus in raw beef and poultry are scarce. The extreme resistance of the spores of this pathogen to heat, desiccation, sanitizers, and irradiation contributes to their distribution, survival, and persistence within the entire food chain, from raw agricultural commodities to finished (nonretorted) products (Smith et al., 2004; Meldrum et al., 2005; Rosenkvist and Hansen, 1995). Bacillus cereus commonly occurs in foods and when consumed at low concentration does not present a public health risk. Nevertheless, foodborne B. cereus illness occurs and continues to increase in Australia (OzFoodNet, 2008, unpublished data, Australian Government Department of Health and Ageing). Illness associated with food products may be underreported as few of those affected seek medical attention owing to the mild nature and short duration of symptoms. The Australia New Zealand Food Standards Code Standard 3.2.2 (Food Standards Australia New Zealand, 2009) includes provisions for a two-step process of cooling foods: cool from 608C to 218C within 2 hours, following by cooling from 218C to 58C in a further 4 hours. There is potential for
1
EML Consulting Services QLD, Tennyson, QLD, Australia. Queensland Health Forensic & Scientific Services, Coopers Plains, QLD, Australia. CSIRO Food and Nutritional Services, Cannon Hill, QLD, Australia.
2 3
867
868
EGLEZOS ET AL.
B. cereus to occur in re-heated foods if this cooling requirement is not followed, as re-heating temperatures are usually not high enough to destroy germination of spore-forming bacteria. The time taken for cooling and re-heating thus provides an opportunity for bacteria to increase in numbers. Composite food products may be at a higher risk of B. cereus contamination due to the use of numerous ingredients through which spores may be introduced. The objective of this study was to investigate the distribution, prevalence, and concentration of B. cereus in a broad range of retail ready-to-eat=ready-toassemble, ready-to-re-heat, and raw foods. Foods for which temperature abuse may be a risk factor for foodborne illness, such as filled savory baked products, were selected to increase the relevance of the study, as were composite food products and foods for which the B. cereus contamination status is not known. Materials and Methods Sampling A total of 1263 retail food samples were obtained from Brisbane (Queensland, Australia) retailers between February 2006 and February 2007. Random samples were drawn, based on availability within each store. No more than 10 samples were taken from each store. These samples were divided into three categories: ready-to-eat=ready-to-assemble foods, ready-to-re-heat foods, and raw foods. Ready-to-eat=readyto-assemble foods comprised 175 pizzas, 350 cooked, cured, chilled, processed meats, 70 sushi samples, 20 sandwiches, 70 powdered skim milk samples, and 70 tortilla shells. Ready-tore-heat foods comprised 157 frozen cooked meat pies, 153 frozen cooked sausage rolls, and 50 shelf stable stir-fry sauces. The raw food category comprised 55 chilled diced chicken samples, 63 chilled unbaked pizza bases, and 30 chilled beef mince samples. Surface temperature measurements were taken at point of sale for chilled products. The temperature of pizzas was not taken as these were made to order and sampled within half an hour of baking. Pizzas were transferred
from the original container into a large sterile stomacher bag. All samples were placed on ice and received within 4 hours into the National Association of Testing Authorities (NATA) accredited laboratory of EML (a private testing laboratory) in Queensland. NATA is Australian Government–endorsed provider of accreditation. Frozen products were received frozen. All chilled and cooked samples that were received into the laboratory above 78C were discarded. The time and temperature combination of 4 hours at 78C was chosen based on the demonstrated inability of food-poisoning strains within the broader B. cereus group (including psychro-tolerant B. cereus strains) to grow at and below this temperature (Guinebretie`re et al., 2008). Microbiological analysis Once received into the laboratory a 10 g sample was removed from each product following the method outlined in Australian Standard 1766.2.6 (Anonymous, 1991), blended with 90 mL of 0.1% sterile peptone solution (Amyl Media, Melbourne, Australia), and stomached for 1 minute. Frozen samples were thawed overnight at 48C before being sampled. Appropriate dilutions were plated onto polymyxin egg yolk mannitol bromothymol blue agar, and analysis for the presence of B. cereus was performed according to AS1766.2.6 (Anonymous, 1991). This standard required incubation of the polymyxin egg yolk mannitol bromothymol blue agar plates at 378C for 24 hours followed by 258C for a further 24 hours, and the presence of B. cereus was then confirmed using microscopy. This technique does not employ the use of a heat treatment and as such the reported result includes both spores and vegetative cells. The limit of detection was 2.0 log10 cfu=g. Results and Discussion Results of the microbiological testing of all samples are summarized in Table 1. No relationship was apparent be-
Table 1. Prevalence and Counts of Bacillus cereus in Retail Food Products Countsb (log cfu=g) Food products Ready-to-eat=ready-to-assemble foods Pizzas Cooked, cured, processed meats Sushi Sandwiches Skim milk powder Tortilla shells Ready-to-re-heat foods Frozen cooked meat pies Frozen cooked sausage rolls Shelf stable stir-fry sauces Raw foods Chilled raw diced chicken Unbaked chilled pizza bases Chilled raw beef mince a
n
Prevalencea % (n)
Mean
Maximum
175 350 70 20 70 70
5.1 (8) 0.3 (1)
